feat: EGOTHOR v4 hot-path additions

.project

@@ -2,21 +2,22 @@
 <projectDescription>
 	<name>Radixor</name>
 	<comment></comment>
-	<projects/>
+	<projects>
+	</projects>
+	<buildSpec>
+		<buildCommand>
+			<name>org.eclipse.jdt.core.javabuilder</name>
+			<arguments>
+			</arguments>
+		</buildCommand>
+		<buildCommand>
+			<name>org.eclipse.buildship.core.gradleprojectbuilder</name>
+			<arguments>
+			</arguments>
+		</buildCommand>
+	</buildSpec>
 	<natures>
 		<nature>org.eclipse.jdt.core.javanature</nature>
 		<nature>org.eclipse.buildship.core.gradleprojectnature</nature>
 	</natures>
-	<buildSpec>
-		<buildCommand>
-			<name>org.eclipse.jdt.core.javabuilder</name>
-			<arguments/>
-		</buildCommand>
-		<buildCommand>
-			<name>org.eclipse.buildship.core.gradleprojectbuilder</name>
-			<arguments/>
-		</buildCommand>
-	</buildSpec>
-	<linkedResources/>
-	<filteredResources/>
 </projectDescription>

docs/compatibility-and-guarantees.md

@@ -75,6 +75,14 @@ The distinction between preferred-result lookup and multi-result lookup is part
 
 That model is part of how the public API should be understood.
 
+Visitor lookup methods such as `getAllNormalized(..., EntrySink, maxResults)` are additive hot-path APIs. They expose the same local ordering and count semantics without allocating result containers, but they do not replace `get()`, `getAll()`, or `getEntries()`.
+
+Compiled `FrequencyTrie` instances are immutable and thread-safe for concurrent reads. Visitor sinks are caller-owned and are not retained by the trie. Stored values passed to sinks are the model-owned trie values; for `FrequencyTrie<String>` patch tries, those patch strings are immutable stored strings rather than fresh per-result strings.
+
+### Stable patch application behavior
+
+`PatchCommandEncoder.apply(...)` remains the compatibility API for string-returning patch application. Buffer-oriented `applyTo(...)` overloads are additive APIs for caller-owned output storage. They do not retain output arrays, report insufficient capacity with `APPLY_INSUFFICIENT_CAPACITY`, and preserve the existing malformed-patch compatibility behavior where `apply(...)` preserves the source.
+
 ### Stable reduction-mode intent
 
 Each public `ReductionMode` constant carries a semantic contract that should remain meaningful across versions.

docs/programmatic-querying-and-ambiguity.md

@@ -33,6 +33,28 @@ import org.egothor.stemmer.ValueCount;
 final List<ValueCount<String>> entries = trie.getEntries("axes");
 ```
 
+### Visitor lookup for hot paths
+
+For allocation-sensitive token loops, use the visitor-style lookup methods. They visit the same ordered local values and counts without allocating a result array, list, or `ValueCount` objects.
+
+```java
+trie.getAll("axes", (patch, count, rank) -> {
+    // rank is zero-based and follows the same ordering as getAll(String).
+    return true; // return false to stop after this callback
+}, 8);
+```
+
+If the caller has already normalized the input exactly as required by `trie.metadata()`, the normalized methods avoid lookup normalization buffers too:
+
+```java
+final char[] token = "axes".toCharArray();
+trie.getAllNormalized(token, 0, token.length, (patch, count, rank) -> {
+    return true;
+}, 8);
+```
+
+`getAllNormalized(...)` bypasses `caseProcessingMode` and `diacriticProcessingMode`; callers are responsible for supplying canonical input. `maxResults == 0` visits nothing, negative values are rejected, and a sink returning `false` stops iteration after the current callback.
+
 ## Apply patch commands
 
 A patch command is not the final stem. It must be applied to the original input token. `PatchCommandEncoder.apply(source, patchCommand)` performs that transformation directly on the serialized command format. If the source is `null`, the method returns `null`. If the patch is `null`, empty, or malformed in compatibility-relevant ways, the original source word is preserved. Equal source and target words are represented by the canonical no-op patch.
@@ -45,6 +67,25 @@ final String patch = trie.get(word);
 final String stem = PatchCommandEncoder.apply(word, patch);
 ```
 
+Hot paths can apply a patch into caller-owned character storage:
+
+```java
+final char[] output = new char[32];
+final int produced = PatchCommandEncoder.applyTo(
+        word,
+        patch,
+        trie.traversalDirection(),
+        output,
+        0,
+        output.length);
+
+if (produced != PatchCommandEncoder.APPLY_INSUFFICIENT_CAPACITY) {
+    final String stem = new String(output, 0, produced);
+}
+```
+
+`applyTo(...)` returns the produced character count on success and `APPLY_INSUFFICIENT_CAPACITY` when the output range is too small. Capacity failure does not write partial output. The source and output ranges of the `char[]` overload must not overlap.
+
 For multiple candidates:
 
 ```java

src/jmh/java/org/egothor/stemmer/benchmark/FrequencyTrieLookupBenchmark.java

@@ -31,11 +31,13 @@
 package org.egothor.stemmer.benchmark;
 
 import java.io.IOException;
+import java.util.List;
 import java.util.concurrent.TimeUnit;
 import org.egothor.stemmer.FrequencyTrie;
 import org.egothor.stemmer.PatchCommandEncoder;
 import org.egothor.stemmer.ReductionMode;
 import org.egothor.stemmer.ReductionSettings;
+import org.egothor.stemmer.ValueCount;
 import org.openjdk.jmh.annotations.Benchmark;
 import org.openjdk.jmh.annotations.BenchmarkMode;
 import org.openjdk.jmh.annotations.Level;
@@ -97,12 +99,45 @@ public class FrequencyTrieLookupBenchmark {
          */
         private String[] lookupKeys;
 
+        /**
+         * Lookup keys as normalized caller-owned character storage.
+         */
+        private char[][] lookupKeyCharacters;
+
         /**
          * Keys that are known to return multiple patch candidates from
          * {@code getAll()}.
          */
         private String[] ambiguousLookupKeys;
 
+        /**
+         * Ambiguous lookup keys as normalized caller-owned character storage.
+         */
+        private char[][] ambiguousLookupKeyCharacters;
+
+        /**
+         * Preferred patches aligned with {@link #lookupKeys}.
+         */
+        private String[] preferredPatches;
+
+        /**
+         * Reusable output buffer for patch application benchmarks.
+         */
+        private char[] outputBuffer;
+
+        /**
+         * Mutable field consumed by visitor sinks.
+         */
+        private int visitorAccumulator;
+
+        /**
+         * Sink used by visitor lookup benchmarks without per-invocation allocation.
+         */
+        private final FrequencyTrie.EntrySink<String> visitorSink = (value, count, rank) -> {
+            this.visitorAccumulator += value.length() + count + rank;
+            return true;
+        };
+
         /**
          * Initializes the benchmark state.
          *
@@ -116,6 +151,23 @@ public class FrequencyTrieLookupBenchmark {
             this.trie = BenchmarkCorpusSupport.compilePatchTrie(corpus.dictionaryText(), settings, true);
             this.lookupKeys = corpus.lookupKeys();
             this.ambiguousLookupKeys = corpus.ambiguousLookupKeys();
+            this.lookupKeyCharacters = toCharArrays(this.lookupKeys);
+            this.ambiguousLookupKeyCharacters = toCharArrays(this.ambiguousLookupKeys);
+            this.preferredPatches = new String[this.lookupKeys.length];
+            int maxKeyLength = 0;
+            for (int index = 0; index < this.lookupKeys.length; index++) {
+                this.preferredPatches[index] = this.trie.get(this.lookupKeys[index]);
+                maxKeyLength = Math.max(maxKeyLength, this.lookupKeys[index].length());
+            }
+            this.outputBuffer = new char[maxKeyLength + 32];
+        }
+
+        private static char[][] toCharArrays(final String[] values) {
+            final char[][] characters = new char[values.length][];
+            for (int index = 0; index < values.length; index++) {
+                characters[index] = values[index].toCharArray();
+            }
+            return characters;
         }
     }
 
@@ -155,6 +207,61 @@ public class FrequencyTrieLookupBenchmark {
         }
     }
 
+    /**
+     * Measures retrieval of all patch candidates through caller-owned normalized
+     * character storage and a visitor sink.
+     *
+     * @param state prepared lookup state
+     * @param blackhole sink preventing dead-code elimination
+     */
+    @Benchmark
+    public void lookupAllPatchesWithNormalizedCharVisitor(final LookupState state, final Blackhole blackhole) {
+        final char[][] keys = state.ambiguousLookupKeyCharacters;
+        for (char[] key : keys) {
+            final int count = state.trie.getAllNormalized(key, 0, key.length, state.visitorSink, Integer.MAX_VALUE);
+            if (count < 2) {
+                throw new IllegalStateException("Expected multiple patches for benchmark key.");
+            }
+        }
+        blackhole.consume(state.visitorAccumulator);
+    }
+
+    /**
+     * Measures counted candidate retrieval through the allocating entry API.
+     *
+     * @param state prepared lookup state
+     * @param blackhole sink preventing dead-code elimination
+     */
+    @Benchmark
+    public void lookupPatchEntries(final LookupState state, final Blackhole blackhole) {
+        final String[] keys = state.ambiguousLookupKeys;
+        for (String key : keys) {
+            final List<ValueCount<String>> entries = state.trie.getEntries(key);
+            if (entries.size() < 2) {
+                throw new IllegalStateException("Expected multiple entries for key " + key + '.');
+            }
+            blackhole.consume(entries);
+        }
+    }
+
+    /**
+     * Measures counted candidate retrieval through the visitor API.
+     *
+     * @param state prepared lookup state
+     * @param blackhole sink preventing dead-code elimination
+     */
+    @Benchmark
+    public void lookupPatchEntriesWithVisitor(final LookupState state, final Blackhole blackhole) {
+        final char[][] keys = state.ambiguousLookupKeyCharacters;
+        for (char[] key : keys) {
+            final int count = state.trie.getAllNormalized(key, 0, key.length, state.visitorSink, Integer.MAX_VALUE);
+            if (count < 2) {
+                throw new IllegalStateException("Expected multiple entries for benchmark key.");
+            }
+        }
+        blackhole.consume(state.visitorAccumulator);
+    }
+
     /**
      * Measures end-to-end preferred stemming from lookup plus patch application.
      *
@@ -170,6 +277,28 @@ public class FrequencyTrieLookupBenchmark {
         }
     }
 
+    /**
+     * Measures patch application into caller-owned output storage.
+     *
+     * @param state prepared lookup state
+     * @param blackhole sink preventing dead-code elimination
+     */
+    @Benchmark
+    public void applyPreferredPatchToBuffer(final LookupState state, final Blackhole blackhole) {
+        final String[] keys = state.lookupKeys;
+        final String[] patches = state.preferredPatches;
+        final char[] output = state.outputBuffer;
+        for (int index = 0; index < keys.length; index++) {
+            final int length = PatchCommandEncoder.applyTo(keys[index], patches[index],
+                    state.trie.traversalDirection(), output, 0, output.length);
+            if (length == PatchCommandEncoder.APPLY_INSUFFICIENT_CAPACITY) {
+                throw new IllegalStateException("Output buffer too small for key " + keys[index] + '.');
+            }
+            blackhole.consume(length);
+            blackhole.consume(output[0]);
+        }
+    }
+
     /**
      * Measures end-to-end full candidate stemming from {@code getAll()} plus
      * patch application.

src/main/java/org/egothor/stemmer/CaseProcessingMode.java

@@ -48,8 +48,7 @@ public enum CaseProcessingMode {
     AS_IS,
 
     /**
-     * Normalizes all dictionary content to lower case using
+     * Normalizes all dictionary content to lower case using {@link Locale#ROOT}.
-     * {@link Locale#ROOT}.
      */
     LOWERCASE_WITH_LOCALE_ROOT
 }

src/main/java/org/egothor/stemmer/DiacriticStripper.java

@@ -93,12 +93,12 @@ final class DiacriticStripper {
     }
 
     /**
-     * Removes supported diacritic marks and common Latin ligatures from the supplied
+     * Removes supported diacritic marks and common Latin ligatures from the
-     * text.
+     * supplied text.
      *
      * <p>
-     * The method returns the original {@link String} instance when no replacement is
+     * The method returns the original {@link String} instance when no replacement
-     * required, avoiding an unnecessary allocation on the common ASCII path.
+     * is required, avoiding an unnecessary allocation on the common ASCII path.
      * </p>
      *
      * @param input text to normalize

src/main/java/org/egothor/stemmer/FrequencyTrie.java

@@ -119,7 +119,8 @@ public final class FrequencyTrie<V> {
     private final boolean removeDiacritics;
 
     /**
-     * Shared empty array instance for empty lookup results from {@link #getAll(String)}.
+     * Shared empty array instance for empty lookup results from
+     * {@link #getAll(String)}.
      */
     private final V[] emptyValues;
 
@@ -165,13 +166,18 @@ public final class FrequencyTrie<V> {
      */
     private static final int CASE_VERSION = 4;
 
+    /**
+     * Argument name for lookup keys.
+     */
+    private static final String ARG_KEY = "key";
+
     /**
      * Default dense child lookup span in code points used when materializing
      * compiled nodes without an explicit override.
      * <p>
-     * Increasing this value increases the chance of direct array indexing for
+     * Increasing this value increases the chance of direct array indexing for child
-     * child lookup at runtime at the cost of per-node dense table memory for
+     * lookup at runtime at the cost of per-node dense table memory for compact
-     * compact character spans.
+     * character spans.
      * </p>
      */
     public static final int DEFAULT_MAX_EXPANDED_INDEX = 512;
@@ -191,6 +197,30 @@ public final class FrequencyTrie<V> {
         return STREAM_VERSION;
     }
 
+    /**
+     * Receives trie values during visitor-style lookup.
+     *
+     * <p>
+     * Implementations are caller-owned and are not retained by the trie. Returning
+     * {@code false} stops iteration after the current callback.
+     * </p>
+     *
+     * @param <V> value type
+     */
+    @FunctionalInterface
+    public interface EntrySink<V> {
+
+        /**
+         * Accepts one ordered local value.
+         *
+         * @param value stored value
+         * @param count stored local occurrence count
+         * @param rank  zero-based rank in deterministic local ordering
+         * @return {@code true} to continue iteration, {@code false} to stop
+         */
+        boolean accept(V value, int count, int rank);
+    }
+
     /**
      * Creates a new compiled trie instance.
      *
@@ -229,7 +259,7 @@ public final class FrequencyTrie<V> {
      * @throws NullPointerException if {@code key} is {@code null}
      */
     public V get(final String key) {
-        Objects.requireNonNull(key, "key");
+        Objects.requireNonNull(key, ARG_KEY);
         final CompiledNode<V> node = findNode(normalizeLookupKey(key));
         if (node == null) {
             return null;
@@ -266,7 +296,7 @@ public final class FrequencyTrie<V> {
      */
     @SuppressWarnings("PMD.MethodReturnsInternalArray")
     public V[] getAll(final String key) {
-        Objects.requireNonNull(key, "key");
+        Objects.requireNonNull(key, ARG_KEY);
         final CompiledNode<V> node = findNode(normalizeLookupKey(key));
         if (node == null) {
             return this.emptyValues;
@@ -301,7 +331,7 @@ public final class FrequencyTrie<V> {
      * @throws NullPointerException if {@code key} is {@code null}
      */
     public List<ValueCount<V>> getEntries(final String key) {
-        Objects.requireNonNull(key, "key");
+        Objects.requireNonNull(key, ARG_KEY);
         final CompiledNode<V> node = findNode(normalizeLookupKey(key));
         if (node == null) {
             return List.of();
@@ -325,6 +355,132 @@ public final class FrequencyTrie<V> {
         return Collections.unmodifiableList(entries);
     }
 
+    /**
+     * Visits all values stored at the node addressed by an already-normalized
+     * {@code char[]} key slice.
+     *
+     * <p>
+     * This method bypasses {@link TrieMetadata#caseProcessingMode()} and
+     * {@link TrieMetadata#diacriticProcessingMode()}. The caller must provide input
+     * normalized exactly as required by this trie's metadata. The trie is immutable
+     * and thread-safe for concurrent reads; the supplied sink is caller-owned and
+     * is not retained.
+     * </p>
+     *
+     * @param key        normalized key storage
+     * @param offset     first character offset
+     * @param length     number of characters to read
+     * @param sink       value sink
+     * @param maxResults maximum number of results to visit
+     * @return number of visited values
+     * @throws NullPointerException      if {@code key} or {@code sink} is
+     *                                   {@code null}
+     * @throws IndexOutOfBoundsException if the key slice is invalid
+     * @throws IllegalArgumentException  if {@code maxResults} is negative
+     */
+    public int getAllNormalized(final char[] key, final int offset, final int length, final EntrySink<? super V> sink,
+            final int maxResults) {
+        Objects.requireNonNull(key, ARG_KEY);
+        Objects.requireNonNull(sink, "sink");
+        Objects.checkFromIndexSize(offset, length, key.length);
+        validateMaxResults(maxResults);
+        if (maxResults == 0) {
+            return 0;
+        }
+        return visitNode(findNode(key, offset, length), sink, maxResults);
+    }
+
+    /**
+     * Visits all values stored at the node addressed by an already-normalized
+     * character sequence.
+     *
+     * @param key        normalized key
+     * @param sink       value sink
+     * @param maxResults maximum number of results to visit
+     * @return number of visited values
+     * @throws NullPointerException     if {@code key} or {@code sink} is
+     *                                  {@code null}
+     * @throws IllegalArgumentException if {@code maxResults} is negative
+     * @see #getAllNormalized(char[], int, int, EntrySink, int)
+     */
+    public int getAllNormalized(final CharSequence key, final EntrySink<? super V> sink, final int maxResults) {
+        Objects.requireNonNull(key, ARG_KEY);
+        Objects.requireNonNull(sink, "sink");
+        validateMaxResults(maxResults);
+        if (maxResults == 0) {
+            return 0;
+        }
+        return visitNode(findNode(key), sink, maxResults);
+    }
+
+    /**
+     * Visits the first value stored at the node addressed by an already-normalized
+     * {@code char[]} key slice.
+     *
+     * @param key    normalized key storage
+     * @param offset first character offset
+     * @param length number of characters to read
+     * @param sink   value sink
+     * @return {@code true} when a value was visited, otherwise {@code false}
+     * @see #getAllNormalized(char[], int, int, EntrySink, int)
+     */
+    public boolean getFirstNormalized(final char[] key, final int offset, final int length,
+            final EntrySink<? super V> sink) {
+        return getAllNormalized(key, offset, length, sink, 1) == 1;
+    }
+
+    /**
+     * Visits the first value stored at the node addressed by an already-normalized
+     * character sequence.
+     *
+     * @param key  normalized key
+     * @param sink value sink
+     * @return {@code true} when a value was visited, otherwise {@code false}
+     * @see #getAllNormalized(CharSequence, EntrySink, int)
+     */
+    public boolean getFirstNormalized(final CharSequence key, final EntrySink<? super V> sink) {
+        return getAllNormalized(key, sink, 1) == 1;
+    }
+
+    /**
+     * Visits all values stored at the node addressed by the supplied key, applying
+     * metadata-driven lookup normalization when required.
+     *
+     * <p>
+     * This method preserves the same lookup normalization semantics as
+     * {@link #getAll(String)}. It may allocate when metadata requires lowercase or
+     * diacritic normalization.
+     * </p>
+     *
+     * @param key        key to resolve
+     * @param sink       value sink
+     * @param maxResults maximum number of results to visit
+     * @return number of visited values
+     */
+    public int getAll(final CharSequence key, final EntrySink<? super V> sink, final int maxResults) {
+        Objects.requireNonNull(key, ARG_KEY);
+        Objects.requireNonNull(sink, "sink");
+        validateMaxResults(maxResults);
+        if (maxResults == 0) {
+            return 0;
+        }
+        final CharSequence normalized = normalizeLookupKey(key);
+        return visitNode(findNode(normalized), sink, maxResults);
+    }
+
+    /**
+     * Visits the first value stored at the node addressed by the supplied key,
+     * applying metadata-driven lookup normalization when required.
+     *
+     * @param key  key to resolve
+     * @param sink value sink
+     * @return {@code true} when a value was visited, otherwise {@code false}
+     * @see #getAll(CharSequence, EntrySink, int)
+     */
+    public boolean getFirst(final CharSequence key, final EntrySink<? super V> sink) {
+        return getAll(key, sink, 1) == 1;
+    }
+
     /**
      * Returns the logical key traversal direction used by this trie.
      *
@@ -431,16 +587,17 @@ public final class FrequencyTrie<V> {
      * dense child-index span configuration.
      * <p>
      * This setting is applied only while materializing the in-memory compiled
-     * representation during load. It is not serialized in {@link TrieMetadata},
+     * representation during load. It is not serialized in {@link TrieMetadata}, so
-     * so each load can independently choose its own runtime lookup trade-off.
+     * each load can independently choose its own runtime lookup trade-off.
      * </p>
      *
-     * @param inputStream       source input stream
+     * @param inputStream      source input stream
-     * @param arrayFactory      array factory used to create typed arrays
+     * @param arrayFactory     array factory used to create typed arrays
-     * @param valueCodec        codec used to read values
+     * @param valueCodec       codec used to read values
-     * @param maxExpandedIndex  dense lookup span override; zero disables dense lookup,
+     * @param maxExpandedIndex dense lookup span override; zero disables dense
-     *                          negative values use {@link #DEFAULT_MAX_EXPANDED_INDEX}
+     *                         lookup, negative values use
-     * @param <V>               value type
+     *                         {@link #DEFAULT_MAX_EXPANDED_INDEX}
+     * @param <V>              value type
      * @return deserialized compiled trie
      * @throws NullPointerException if any argument is {@code null}
      * @throws IOException          if reading fails or the binary format is invalid
@@ -573,7 +730,8 @@ public final class FrequencyTrie<V> {
 
             final TrieMetadata sourceMetadata = readMetadata(dataInput, version);
             final int effectiveMaxExpandedIndex = maxExpandedIndex >= 0 ? maxExpandedIndex : DEFAULT_MAX_EXPANDED_INDEX;
-            final CompiledNode<V>[] nodes = readNodes(dataInput, arrayFactory, valueCodec, nodeCount, effectiveMaxExpandedIndex);
+            final CompiledNode<V>[] nodes = readNodes(dataInput, arrayFactory, valueCodec, nodeCount,
+                    effectiveMaxExpandedIndex);
             final CompiledNode<V> rootNode = nodes[rootNodeId];
 
             if (LOGGER.isLoggable(Level.FINE)) {
@@ -584,12 +742,12 @@ public final class FrequencyTrie<V> {
         }
 
         private static DataInputStream wrapInputStream(final InputStream inputStream) {
-            return inputStream instanceof DataInputStream
+            return inputStream instanceof DataInputStream ? (DataInputStream) inputStream
-                    ? (DataInputStream) inputStream
                     : new DataInputStream(inputStream);
         }
 
-        private static TrieMetadata readMetadata(final DataInputStream dataInput, final int version) throws IOException {
+        private static TrieMetadata readMetadata(final DataInputStream dataInput, final int version)
+                throws IOException {
             if (version == STREAM_VERSION) {
                 return readTextMetadata(dataInput);
             }
@@ -600,12 +758,12 @@ public final class FrequencyTrie<V> {
             }
 
             final ReductionSettings reductionSettings = readReductionSettings(dataInput);
-            final DiacriticProcessingMode diacriticProcessingMode = readEnumByOrdinal(dataInput, DiacriticProcessingMode.values(),
+            final DiacriticProcessingMode diacriticProcessingMode = readEnumByOrdinal(dataInput,
-                    "diacritic processing mode");
+                    DiacriticProcessingMode.values(), "diacritic processing mode");
-            final CaseProcessingMode caseProcessingMode = version >= CASE_VERSION
+            final CaseProcessingMode caseProcessingMode = version >= CASE_VERSION ? readCaseProcessingMode(dataInput)
-                    ? readCaseProcessingMode(dataInput)
                     : CaseProcessingMode.LOWERCASE_WITH_LOCALE_ROOT;
-            return new TrieMetadata(version, traversalDirection, reductionSettings, diacriticProcessingMode, caseProcessingMode);
+            return new TrieMetadata(version, traversalDirection, reductionSettings, diacriticProcessingMode,
+                    caseProcessingMode);
         }
 
         private static TrieMetadata readTextMetadata(final DataInputStream dataInput) throws IOException {
@@ -644,8 +802,9 @@ public final class FrequencyTrie<V> {
             return values[ordinal];
         }
 
-        private static <V> CompiledNode<V>[] readNodes(final DataInputStream dataInput, final IntFunction<V[]> arrayFactory,
+        private static <V> CompiledNode<V>[] readNodes(final DataInputStream dataInput,
-                final ValueStreamCodec<V> valueCodec, final int nodeCount, final int maxExpandedIndex) throws IOException {
+                final IntFunction<V[]> arrayFactory, final ValueStreamCodec<V> valueCodec, final int nodeCount,
+                final int maxExpandedIndex) throws IOException {
             final char[][] edgeLabelsByNode = new char[nodeCount][];
             final int[][] childNodeIdsByNode = new int[nodeCount][];
             @SuppressWarnings("unchecked")
@@ -700,14 +859,16 @@ public final class FrequencyTrie<V> {
 
         private static <V> CompiledNode<V> resolveNode(final int nodeIndex, final char[][] edgeLabelsByNode,
                 final int[][] childNodeIdsByNode, final V[][] orderedValuesByNode, final int[][] orderedCountsByNode,
-                final CompiledNode<V>[] nodes, final boolean[] inProgress, final int maxExpandedIndex) throws IOException {
+                final CompiledNode<V>[] nodes, final boolean[] inProgress, final int maxExpandedIndex)
+                throws IOException {
             final CompiledNode<V> cachedNode = nodes[nodeIndex];
             if (cachedNode != null) {
                 return cachedNode;
             }
 
             if (inProgress[nodeIndex]) {
-                throw new IOException("Invalid serialized node graph: cyclic reference detected at node " + nodeIndex + '.');
+                throw new IOException(
+                        "Invalid serialized node graph: cyclic reference detected at node " + nodeIndex + '.');
             }
             inProgress[nodeIndex] = true;
             try {
@@ -720,16 +881,15 @@ public final class FrequencyTrie<V> {
                 for (int edgeIndex = 0; edgeIndex < edgeCount; edgeIndex++) {
                     final int childNodeId = childNodeIds[edgeIndex];
                     if (childNodeId < 0 || childNodeId >= edgeLabelsByNode.length) {
-                        throw new IOException(
+                        throw new IOException("Invalid child node id at node " + nodeIndex + ", edge index " + edgeIndex
-                                "Invalid child node id at node " + nodeIndex + ", edge index " + edgeIndex + ": "
+                                + ": " + childNodeId);
-                                        + childNodeId);
                     }
                     children[edgeIndex] = resolveNode(childNodeId, edgeLabelsByNode, childNodeIdsByNode,
                             orderedValuesByNode, orderedCountsByNode, nodes, inProgress, maxExpandedIndex);
                 }
 
-                final CompiledNode<V> node = new CompiledNode<>(edgeLabels, children, orderedValuesByNode[nodeIndex], maxExpandedIndex,
+                final CompiledNode<V> node = new CompiledNode<>(edgeLabels, children, orderedValuesByNode[nodeIndex],
-                        orderedCountsByNode[nodeIndex]);
+                        maxExpandedIndex, orderedCountsByNode[nodeIndex]);
                 nodes[nodeIndex] = node;
                 return node;
             } finally {
@@ -740,8 +900,9 @@ public final class FrequencyTrie<V> {
         private static void validateSerializedEdges(final int nodeIndex, final char... edgeLabels) throws IOException {
             for (int edgeIndex = 1; edgeIndex < edgeLabels.length; edgeIndex++) {
                 if (edgeLabels[edgeIndex - 1] >= edgeLabels[edgeIndex]) {
-                    throw new IOException("Edge labels must be strictly ascending at node " + nodeIndex + ", edge index "
+                    throw new IOException(
-                            + edgeIndex + ": '" + edgeLabels[edgeIndex - 1] + "' then '" + edgeLabels[edgeIndex] + "'.");
+                            "Edge labels must be strictly ascending at node " + nodeIndex + ", edge index " + edgeIndex
+                                    + ": '" + edgeLabels[edgeIndex - 1] + "' then '" + edgeLabels[edgeIndex] + "'.");
                 }
             }
         }
@@ -754,6 +915,16 @@ public final class FrequencyTrie<V> {
      * @return compiled node, or {@code null} if the path does not exist
      */
     private CompiledNode<V> findNode(final String key) {
+        return findNode((CharSequence) key);
+    }
+
+    /**
+     * Locates the compiled node for the supplied key.
+     *
+     * @param key already-normalized key to resolve
+     * @return compiled node, or {@code null} if the path does not exist
+     */
+    private CompiledNode<V> findNode(final CharSequence key) {
         CompiledNode<V> current = this.root;
         if (this.lookupTraversalDirection == WordTraversalDirection.BACKWARD) {
             for (int traversalOffset = key.length() - 1; traversalOffset >= 0; traversalOffset--) {
@@ -774,6 +945,77 @@ public final class FrequencyTrie<V> {
         return current;
     }
 
+    /**
+     * Locates the compiled node for the supplied key slice.
+     *
+     * @param key    already-normalized key storage
+     * @param offset first character offset
+     * @param length number of characters to read
+     * @return compiled node, or {@code null} if the path does not exist
+     */
+    private CompiledNode<V> findNode(final char[] key, final int offset, final int length) {
+        CompiledNode<V> current = this.root;
+        if (this.lookupTraversalDirection == WordTraversalDirection.BACKWARD) {
+            for (int traversalOffset = offset + length - 1; traversalOffset >= offset; traversalOffset--) {
+                current = current.findChild(key[traversalOffset]);
+                if (current == null) {
+                    return null;
+                }
+            }
+            return current;
+        }
+
+        final int endExclusive = offset + length;
+        for (int traversalOffset = offset; traversalOffset < endExclusive; traversalOffset++) {
+            current = current.findChild(key[traversalOffset]);
+            if (current == null) {
+                return null;
+            }
+        }
+        return current;
+    }
+
+    /**
+     * Visits node-local values without allocating result containers.
+     *
+     * @param node       resolved node, or {@code null}
+     * @param sink       value sink
+     * @param maxResults maximum values to visit
+     * @return number of visited values
+     */
+    private int visitNode(final CompiledNode<V> node, final EntrySink<? super V> sink, final int maxResults) {
+        if (node == null) {
+            return 0;
+        }
+
+        final V[] orderedValues = node.orderedValues();
+        final int valueCount = Math.min(orderedValues.length, maxResults);
+        if (valueCount == 0) {
+            return 0;
+        }
+
+        final int[] orderedCounts = node.orderedCounts();
+        int visited = 0;
+        for (int rank = 0; rank < valueCount; rank++) {
+            visited++;
+            if (!sink.accept(orderedValues[rank], orderedCounts[rank], rank)) {
+                break;
+            }
+        }
+        return visited;
+    }
+
+    /**
+     * Validates visitor maximum result count.
+     *
+     * @param maxResults maximum result count
+     */
+    private static void validateMaxResults(final int maxResults) {
+        if (maxResults < 0) {
+            throw new IllegalArgumentException("maxResults must be non-negative.");
+        }
+    }
+
     /**
      * Applies lookup-time case normalization according to persisted metadata.
      *
@@ -781,11 +1023,21 @@ public final class FrequencyTrie<V> {
      * @return normalized key for trie traversal
      */
     private String normalizeLookupKey(final String key) {
+        return normalizeLookupKey((CharSequence) key).toString();
+    }
+
+    /**
+     * Applies lookup-time normalization according to persisted metadata.
+     *
+     * @param key lookup key
+     * @return normalized key for trie traversal
+     */
+    private CharSequence normalizeLookupKey(final CharSequence key) {
         if (!this.lowercasesLookupKeys && !this.removeDiacritics) {
             return key;
         }
 
-        String normalized = key;
+        String normalized = key.toString();
         if (this.lowercasesLookupKeys) {
             normalized = normalized.toLowerCase(Locale.ROOT);
         }
@@ -846,9 +1098,9 @@ public final class FrequencyTrie<V> {
         /**
          * Dense edge lookup span threshold.
          * <p>
-         * This value controls a speed/memory trade-off during freezing:
+         * This value controls a speed/memory trade-off during freezing: dense child
-         * dense child lookup tables are allocated only for nodes whose child
+         * lookup tables are allocated only for nodes whose child labels fit in this
-         * labels fit in this span.
+         * span.
          * </p>
          */
         private final int maxExpandedIndex;
@@ -925,8 +1177,8 @@ public final class FrequencyTrie<V> {
 
         /**
          * Creates a new builder with the provided settings, explicit traversal
-         * direction, explicit case processing mode, explicit diacritic processing
+         * direction, explicit case processing mode, explicit diacritic processing mode,
-         * mode, and an explicit dense child lookup threshold.
+         * and an explicit dense child lookup threshold.
          *
          * @param arrayFactory            array factory
          * @param reductionSettings       reduction configuration
@@ -934,10 +1186,10 @@ public final class FrequencyTrie<V> {
          * @param caseProcessingMode      dictionary case processing mode
          * @param diacriticProcessingMode dictionary diacritic processing mode
          * @param maxExpandedIndex        dense lookup span override; zero disables
-         *                               dense lookup. Larger values increase direct
+         *                                dense lookup. Larger values increase direct
-         *                               indexing opportunities while potentially
+         *                                indexing opportunities while potentially
-         *                               increasing materialization memory in nodes
+         *                                increasing materialization memory in nodes
-         *                               whose edge label span is within the limit.
+         *                                whose edge label span is within the limit.
          * @throws NullPointerException if any argument is {@code null}
          */
         public Builder(final IntFunction<V[]> arrayFactory, final ReductionSettings reductionSettings,
@@ -1052,7 +1304,7 @@ public final class FrequencyTrie<V> {
          * @throws IllegalArgumentException if {@code count} is less than {@code 1}
          */
         public Builder<V> put(final String key, final V value, final int count) {
-            Objects.requireNonNull(key, "key");
+            Objects.requireNonNull(key, ARG_KEY);
             Objects.requireNonNull(value, "value");
 
             if (count < 1) { // NOPMD

src/main/java/org/egothor/stemmer/FrequencyTrieBuilders.java

@@ -119,11 +119,11 @@ public final class FrequencyTrieBuilders {
      * Copies one compiled node and all reachable descendants into the target
      * builder.
      *
-     * @param node       current compiled node
+     * @param node               current compiled node
-     * @param keyBuilder current key builder
+     * @param keyBuilder         current key builder
      * @param builder            target mutable builder
      * @param traversalDirection logical key traversal direction used by the source
-     * @param <V>                 value type
+     * @param <V>                value type
      */
     private static <V> void copyNode(final CompiledNode<V> node, final StringBuilder keyBuilder,
             final FrequencyTrie.Builder<V> builder, final WordTraversalDirection traversalDirection) {

src/main/java/org/egothor/stemmer/PatchCommandEncoder.java

@@ -67,7 +67,7 @@ import java.util.concurrent.locks.ReentrantLock;
  * instance can still be used safely when needed.
  * </p>
  */
-@SuppressWarnings("PMD.CyclomaticComplexity")
+@SuppressWarnings({ "PMD.AvoidLiteralsInIfCondition", "PMD.CyclomaticComplexity", "PMD.ForLoopVariableCount" })
 public final class PatchCommandEncoder {
 
     /**
@@ -121,6 +121,13 @@ public final class PatchCommandEncoder {
      */
     /* default */ static final String NOOP_PATCH = String.valueOf(new char[] { NOOP_OPCODE, NOOP_ARGUMENT });
 
+    /**
+     * Return value used by
+     * {@link #applyTo(CharSequence, String, WordTraversalDirection, char[], int, int)}
+     * when the caller-owned output range is too small for the transformed text.
+     */
+    public static final int APPLY_INSUFFICIENT_CAPACITY = -1;
+
     /**
      * Prefix used in unsupported NOOP patch argument exceptions.
      */
@@ -346,6 +353,78 @@ public final class PatchCommandEncoder {
         return applyStrategyFor(traversalDirection).apply(source, patchCommand);
     }
 
+    /**
+     * Applies a compact patch command into a caller-owned output buffer.
+     *
+     * <p>
+     * The output array is not retained. Capacity failure is reported by
+     * {@link #APPLY_INSUFFICIENT_CAPACITY} and leaves the output range unchanged.
+     * Malformed compatibility cases preserve the source exactly as
+     * {@link #apply(String, String, WordTraversalDirection)} does.
+     * </p>
+     *
+     * @param source             original source text
+     * @param patchCommand       compact patch command
+     * @param traversalDirection traversal direction used by the patch command
+     * @param output             caller-owned output storage
+     * @param outputOffset       first writable output offset
+     * @param outputLength       writable output capacity
+     * @return produced character count, or {@link #APPLY_INSUFFICIENT_CAPACITY}
+     *         when {@code outputLength} is too small
+     */
+    public static int applyTo(final CharSequence source, final String patchCommand,
+            final WordTraversalDirection traversalDirection, final char[] output, final int outputOffset,
+            final int outputLength) {
+        Objects.requireNonNull(source, "source");
+        Objects.requireNonNull(traversalDirection, "traversalDirection");
+        Objects.requireNonNull(output, "output");
+        Objects.checkFromIndexSize(outputOffset, outputLength, output.length);
+
+        final int sourceLength = source.length();
+        final int producedLength = computeAppliedLength(sourceLength, patchCommand, traversalDirection);
+        if (producedLength > outputLength) {
+            return APPLY_INSUFFICIENT_CAPACITY;
+        }
+        applyToOutput(source, 0, sourceLength, patchCommand, traversalDirection, output, outputOffset, producedLength);
+        return producedLength;
+    }
+
+    /**
+     * Applies a compact patch command from a caller-owned source slice into a
+     * caller-owned output buffer.
+     *
+     * @param source             source storage
+     * @param sourceOffset       first source character offset
+     * @param sourceLength       number of source characters
+     * @param patchCommand       compact patch command
+     * @param traversalDirection traversal direction used by the patch command
+     * @param output             caller-owned output storage
+     * @param outputOffset       first writable output offset
+     * @param outputLength       writable output capacity
+     * @return produced character count, or {@link #APPLY_INSUFFICIENT_CAPACITY}
+     *         when {@code outputLength} is too small
+     * @throws IllegalArgumentException when source and output ranges overlap in the
+     *                                  same array
+     */
+    public static int applyTo(final char[] source, final int sourceOffset, final int sourceLength,
+            final String patchCommand, final WordTraversalDirection traversalDirection, final char[] output,
+            final int outputOffset, final int outputLength) {
+        Objects.requireNonNull(source, "source");
+        Objects.requireNonNull(traversalDirection, "traversalDirection");
+        Objects.requireNonNull(output, "output");
+        Objects.checkFromIndexSize(sourceOffset, sourceLength, source.length);
+        Objects.checkFromIndexSize(outputOffset, outputLength, output.length);
+        validateNonOverlappingRanges(source, sourceOffset, sourceLength, output, outputOffset, outputLength);
+
+        final int producedLength = computeAppliedLength(sourceLength, patchCommand, traversalDirection);
+        if (producedLength > outputLength) {
+            return APPLY_INSUFFICIENT_CAPACITY;
+        }
+        applyToOutput(source, sourceOffset, sourceLength, patchCommand, traversalDirection, output, outputOffset,
+                producedLength);
+        return producedLength;
+    }
+
     /**
      * Encodes a patch command using the historical backward Egothor semantics.
      *
@@ -409,7 +488,6 @@ public final class PatchCommandEncoder {
      * @param patchCommand compact patch command
      * @return transformed word, or {@code null} when {@code source} is {@code null}
      */
-    @SuppressWarnings({ "PMD.CyclomaticComplexity", "PMD.AvoidLiteralsInIfCondition" })
     private static String applyBackward(final String source, final String patchCommand) {
         if (source == null) {
             return null;
@@ -435,7 +513,7 @@ public final class PatchCommandEncoder {
         int position = result.length() - 1;
 
         try {
-            for (int patchIndex = 0, patchLength = patchCommand.length(); patchIndex < patchLength; patchIndex += 2) { // NOPMD
+            for (int patchIndex = 0, patchLength = patchCommand.length(); patchIndex < patchLength; patchIndex += 2) {
                 final char opcode = patchCommand.charAt(patchIndex);
                 final char argument = patchCommand.charAt(patchIndex + 1);
 
@@ -493,7 +571,6 @@ public final class PatchCommandEncoder {
      * @param patchCommand compact patch command
      * @return transformed word, or {@code null} when {@code source} is {@code null}
      */
-    @SuppressWarnings({ "PMD.CyclomaticComplexity", "PMD.AvoidLiteralsInIfCondition" })
     private static String applyForward(final String source, final String patchCommand) {
         if (source == null) {
             return null;
@@ -519,7 +596,7 @@ public final class PatchCommandEncoder {
         int position = 0;
 
         try {
-            for (int patchIndex = 0, patchLength = patchCommand.length(); patchIndex < patchLength; patchIndex += 2) { // NOPMD
+            for (int patchIndex = 0, patchLength = patchCommand.length(); patchIndex < patchLength; patchIndex += 2) {
                 final char opcode = patchCommand.charAt(patchIndex);
                 final char argument = patchCommand.charAt(patchIndex + 1);
 
@@ -681,7 +758,7 @@ public final class PatchCommandEncoder {
      */
     private static String applyBackwardToEmptySource(final StringBuilder result, final String patchCommand) {
         try {
-            for (int patchIndex = 0, patchLength = patchCommand.length(); patchIndex < patchLength; patchIndex += 2) { // NOPMD
+            for (int patchIndex = 0, patchLength = patchCommand.length(); patchIndex < patchLength; patchIndex += 2) {
                 final char opcode = patchCommand.charAt(patchIndex);
                 final char argument = patchCommand.charAt(patchIndex + 1);
 
@@ -722,7 +799,7 @@ public final class PatchCommandEncoder {
      */
     private static String applyForwardToEmptySource(final StringBuilder result, final String patchCommand) {
         try {
-            for (int patchIndex = 0, patchLength = patchCommand.length(); patchIndex < patchLength; patchIndex += 2) { // NOPMD
+            for (int patchIndex = 0, patchLength = patchCommand.length(); patchIndex < patchLength; patchIndex += 2) {
                 final char opcode = patchCommand.charAt(patchIndex);
                 final char argument = patchCommand.charAt(patchIndex + 1);
 
@@ -753,6 +830,711 @@ public final class PatchCommandEncoder {
         return result.toString();
     }
 
+    /**
+     * Computes the transformed length or the preserved source length for malformed
+     * compatibility cases.
+     *
+     * @param sourceLength       source length
+     * @param patchCommand       patch command
+     * @param traversalDirection traversal direction
+     * @return produced length
+     */
+    private static int computeAppliedLength(final int sourceLength, final String patchCommand,
+            final WordTraversalDirection traversalDirection) {
+        if (patchCommand == null || patchCommand.isEmpty() || NOOP_PATCH.equals(patchCommand)
+                || (patchCommand.length() & 1) != 0) {
+            return sourceLength;
+        }
+        if (traversalDirection == WordTraversalDirection.BACKWARD) {
+            return computeBackwardAppliedLength(sourceLength, patchCommand);
+        }
+        return computeForwardAppliedLength(sourceLength, patchCommand);
+    }
+
+    /**
+     * Computes the backward traversal output length.
+     *
+     * @param sourceLength source length
+     * @param patchCommand patch command
+     * @return produced length
+     */
+    private static int computeBackwardAppliedLength(final int sourceLength, final String patchCommand) {
+        if (patchCommand.length() == 2) {
+            return computeSingleBackwardAppliedLength(sourceLength, patchCommand.charAt(0), patchCommand.charAt(1));
+        }
+        if (sourceLength == 0) {
+            return computeBackwardEmptyAppliedLength(patchCommand);
+        }
+
+        int currentLength = sourceLength;
+        int position = sourceLength - 1;
+        for (int patchIndex = 0, patchLength = patchCommand.length(); patchIndex < patchLength; patchIndex += 2) {
+            final char opcode = patchCommand.charAt(patchIndex);
+            final char argument = patchCommand.charAt(patchIndex + 1);
+
+            switch (opcode) {
+                case SKIP_OPCODE:
+                    final int skipCount = decodeEncodedCount(argument);
+                    if (skipCount < 1) {
+                        return sourceLength;
+                    }
+                    position = position - skipCount + 1;
+                    break;
+
+                case REPLACE_OPCODE:
+                    if (position < 0 || position >= currentLength) {
+                        return sourceLength;
+                    }
+                    break;
+
+                case DELETE_OPCODE:
+                    final int deleteCount = decodeEncodedCount(argument);
+                    if (deleteCount < 1) {
+                        return sourceLength;
+                    }
+                    final int deleteEndExclusive = position + 1;
+                    position -= deleteCount - 1;
+                    if (position < 0 || deleteEndExclusive > currentLength || position > deleteEndExclusive) {
+                        return sourceLength;
+                    }
+                    currentLength -= deleteEndExclusive - position;
+                    break;
+
+                case INSERT_OPCODE:
+                    if (position < -1 || position >= currentLength) {
+                        return sourceLength;
+                    }
+                    currentLength++;
+                    position++;
+                    break;
+
+                case NOOP_OPCODE:
+                    if (argument != NOOP_ARGUMENT) {
+                        throw new IllegalArgumentException(MSG_NOOP + argument);
+                    }
+                    return sourceLength;
+
+                default:
+                    throw new IllegalArgumentException(MSG_OPCODE + opcode);
+            }
+
+            position--;
+        }
+        return currentLength;
+    }
+
+    /**
+     * Computes the forward traversal output length.
+     *
+     * @param sourceLength source length
+     * @param patchCommand patch command
+     * @return produced length
+     */
+    private static int computeForwardAppliedLength(final int sourceLength, final String patchCommand) {
+        if (patchCommand.length() == 2) {
+            return computeSingleForwardAppliedLength(sourceLength, patchCommand.charAt(0), patchCommand.charAt(1));
+        }
+        if (sourceLength == 0) {
+            return computeForwardEmptyAppliedLength(patchCommand);
+        }
+
+        int currentLength = sourceLength;
+        int position = 0;
+        for (int patchIndex = 0, patchLength = patchCommand.length(); patchIndex < patchLength; patchIndex += 2) {
+            final char opcode = patchCommand.charAt(patchIndex);
+            final char argument = patchCommand.charAt(patchIndex + 1);
+
+            switch (opcode) {
+                case SKIP_OPCODE:
+                    final int skipCount = decodeEncodedCount(argument);
+                    if (skipCount < 1) {
+                        return sourceLength;
+                    }
+                    position = position + skipCount - 1;
+                    break;
+
+                case REPLACE_OPCODE:
+                    if (position < 0 || position >= currentLength) {
+                        return sourceLength;
+                    }
+                    break;
+
+                case DELETE_OPCODE:
+                    final int deleteCount = decodeEncodedCount(argument);
+                    if (deleteCount < 1 || position < 0 || position + deleteCount > currentLength) {
+                        return sourceLength;
+                    }
+                    currentLength -= deleteCount;
+                    position--;
+                    break;
+
+                case INSERT_OPCODE:
+                    if (position < 0 || position > currentLength) {
+                        return sourceLength;
+                    }
+                    currentLength++;
+                    break;
+
+                case NOOP_OPCODE:
+                    if (argument != NOOP_ARGUMENT) {
+                        throw new IllegalArgumentException(MSG_NOOP + argument);
+                    }
+                    return sourceLength;
+
+                default:
+                    throw new IllegalArgumentException(MSG_OPCODE + opcode);
+            }
+
+            position++;
+        }
+        return currentLength;
+    }
+
+    /**
+     * Computes a single backward instruction output length.
+     *
+     * @param sourceLength source length
+     * @param opcode       opcode
+     * @param argument     argument
+     * @return produced length
+     */
+    private static int computeSingleBackwardAppliedLength(final int sourceLength, final char opcode,
+            final char argument) {
+        final int encodedValue;
+        switch (opcode) {
+            case DELETE_OPCODE:
+                encodedValue = decodeEncodedCount(argument);
+                return encodedValue < 1 || encodedValue > sourceLength ? sourceLength : sourceLength - encodedValue;
+            case INSERT_OPCODE:
+                return sourceLength + 1;
+            case REPLACE_OPCODE:
+            case SKIP_OPCODE:
+                return sourceLength;
+            case NOOP_OPCODE:
+                if (argument != NOOP_ARGUMENT) {
+                    throw new IllegalArgumentException(MSG_NOOP + argument);
+                }
+                return sourceLength;
+            default:
+                throw new IllegalArgumentException(MSG_OPCODE + opcode);
+        }
+    }
+
+    /**
+     * Computes a single forward instruction output length.
+     *
+     * @param sourceLength source length
+     * @param opcode       opcode
+     * @param argument     argument
+     * @return produced length
+     */
+    private static int computeSingleForwardAppliedLength(final int sourceLength, final char opcode,
+            final char argument) {
+        return computeSingleBackwardAppliedLength(sourceLength, opcode, argument);
+    }
+
+    /**
+     * Computes output length for an empty source in backward traversal.
+     *
+     * @param patchCommand patch command
+     * @return produced length
+     */
+    private static int computeBackwardEmptyAppliedLength(final String patchCommand) {
+        int currentLength = 0;
+        for (int patchIndex = 0, patchLength = patchCommand.length(); patchIndex < patchLength; patchIndex += 2) {
+            final char opcode = patchCommand.charAt(patchIndex);
+            final char argument = patchCommand.charAt(patchIndex + 1);
+            switch (opcode) {
+                case INSERT_OPCODE:
+                    currentLength++;
+                    break;
+                case SKIP_OPCODE:
+                case REPLACE_OPCODE:
+                case DELETE_OPCODE:
+                    return 0;
+                case NOOP_OPCODE:
+                    if (argument != NOOP_ARGUMENT) {
+                        throw new IllegalArgumentException(MSG_NOOP + argument);
+                    }
+                    return 0;
+                default:
+                    throw new IllegalArgumentException(MSG_OPCODE + opcode);
+            }
+        }
+        return currentLength;
+    }
+
+    /**
+     * Computes output length for an empty source in forward traversal.
+     *
+     * @param patchCommand patch command
+     * @return produced length
+     */
+    private static int computeForwardEmptyAppliedLength(final String patchCommand) {
+        return computeBackwardEmptyAppliedLength(patchCommand);
+    }
+
+    /**
+     * Applies an already-sized patch into caller output.
+     *
+     * @param source             source text
+     * @param sourceOffset       source offset
+     * @param sourceLength       source length
+     * @param patchCommand       patch command
+     * @param traversalDirection traversal direction
+     * @param output             output storage
+     * @param outputOffset       output offset
+     * @param producedLength     already-validated produced length
+     */
+    private static void applyToOutput(final CharSequence source, final int sourceOffset, final int sourceLength,
+            final String patchCommand, final WordTraversalDirection traversalDirection, final char[] output,
+            final int outputOffset, final int producedLength) {
+        if (isPreservedSource(sourceLength, producedLength, patchCommand, traversalDirection)) {
+            copySource(source, sourceOffset, sourceLength, output, outputOffset);
+            return;
+        }
+
+        if (sourceLength > 0) {
+            copySource(source, sourceOffset, sourceLength, output, outputOffset);
+        }
+
+        if (traversalDirection == WordTraversalDirection.BACKWARD) {
+            applyBackwardToOutput(sourceLength, patchCommand, output, outputOffset);
+        } else {
+            applyForwardToOutput(sourceLength, patchCommand, output, outputOffset);
+        }
+    }
+
+    /**
+     * Applies an already-sized patch into caller output.
+     *
+     * @param source             source storage
+     * @param sourceOffset       source offset
+     * @param sourceLength       source length
+     * @param patchCommand       patch command
+     * @param traversalDirection traversal direction
+     * @param output             output storage
+     * @param outputOffset       output offset
+     * @param producedLength     already-validated produced length
+     */
+    private static void applyToOutput(final char[] source, final int sourceOffset, final int sourceLength,
+            final String patchCommand, final WordTraversalDirection traversalDirection, final char[] output,
+            final int outputOffset, final int producedLength) {
+        if (isPreservedSource(sourceLength, producedLength, patchCommand, traversalDirection)) {
+            System.arraycopy(source, sourceOffset, output, outputOffset, sourceLength);
+            return;
+        }
+
+        if (sourceLength > 0) {
+            System.arraycopy(source, sourceOffset, output, outputOffset, sourceLength);
+        }
+
+        if (traversalDirection == WordTraversalDirection.BACKWARD) {
+            applyBackwardToOutput(sourceLength, patchCommand, output, outputOffset);
+        } else {
+            applyForwardToOutput(sourceLength, patchCommand, output, outputOffset);
+        }
+    }
+
+    /**
+     * Determines whether the output is exactly the original source.
+     *
+     * @param sourceLength       source length
+     * @param producedLength     produced length
+     * @param patchCommand       patch command
+     * @param traversalDirection traversal direction
+     * @return {@code true} if copying the source is sufficient
+     */
+    private static boolean isPreservedSource(final int sourceLength, final int producedLength,
+            final String patchCommand, final WordTraversalDirection traversalDirection) {
+        return producedLength == sourceLength
+                && isKnownPreserveOnlyPatch(sourceLength, patchCommand, traversalDirection);
+    }
+
+    /**
+     * Returns whether equal length also means no mutation is needed.
+     *
+     * @param sourceLength       source length
+     * @param patchCommand       patch command
+     * @param traversalDirection traversal direction
+     * @return {@code true} when the command preserves source content
+     */
+    private static boolean isKnownPreserveOnlyPatch(final int sourceLength, final String patchCommand,
+            final WordTraversalDirection traversalDirection) {
+        if (patchCommand == null || patchCommand.isEmpty() || NOOP_PATCH.equals(patchCommand)
+                || (patchCommand.length() & 1) != 0) {
+            return true;
+        }
+        if (patchCommand.length() == 2) {
+            return isSingleInstructionPreserveOnly(sourceLength, patchCommand.charAt(0), patchCommand.charAt(1));
+        }
+        if (sourceLength == 0) {
+            return hasEmptySourcePreserveOnlyPatch(patchCommand);
+        }
+        return traversalDirection == WordTraversalDirection.BACKWARD
+                ? hasBackwardPreserveOnlyPatch(sourceLength, patchCommand)
+                : hasForwardPreserveOnlyPatch(sourceLength, patchCommand);
+    }
+
+    /**
+     * Tests whether a single instruction preserves the source content.
+     *
+     * @param sourceLength source length
+     * @param opcode       opcode
+     * @param argument     argument
+     * @return {@code true} when no mutation should be applied
+     */
+    private static boolean isSingleInstructionPreserveOnly(final int sourceLength, final char opcode,
+            final char argument) {
+        switch (opcode) {
+            case DELETE_OPCODE:
+                final int encodedValue = decodeEncodedCount(argument);
+                return encodedValue < 1 || encodedValue > sourceLength;
+            case INSERT_OPCODE:
+                return false;
+            case REPLACE_OPCODE:
+                return sourceLength == 0;
+            case SKIP_OPCODE:
+                return true;
+            case NOOP_OPCODE:
+                if (argument != NOOP_ARGUMENT) {
+                    throw new IllegalArgumentException(MSG_NOOP + argument);
+                }
+                return true;
+            default:
+                throw new IllegalArgumentException(MSG_OPCODE + opcode);
+        }
+    }
+
+    /**
+     * Tests whether an empty-source patch preserves the source.
+     *
+     * @param patchCommand patch command
+     * @return {@code true} when no mutation should be applied
+     */
+    private static boolean hasEmptySourcePreserveOnlyPatch(final String patchCommand) {
+        for (int patchIndex = 0, patchLength = patchCommand.length(); patchIndex < patchLength; patchIndex += 2) {
+            final char opcode = patchCommand.charAt(patchIndex);
+            final char argument = patchCommand.charAt(patchIndex + 1);
+            switch (opcode) {
+                case INSERT_OPCODE:
+                    break;
+                case SKIP_OPCODE:
+                case REPLACE_OPCODE:
+                case DELETE_OPCODE:
+                    return true;
+                case NOOP_OPCODE:
+                    if (argument != NOOP_ARGUMENT) {
+                        throw new IllegalArgumentException(MSG_NOOP + argument);
+                    }
+                    return true;
+                default:
+                    throw new IllegalArgumentException(MSG_OPCODE + opcode);
+            }
+        }
+        return false;
+    }
+
+    /**
+     * Tests whether a backward patch preserves the source because it is malformed
+     * or a NOOP.
+     *
+     * @param sourceLength source length
+     * @param patchCommand patch command
+     * @return {@code true} when no mutation should be applied
+     */
+    private static boolean hasBackwardPreserveOnlyPatch(final int sourceLength, final String patchCommand) {
+        int currentLength = sourceLength;
+        int position = sourceLength - 1;
+        for (int patchIndex = 0, patchLength = patchCommand.length(); patchIndex < patchLength; patchIndex += 2) {
+            final char opcode = patchCommand.charAt(patchIndex);
+            final char argument = patchCommand.charAt(patchIndex + 1);
+            switch (opcode) {
+                case SKIP_OPCODE:
+                    final int skipCount = decodeEncodedCount(argument);
+                    if (skipCount < 1) {
+                        return true;
+                    }
+                    position = position - skipCount + 1;
+                    break;
+                case REPLACE_OPCODE:
+                    if (position < 0 || position >= currentLength) {
+                        return true;
+                    }
+                    break;
+                case DELETE_OPCODE:
+                    final int deleteCount = decodeEncodedCount(argument);
+                    if (deleteCount < 1) {
+                        return true;
+                    }
+                    final int deleteEndExclusive = position + 1;
+                    position -= deleteCount - 1;
+                    if (position < 0 || deleteEndExclusive > currentLength || position > deleteEndExclusive) {
+                        return true;
+                    }
+                    currentLength -= deleteEndExclusive - position;
+                    break;
+                case INSERT_OPCODE:
+                    if (position < -1 || position >= currentLength) {
+                        return true;
+                    }
+                    currentLength++;
+                    position++;
+                    break;
+                case NOOP_OPCODE:
+                    if (argument != NOOP_ARGUMENT) {
+                        throw new IllegalArgumentException(MSG_NOOP + argument);
+                    }
+                    return true;
+                default:
+                    throw new IllegalArgumentException(MSG_OPCODE + opcode);
+            }
+            position--;
+        }
+        return false;
+    }
+
+    /**
+     * Tests whether a forward patch preserves the source because it is malformed or
+     * a NOOP.
+     *
+     * @param sourceLength source length
+     * @param patchCommand patch command
+     * @return {@code true} when no mutation should be applied
+     */
+    private static boolean hasForwardPreserveOnlyPatch(final int sourceLength, final String patchCommand) {
+        int currentLength = sourceLength;
+        int position = 0;
+        for (int patchIndex = 0, patchLength = patchCommand.length(); patchIndex < patchLength; patchIndex += 2) {
+            final char opcode = patchCommand.charAt(patchIndex);
+            final char argument = patchCommand.charAt(patchIndex + 1);
+            switch (opcode) {
+                case SKIP_OPCODE:
+                    final int skipCount = decodeEncodedCount(argument);
+                    if (skipCount < 1) {
+                        return true;
+                    }
+                    position = position + skipCount - 1;
+                    break;
+                case REPLACE_OPCODE:
+                    if (position < 0 || position >= currentLength) {
+                        return true;
+                    }
+                    break;
+                case DELETE_OPCODE:
+                    final int deleteCount = decodeEncodedCount(argument);
+                    if (deleteCount < 1 || position < 0 || position + deleteCount > currentLength) {
+                        return true;
+                    }
+                    currentLength -= deleteCount;
+                    position--;
+                    break;
+                case INSERT_OPCODE:
+                    if (position < 0 || position > currentLength) {
+                        return true;
+                    }
+                    currentLength++;
+                    break;
+                case NOOP_OPCODE:
+                    if (argument != NOOP_ARGUMENT) {
+                        throw new IllegalArgumentException(MSG_NOOP + argument);
+                    }
+                    return true;
+                default:
+                    throw new IllegalArgumentException(MSG_OPCODE + opcode);
+            }
+            position++;
+        }
+        return false;
+    }
+
+    /**
+     * Copies source characters from a sequence.
+     *
+     * @param source       source text
+     * @param sourceOffset source offset
+     * @param sourceLength source length
+     * @param output       output storage
+     * @param outputOffset output offset
+     */
+    private static void copySource(final CharSequence source, final int sourceOffset, final int sourceLength,
+            final char[] output, final int outputOffset) {
+        for (int index = 0; index < sourceLength; index++) {
+            output[outputOffset + index] = source.charAt(sourceOffset + index);
+        }
+    }
+
+    /**
+     * Applies a backward patch after validation.
+     *
+     * @param sourceLength source length
+     * @param patchCommand patch command
+     * @param output       output storage initialized with source
+     * @param outputOffset output offset
+     */
+    private static void applyBackwardToOutput(final int sourceLength, final String patchCommand, final char[] output,
+            final int outputOffset) {
+        if (sourceLength == 0) {
+            applyBackwardEmptyToOutput(patchCommand, output, outputOffset);
+            return;
+        }
+
+        int currentLength = sourceLength;
+        int position = sourceLength - 1;
+        for (int patchIndex = 0, patchLength = patchCommand.length(); patchIndex < patchLength; patchIndex += 2) {
+            final char opcode = patchCommand.charAt(patchIndex);
+            final char argument = patchCommand.charAt(patchIndex + 1);
+
+            switch (opcode) {
+                case SKIP_OPCODE:
+                    position = position - decodeEncodedCount(argument) + 1;
+                    break;
+
+                case REPLACE_OPCODE:
+                    output[outputOffset + position] = argument;
+                    break;
+
+                case DELETE_OPCODE:
+                    final int deleteEndExclusive = position + 1;
+                    position -= decodeEncodedCount(argument) - 1;
+                    System.arraycopy(output, outputOffset + deleteEndExclusive, output, outputOffset + position,
+                            currentLength - deleteEndExclusive);
+                    currentLength -= deleteEndExclusive - position;
+                    break;
+
+                case INSERT_OPCODE:
+                    final int insertIndex = position + 1;
+                    System.arraycopy(output, outputOffset + insertIndex, output, outputOffset + insertIndex + 1,
+                            currentLength - insertIndex);
+                    output[outputOffset + insertIndex] = argument;
+                    currentLength++;
+                    position++;
+                    break;
+
+                case NOOP_OPCODE:
+                    return;
+
+                default:
+                    throw new AssertionError("Patch command was not validated.");
+            }
+
+            position--;
+        }
+    }
+
+    /**
+     * Applies a forward patch after validation.
+     *
+     * @param sourceLength source length
+     * @param patchCommand patch command
+     * @param output       output storage initialized with source
+     * @param outputOffset output offset
+     */
+    private static void applyForwardToOutput(final int sourceLength, final String patchCommand, final char[] output,
+            final int outputOffset) {
+        if (sourceLength == 0) {
+            applyForwardEmptyToOutput(patchCommand, output, outputOffset);
+            return;
+        }
+
+        int currentLength = sourceLength;
+        int position = 0;
+        for (int patchIndex = 0, patchLength = patchCommand.length(); patchIndex < patchLength; patchIndex += 2) {
+            final char opcode = patchCommand.charAt(patchIndex);
+            final char argument = patchCommand.charAt(patchIndex + 1);
+
+            switch (opcode) {
+                case SKIP_OPCODE:
+                    position = position + decodeEncodedCount(argument) - 1;
+                    break;
+
+                case REPLACE_OPCODE:
+                    output[outputOffset + position] = argument;
+                    break;
+
+                case DELETE_OPCODE:
+                    final int deleteCount = decodeEncodedCount(argument);
+                    System.arraycopy(output, outputOffset + position + deleteCount, output, outputOffset + position,
+                            currentLength - position - deleteCount);
+                    currentLength -= deleteCount;
+                    position--;
+                    break;
+
+                case INSERT_OPCODE:
+                    System.arraycopy(output, outputOffset + position, output, outputOffset + position + 1,
+                            currentLength - position);
+                    output[outputOffset + position] = argument;
+                    currentLength++;
+                    break;
+
+                case NOOP_OPCODE:
+                    return;
+
+                default:
+                    throw new AssertionError("Patch command was not validated.");
+            }
+
+            position++;
+        }
+    }
+
+    /**
+     * Applies an empty-source backward patch after validation.
+     *
+     * @param patchCommand patch command
+     * @param output       output storage
+     * @param outputOffset output offset
+     */
+    private static void applyBackwardEmptyToOutput(final String patchCommand, final char[] output,
+            final int outputOffset) {
+        int currentLength = 0;
+        for (int patchIndex = 0, patchLength = patchCommand.length(); patchIndex < patchLength; patchIndex += 2) {
+            final char argument = patchCommand.charAt(patchIndex + 1);
+            System.arraycopy(output, outputOffset, output, outputOffset + 1, currentLength);
+            output[outputOffset] = argument;
+            currentLength++;
+        }
+    }
+
+    /**
+     * Applies an empty-source forward patch after validation.
+     *
+     * @param patchCommand patch command
+     * @param output       output storage
+     * @param outputOffset output offset
+     */
+    private static void applyForwardEmptyToOutput(final String patchCommand, final char[] output,
+            final int outputOffset) {
+        int currentLength = 0;
+        for (int patchIndex = 0, patchLength = patchCommand.length(); patchIndex < patchLength; patchIndex += 2) {
+            output[outputOffset + currentLength] = patchCommand.charAt(patchIndex + 1);
+            currentLength++;
+        }
+    }
+
+    /**
+     * Validates that source and output slices do not overlap when backed by the
+     * same array.
+     *
+     * @param source       source storage
+     * @param sourceOffset source offset
+     * @param sourceLength source length
+     * @param output       output storage
+     * @param outputOffset output offset
+     * @param outputLength output length
+     */
+    private static void validateNonOverlappingRanges(final char[] source, final int sourceOffset,
+            final int sourceLength, final char[] output, final int outputOffset, final int outputLength) {
+        if (!source.equals(output) || sourceLength == 0 || outputLength == 0) {
+            return;
+        }
+        final int sourceEnd = sourceOffset + sourceLength;
+        final int outputEnd = outputOffset + outputLength;
+        if (sourceOffset < outputEnd && outputOffset < sourceEnd) {
+            throw new IllegalArgumentException("source and output ranges must not overlap.");
+        }
+    }
+
     /**
      * Returns the direction-specialized apply strategy.
      *
@@ -769,7 +1551,6 @@ public final class PatchCommandEncoder {
      * @param argument serialized count argument
      * @return decoded positive count, or {@code -1} when the argument is malformed
      */
-    @SuppressWarnings("PMD.AvoidLiteralsInIfCondition")
     private static int decodeEncodedCount(final char argument) {
         if (argument < 'a') {
             return -1;

src/main/java/org/egothor/stemmer/StemmerPatchTrieBinaryIO.java

@@ -95,8 +95,8 @@ public final class StemmerPatchTrieBinaryIO {
     }
 
     /**
-     * Reads a GZip-compressed binary patch-command trie from a filesystem path
+     * Reads a GZip-compressed binary patch-command trie from a filesystem path with
-     * with an optional dense child lookup span override.
+     * an optional dense child lookup span override.
      * <p>
      * This is a runtime-only tuning parameter. The dense-span setting is not
      * persisted in the file and does not change the compiled metadata.
@@ -183,14 +183,15 @@ public final class StemmerPatchTrieBinaryIO {
      * persisted in the file and does not change the compiled metadata.
      * </p>
      *
-     * @param inputStream     source stream
+     * @param inputStream      source stream
      * @param maxExpandedIndex dense lookup span override; negative values use
      *                         {@link FrequencyTrie#DEFAULT_MAX_EXPANDED_INDEX}
      * @return deserialized trie
      * @throws NullPointerException if {@code inputStream} is {@code null}
      * @throws IOException          if reading or decompression fails
      */
-    public static FrequencyTrie<String> read(final InputStream inputStream, final int maxExpandedIndex) throws IOException {
+    public static FrequencyTrie<String> read(final InputStream inputStream, final int maxExpandedIndex)
+            throws IOException {
         Objects.requireNonNull(inputStream, "inputStream");
 
         try (GZIPInputStream gzipInputStream = new GZIPInputStream(new BufferedInputStream(inputStream));

src/main/java/org/egothor/stemmer/StemmerPatchTrieLoader.java

@@ -461,7 +461,7 @@ public final class StemmerPatchTrieLoader {
     public static FrequencyTrie<String> load(final Path path, final boolean storeOriginal,
             final ReductionSettings reductionSettings, final WordTraversalDirection traversalDirection,
             final CaseProcessingMode caseProcessingMode, final DiacriticProcessingMode diacriticProcessingMode)
-        throws IOException {
+            throws IOException {
         Objects.requireNonNull(path, PARAMETER_PATH);
         final TrieMetadata metadata = metadataForCompilation(traversalDirection, reductionSettings, caseProcessingMode,
                 diacriticProcessingMode);
@@ -816,7 +816,8 @@ public final class StemmerPatchTrieLoader {
      * @throws IOException          if the file cannot be opened, decompressed, or
      *                              read
      */
-    public static FrequencyTrie<String> loadBinary(final String fileName, final int maxExpandedIndex) throws IOException {
+    public static FrequencyTrie<String> loadBinary(final String fileName, final int maxExpandedIndex)
+            throws IOException {
         Objects.requireNonNull(fileName, FILENAME_REQUIRED);
         return StemmerPatchTrieBinaryIO.read(fileName, maxExpandedIndex);
     }

src/main/java/org/egothor/stemmer/package-info.java

@@ -58,17 +58,17 @@
  * {@link org.egothor.stemmer.StemmerPatchTrieLoader}, which reads the
  * traditional line-oriented tab-separated values resource format in which each
  * non-empty logical line starts with a canonical stem followed by known surface
- * variants in subsequent tab-separated columns.
+ * variants in subsequent tab-separated columns. Parsing is delegated to
- * Parsing is delegated to {@link org.egothor.stemmer.StemmerDictionaryParser},
+ * {@link org.egothor.stemmer.StemmerDictionaryParser}, which applies
- * which applies configurable case processing through
+ * configurable case processing through
  * {@link org.egothor.stemmer.CaseProcessingMode} (default:
  * {@link org.egothor.stemmer.CaseProcessingMode#LOWERCASE_WITH_LOCALE_ROOT}),
  * supports whole-line as well as trailing remarks introduced by {@code #} or
  * {@code //}, and currently ignores dictionary items containing Unicode
  * whitespace characters while reporting them through warning-level diagnostics.
- * During loading, each variant is converted into a patch command
+ * During loading, each variant is converted into a patch command targeting the
- * targeting the canonical stem, and the stem itself may optionally be stored
+ * canonical stem, and the stem itself may optionally be stored under the
- * under the canonical no-operation patch.
+ * canonical no-operation patch.
  * </p>
  *
  * <p>

src/main/java/org/egothor/stemmer/trie/CompiledNode.java

@@ -48,8 +48,8 @@ import java.util.Objects;
 public final class CompiledNode<V> {
 
     /**
-     * Default dense child lookup span in characters used when an explicit override is
+     * Default dense child lookup span in characters used when an explicit override
-     * not provided.
+     * is not provided.
      */
     public static final int DEFAULT_MAX_EXPANDED_INDEX = 512;
 
@@ -71,8 +71,8 @@ public final class CompiledNode<V> {
     /**
      * Dense child lookup table used when labels fit into a compact char interval.
      * <p>
-     * The table enables direct O(1) indexing for child lookup and is allocated
+     * The table enables direct O(1) indexing for child lookup and is allocated only
-     * only when the character span of this node's edges is within the configured
+     * when the character span of this node's edges is within the configured
      * threshold.
      * </p>
      */
@@ -111,8 +111,8 @@ public final class CompiledNode<V> {
      *
      * @param maxExpandedIndex upper bound for the dense lookup interval size; zero
      *                         disables dense lookup. Larger values improve
-     *                         direct-index likelihood while increasing dense
+     *                         direct-index likelihood while increasing dense table
-     *                         table memory in compact-label nodes.
+     *                         memory in compact-label nodes.
      * @throws NullPointerException     if any array argument is {@code null}
      * @throws IllegalArgumentException if the edge-related arrays or value-related
      *                                  arrays do not have matching lengths or the
@@ -288,7 +288,8 @@ public final class CompiledNode<V> {
     }
 
     /**
-     * Returns a small memory-related metric describing this node's dense table size.
+     * Returns a small memory-related metric describing this node's dense table
+     * size.
      *
      * @return number of dense table slots, or {@code 0} when dense lookup is not
      *         enabled
@@ -328,8 +329,9 @@ public final class CompiledNode<V> {
             return false;
         }
         return Arrays.equals(this.edgeLabels, other.edgeLabels) && Arrays.equals(this.children, other.children)
-                && Arrays.equals(this.orderedValues, other.orderedValues) && Arrays.equals(this.orderedCounts, other.orderedCounts)
+                && Arrays.equals(this.orderedValues, other.orderedValues)
-                && this.denseEdgeMin == other.denseEdgeMin && Arrays.equals(this.denseChildren, other.denseChildren);
+                && Arrays.equals(this.orderedCounts, other.orderedCounts) && this.denseEdgeMin == other.denseEdgeMin
+                && Arrays.equals(this.denseChildren, other.denseChildren);
     }
 
     /**
@@ -339,9 +341,8 @@ public final class CompiledNode<V> {
      */
     @Override
     public String toString() {
-        return "CompiledNode{"
+        return "CompiledNode{" + "edgeCount=" + this.edgeLabels.length + ", orderedValueCount="
-                + "edgeCount=" + this.edgeLabels.length + ", orderedValueCount=" + this.orderedValues.length
+                + this.orderedValues.length + ", denseTableLength=" + denseTableLength() + '}';
-                + ", denseTableLength=" + denseTableLength() + '}';
     }
 
     /**
@@ -350,8 +351,8 @@ public final class CompiledNode<V> {
      * Lookup order is:
      * <ol>
      * <li>dense array index (if the label interval is compact enough),</li>
-     * <li>small-child linear scan when the fallback node has {@value #LINEAR_CHILD_COUNT_THRESHOLD}
+     * <li>small-child linear scan when the fallback node has
-     * or fewer edges,</li>
+     * {@value #LINEAR_CHILD_COUNT_THRESHOLD} or fewer edges,</li>
      * <li>binary search over sorted labels.</li>
      * </ol>
      * </p>

src/test/java/org/egothor/stemmer/FrequencyTrieTest.java

@@ -45,6 +45,7 @@ import java.io.ByteArrayOutputStream;
 import java.io.DataInputStream;
 import java.io.DataOutputStream;
 import java.io.IOException;
+import java.util.ArrayList;
 import java.util.List;
 
 import org.junit.jupiter.api.DisplayName;
@@ -63,6 +64,7 @@ import org.junit.jupiter.api.Test;
 @Tag("unit")
 @Tag("trie")
 @Tag("frequency-trie")
+@Tag("lookup")
 @DisplayName("FrequencyTrie")
 class FrequencyTrieTest {
 
@@ -398,6 +400,149 @@ class FrequencyTrieTest {
                 () -> assertThrows(UnsupportedOperationException.class, () -> entries.add(new ValueCount<String>("z", 1))));
     }
 
+    /**
+     * Verifies that visitor lookup returns the same deterministic order and counts
+     * as the allocating APIs.
+     */
+    @Test
+    @DisplayName("Visitor lookup matches getAll order and getEntries counts")
+    void visitorLookupMatchesGetAllOrderAndGetEntriesCounts() {
+        final FrequencyTrie.Builder<String> builder = rankedBuilder();
+        builder.put("house", "noun", 3);
+        builder.put("house", "verb", 2);
+        builder.put("house", "adjective", 1);
+        final FrequencyTrie<String> trie = builder.build();
+        final List<String> values = new ArrayList<>();
+        final List<Integer> counts = new ArrayList<>();
+        final List<Integer> ranks = new ArrayList<>();
+
+        final int visited = trie.getAllNormalized("house", (value, count, rank) -> {
+            values.add(value);
+            counts.add(count);
+            ranks.add(rank);
+            return true;
+        }, 10);
+
+        assertAll(() -> assertEquals(3, visited),
+                () -> assertEquals(List.of("noun", "verb", "adjective"), values),
+                () -> assertEquals(List.of(3, 2, 1), counts),
+                () -> assertEquals(List.of(0, 1, 2), ranks));
+    }
+
+    /**
+     * Verifies visitor maximum result and early-stop behavior.
+     */
+    @Test
+    @DisplayName("Visitor lookup honors maxResults and sink early stop")
+    void visitorLookupHonorsMaxResultsAndSinkEarlyStop() {
+        final FrequencyTrie.Builder<String> builder = rankedBuilder();
+        builder.put("house", "noun", 3);
+        builder.put("house", "verb", 2);
+        builder.put("house", "adjective", 1);
+        final FrequencyTrie<String> trie = builder.build();
+        final List<String> limited = new ArrayList<>();
+        final List<String> stopped = new ArrayList<>();
+
+        final int limitedCount = trie.getAllNormalized("house", (value, count, rank) -> {
+            limited.add(value);
+            return true;
+        }, 2);
+        final int stoppedCount = trie.getAllNormalized("house", (value, count, rank) -> {
+            stopped.add(value);
+            return false;
+        }, 10);
+
+        assertAll(() -> assertEquals(2, limitedCount),
+                () -> assertEquals(List.of("noun", "verb"), limited),
+                () -> assertEquals(1, stoppedCount),
+                () -> assertEquals(List.of("noun"), stopped));
+    }
+
+    /**
+     * Verifies visitor zero, negative, missing, and first-result behavior.
+     */
+    @Test
+    @DisplayName("Visitor lookup handles zero, negative, missing, and first-result cases")
+    void visitorLookupHandlesBoundaryCases() {
+        final FrequencyTrie.Builder<String> builder = rankedBuilder();
+        builder.put("house", "noun");
+        final FrequencyTrie<String> trie = builder.build();
+        final int[] calls = new int[1];
+
+        assertAll(() -> assertEquals(0, trie.getAllNormalized("house", (value, count, rank) -> {
+            calls[0]++;
+            return true;
+        }, 0)),
+                () -> assertEquals(0, calls[0]),
+                () -> assertThrows(IllegalArgumentException.class,
+                        () -> trie.getAllNormalized("house", (value, count, rank) -> true, -1)),
+                () -> assertEquals(0, trie.getAllNormalized("missing", (value, count, rank) -> true, 10)),
+                () -> assertFalse(trie.getFirstNormalized("missing", (value, count, rank) -> true)),
+                () -> assertTrue(trie.getFirstNormalized("house", (value, count, rank) -> {
+                    assertEquals("noun", value);
+                    assertEquals(1, count);
+                    assertEquals(0, rank);
+                    return true;
+                })));
+    }
+
+    /**
+     * Verifies visitor API argument validation.
+     */
+    @Test
+    @DisplayName("Visitor lookup rejects null and invalid range arguments")
+    void visitorLookupRejectsNullAndInvalidRangeArguments() {
+        final FrequencyTrie<String> trie = rankedBuilder().build();
+        final char[] key = "house".toCharArray();
+        final FrequencyTrie.EntrySink<String> sink = (value, count, rank) -> true;
+
+        assertAll(() -> assertThrows(NullPointerException.class,
+                () -> trie.getAllNormalized((char[]) null, 0, 0, sink, 1)),
+                () -> assertThrows(NullPointerException.class,
+                        () -> trie.getAllNormalized(key, 0, key.length, null, 1)),
+                () -> assertThrows(IndexOutOfBoundsException.class,
+                        () -> trie.getAllNormalized(key, -1, key.length, sink, 1)),
+                () -> assertThrows(IndexOutOfBoundsException.class,
+                        () -> trie.getAllNormalized(key, 1, key.length, sink, 1)),
+                () -> assertThrows(NullPointerException.class,
+                        () -> trie.getAllNormalized((CharSequence) null, sink, 1)),
+                () -> assertThrows(NullPointerException.class,
+                        () -> trie.getAllNormalized("house", null, 1)),
+                () -> assertThrows(NullPointerException.class,
+                        () -> trie.getAll((CharSequence) null, sink, 1)),
+                () -> assertThrows(NullPointerException.class,
+                        () -> trie.getAll("house", null, 1)),
+                () -> assertThrows(IllegalArgumentException.class,
+                        () -> trie.getAll("house", sink, -1)));
+    }
+
+    /**
+     * Verifies normalized char-array slices and metadata-aware CharSequence visitor
+     * lookup.
+     */
+    @Test
+    @DisplayName("Visitor lookup supports normalized char slices and metadata-aware CharSequence keys")
+    void visitorLookupSupportsCharSlicesAndMetadataAwareKeys() {
+        final FrequencyTrie.Builder<String> builder = new FrequencyTrie.Builder<>(String[]::new,
+                ReductionSettings.withDefaults(ReductionMode.MERGE_SUBTREES_WITH_EQUIVALENT_RANKED_GET_ALL_RESULTS),
+                WordTraversalDirection.BACKWARD, CaseProcessingMode.LOWERCASE_WITH_LOCALE_ROOT);
+        builder.put("house", "noun");
+        final FrequencyTrie<String> trie = builder.build();
+        final char[] padded = "__house__".toCharArray();
+
+        assertAll(() -> assertEquals(1,
+                trie.getAllNormalized(padded, 2, 5, (value, count, rank) -> {
+                    assertEquals("noun", value);
+                    return true;
+                }, 10)),
+                () -> assertFalse(trie.getFirstNormalized("HOUSE", (value, count, rank) -> true),
+                        "Normalized lookup must bypass metadata lowercasing."),
+                () -> assertTrue(trie.getFirst("HOUSE", (value, count, rank) -> {
+                    assertEquals("noun", value);
+                    return true;
+                })));
+    }
+
     /**
      * Verifies that equal frequencies prefer the shorter string representation.
      */

src/test/java/org/egothor/stemmer/PatchCommandEncoderTest.java

@@ -31,6 +31,7 @@
 package org.egothor.stemmer;
 
 import static org.junit.jupiter.api.Assertions.assertAll;
+import static org.junit.jupiter.api.Assertions.assertArrayEquals;
 import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertNotNull;
 import static org.junit.jupiter.api.Assertions.assertNull;
@@ -68,6 +69,8 @@ import org.junit.jupiter.params.provider.MethodSource;
 @Tag("unit")
 @Tag("stemmer")
 @Tag("patch")
+@Tag("encoding")
+@Tag("apply")
 @TestInstance(TestInstance.Lifecycle.PER_CLASS)
 class PatchCommandEncoderTest {
 
@@ -147,6 +150,63 @@ class PatchCommandEncoderTest {
                 Arguments.of(10, "teacher", PatchCommandEncoder.NOOP_PATCH, "teacher"));
     }
 
+    /**
+     * Provides explicit forward-direction single-instruction patch application cases.
+     *
+     * @return test arguments
+     */
+    private static Stream<Arguments> provideForwardSingleInstructionApplyCases() {
+        return Stream.of(
+                // 1
+                Arguments.of(1, "abcd", "Db", "cd"),
+                // 2
+                Arguments.of(2, "abc", "Ia", "aabc"),
+                // 3
+                Arguments.of(3, "abc", "Ra", "abc"),
+                // 4
+                Arguments.of(4, "abc", "-a", "abc"),
+                // 5
+                Arguments.of(5, "abc", PatchCommandEncoder.NOOP_PATCH, "abc"));
+    }
+
+    /**
+     * Provides forward-direction applyTo cases that exercise preserve-only and
+     * non-preserve branches.
+     *
+     * @return test arguments
+     */
+    private static Stream<Arguments> provideForwardApplyToCases() {
+        return Stream.of(
+                // 1
+                Arguments.of(1, "book", "-aRa", "baok"),
+                // 2
+                Arguments.of(2, "abc", "-dRa", "abc"),
+                // 3
+                Arguments.of(3, "abc", "DdRa", "abc"),
+                // 4
+                Arguments.of(4, "abc", "-dIa", "abc"),
+                // 5
+                Arguments.of(5, "abc", "Na-a", "abc"));
+    }
+
+    /**
+     * Provides empty-source forward applyTo cases that cover empty-source
+     * instruction handling.
+     *
+     * @return test arguments
+     */
+    private static Stream<Arguments> provideForwardEmptySourceApplyCases() {
+        return Stream.of(
+                // 1
+                Arguments.of(1, "IaIb", "ab"),
+                // 2
+                Arguments.of(2, "-aRa", ""),
+                // 3
+                Arguments.of(3, "IaRa", ""),
+                // 4
+                Arguments.of(4, "Na-a", ""));
+    }
+
     /**
      * Provides malformed or index-invalid patch inputs that must preserve the
      * original source according to the implementation contract.
@@ -236,12 +296,31 @@ class PatchCommandEncoderTest {
         return new StringBuilder(text).reverse().toString();
     }
 
+    /**
+     * Applies a patch into a right-sized output buffer and returns the produced
+     * string.
+     *
+     * @param source             source text
+     * @param patch              patch command
+     * @param traversalDirection traversal direction
+     * @return transformed text
+     */
+    private static String applyToString(final String source, final String patch,
+            final WordTraversalDirection traversalDirection) {
+        final char[] output = new char[Math.max(source.length() + 16, 16)];
+        final int produced = PatchCommandEncoder.applyTo(source, patch, traversalDirection, output, 0, output.length);
+        return new String(output, 0, produced);
+    }
+
     /**
      * Tests constructor validation and basic instantiation behavior.
      */
     @Nested
     @DisplayName("construction")
     @Tag("construction")
+    @Tag("unit")
+    @Tag("stemmer")
+    @Tag("patch")
     class ConstructionTests {
 
         /**
@@ -327,6 +406,9 @@ class PatchCommandEncoderTest {
     @Nested
     @DisplayName("encode(String, String)")
     @Tag("encoding")
+    @Tag("unit")
+    @Tag("stemmer")
+    @Tag("patch")
     class EncodeTests {
 
         /**
@@ -461,6 +543,9 @@ class PatchCommandEncoderTest {
     @Nested
     @DisplayName("apply(String, String)")
     @Tag("apply")
+    @Tag("unit")
+    @Tag("stemmer")
+    @Tag("patch")
     class ApplyTests {
 
         /**
@@ -535,6 +620,101 @@ class PatchCommandEncoderTest {
             assertEquals("city", PatchCommandEncoder.apply("cities", patch, WordTraversalDirection.FORWARD));
         }
 
+        /**
+         * Verifies explicit single-instruction forward patch application
+         * semantics.
+         *
+         * @param caseId   numeric case identifier
+         * @param source   source word
+         * @param patch    encoded patch command
+         * @param expected expected transformed word
+         */
+        @ParameterizedTest(name = "[{index}] case {0}: forward single instruction apply({1}, {2}) -> {3}")
+        @MethodSource("org.egothor.stemmer.PatchCommandEncoderTest#provideForwardSingleInstructionApplyCases")
+        @DisplayName("applies forward single instruction patches correctly")
+        void shouldApplyForwardSingleInstructionsExplicitly(int caseId, String source, String patch, String expected) {
+            assertEquals(expected, PatchCommandEncoder.apply(source, patch, WordTraversalDirection.FORWARD),
+                    () -> "Case " + caseId + " failed for source='" + source + "', patch='" + patch + "'.");
+        }
+
+        /**
+         * Verifies forward single-instruction malformed commands fail fast.
+         */
+        @Test
+        @DisplayName("throws for unsupported forward opcode and NOOP argument")
+        void shouldThrowForUnsupportedForwardOpcodeAndNoopArgument() {
+            assertAll(() -> assertEquals("Unsupported patch opcode: X",
+                    assertThrows(IllegalArgumentException.class,
+                            () -> PatchCommandEncoder.apply("abc", "Xa", WordTraversalDirection.FORWARD))
+                                    .getMessage()),
+                    () -> assertEquals("Unsupported NOOP patch argument: `",
+                            assertThrows(IllegalArgumentException.class,
+                                    () -> PatchCommandEncoder.apply("abc", "N`", WordTraversalDirection.FORWARD))
+                                            .getMessage()));
+        }
+
+        /**
+         * Verifies explicit forward-applyTo cases that exercise preserve-only and
+         * non-preserve branches.
+         *
+         * @param caseId   numeric case identifier
+         * @param source   source word
+         * @param patch    encoded patch command
+         * @param expected expected transformed word
+         */
+        @ParameterizedTest(name = "[{index}] case {0}: applyToForward({1}, {2}) -> {3}")
+        @MethodSource("org.egothor.stemmer.PatchCommandEncoderTest#provideForwardApplyToCases")
+        @DisplayName("applyTo handles forward preserve-only and mutation branches")
+        void shouldApplyToForwardPreserveAndMutationBranches(int caseId, String source, String patch, String expected) {
+            final char[] output = new char[Math.max(source.length() + 16, 16)];
+
+            final int produced = PatchCommandEncoder.applyTo(source, patch, WordTraversalDirection.FORWARD, output, 0,
+                    output.length);
+
+            assertAll(
+                    () -> assertEquals(expected.length(), produced,
+                            () -> "Case " + caseId + " produced wrong length."),
+                    () -> assertEquals(expected, new String(output, 0, produced),
+                            () -> "Case " + caseId + " failed for patch='" + patch + "'."));
+        }
+
+        /**
+         * Verifies empty-source forward applyTo behavior for insert-only and malformed
+         * instructions.
+         *
+         * @param caseId numeric case identifier
+         * @param patch  encoded patch command
+         * @param expected expected transformed word
+         */
+        @ParameterizedTest(name = "[{index}] case {0}: applyToForward(\"\", {1}) -> \"{2}\"")
+        @MethodSource("org.egothor.stemmer.PatchCommandEncoderTest#provideForwardEmptySourceApplyCases")
+        @DisplayName("applies forward empty-source patches correctly")
+        void shouldApplyToForwardEmptySourceCases(int caseId, String patch, String expected) {
+            final char[] output = new char[Math.max(expected.length() + 16, 16)];
+
+            final int produced = PatchCommandEncoder.applyTo("", patch, WordTraversalDirection.FORWARD, output, 0,
+                    output.length);
+
+            assertAll(
+                    () -> assertEquals(expected.length(), produced,
+                            () -> "Case " + caseId + " produced wrong length."),
+                    () -> assertEquals(expected, new String(output, 0, produced),
+                            () -> "Case " + caseId + " failed for patch='" + patch + "'."));
+        }
+
+        /**
+         * Verifies malformed empty-source forward patches fail fast and preserve
+         * empty-source semantics.
+         */
+        @Test
+        @DisplayName("throws for unsupported NOOP argument on empty-source forward patch")
+        void shouldThrowForUnsupportedNoopArgumentOnForwardEmptySource() {
+            IllegalArgumentException exception = assertThrows(IllegalArgumentException.class,
+                    () -> PatchCommandEncoder.apply("", "N`Ra", WordTraversalDirection.FORWARD));
+
+            assertEquals("Unsupported NOOP patch argument: `", exception.getMessage());
+        }
+
         /**
          * Verifies explicit patch application cases.
          *
@@ -590,6 +770,181 @@ class PatchCommandEncoderTest {
             assertEquals(source, PatchCommandEncoder.apply(source, malformedPatch), () -> "Case " + caseId
                     + " failed for source='" + source + "', malformedPatch='" + malformedPatch + "'.");
         }
+
+        /**
+         * Verifies buffer application against string-returning application.
+         *
+         * @param caseId   numeric case identifier
+         * @param source   source word
+         * @param patch    patch command
+         * @param expected expected transformed word
+         */
+        @ParameterizedTest(name = "[{index}] case {0}: applyTo({1}, {2}) -> {3}")
+        @MethodSource("org.egothor.stemmer.PatchCommandEncoderTest#provideApplyCases")
+        @DisplayName("applyTo matches apply for explicit backward patch commands")
+        void shouldApplyToBufferLikeApplyForBackwardCommands(int caseId, String source, String patch, String expected) {
+            final char[] output = "___..............".toCharArray();
+
+            final int produced = PatchCommandEncoder.applyTo(source, patch, WordTraversalDirection.BACKWARD, output, 3,
+                    output.length - 3);
+
+            assertAll(() -> assertEquals(expected.length(), produced, () -> "Case " + caseId + " produced wrong length."),
+                    () -> assertEquals(expected, new String(output, 3, produced)));
+        }
+
+        /**
+         * Verifies char-array source slices.
+         */
+        @Test
+        @DisplayName("applyTo supports char-array source slices")
+        void shouldApplyToCharArraySourceSlice() {
+            final char[] source = "__teacher__".toCharArray();
+            final char[] output = new char[16];
+
+            final int produced = PatchCommandEncoder.applyTo(source, 2, 7, "Db", WordTraversalDirection.BACKWARD,
+                    output, 1, output.length - 1);
+
+            assertAll(() -> assertEquals(5, produced), () -> assertEquals("teach", new String(output, 1, produced)));
+        }
+
+        /**
+         * Verifies null and range validation for buffer application.
+         */
+        @Test
+        @DisplayName("applyTo rejects null and invalid range arguments")
+        void shouldRejectNullAndInvalidRangeArguments() {
+            final char[] source = "teacher".toCharArray();
+            final char[] output = new char[16];
+
+            assertAll(() -> assertThrows(NullPointerException.class,
+                    () -> PatchCommandEncoder.applyTo((CharSequence) null, "Db", WordTraversalDirection.BACKWARD,
+                            output, 0, output.length)),
+                    () -> assertThrows(NullPointerException.class,
+                            () -> PatchCommandEncoder.applyTo("teacher", "Db", null, output, 0, output.length)),
+                    () -> assertThrows(NullPointerException.class,
+                            () -> PatchCommandEncoder.applyTo("teacher", "Db", WordTraversalDirection.BACKWARD, null,
+                                    0, output.length)),
+                    () -> assertThrows(IndexOutOfBoundsException.class,
+                            () -> PatchCommandEncoder.applyTo("teacher", "Db", WordTraversalDirection.BACKWARD,
+                                    output, -1, output.length)),
+                    () -> assertThrows(NullPointerException.class,
+                            () -> PatchCommandEncoder.applyTo((char[]) null, 0, 7, "Db",
+                                    WordTraversalDirection.BACKWARD, output, 0, output.length)),
+                    () -> assertThrows(IndexOutOfBoundsException.class,
+                            () -> PatchCommandEncoder.applyTo(source, 1, source.length, "Db",
+                                    WordTraversalDirection.BACKWARD, output, 0, output.length)));
+        }
+
+        /**
+         * Verifies null, empty, and canonical NOOP patch preservation.
+         */
+        @Test
+        @DisplayName("applyTo preserves source for null, empty, and canonical NOOP patches")
+        void shouldApplyToPreserveSourceForEmptyCompatibilityPatches() {
+            final char[] nullPatchOutput = new char[8];
+            final char[] emptyPatchOutput = new char[8];
+            final char[] noopOutput = new char[8];
+
+            final int nullPatchLength = PatchCommandEncoder.applyTo("teacher", null, WordTraversalDirection.BACKWARD,
+                    nullPatchOutput, 0, nullPatchOutput.length);
+            final int emptyPatchLength = PatchCommandEncoder.applyTo("teacher", "", WordTraversalDirection.BACKWARD,
+                    emptyPatchOutput, 0, emptyPatchOutput.length);
+            final int noopLength = PatchCommandEncoder.applyTo("teacher", PatchCommandEncoder.NOOP_PATCH,
+                    WordTraversalDirection.BACKWARD, noopOutput, 0, noopOutput.length);
+
+            assertAll(() -> assertEquals(7, nullPatchLength),
+                    () -> assertEquals("teacher", new String(nullPatchOutput, 0, nullPatchLength)),
+                    () -> assertEquals(7, emptyPatchLength),
+                    () -> assertEquals("teacher", new String(emptyPatchOutput, 0, emptyPatchLength)),
+                    () -> assertEquals(7, noopLength),
+                    () -> assertEquals("teacher", new String(noopOutput, 0, noopLength)));
+        }
+
+        /**
+         * Verifies insufficient capacity behavior.
+         */
+        @Test
+        @DisplayName("applyTo reports insufficient capacity without writing output")
+        void shouldReportInsufficientCapacityWithoutWritingOutput() {
+            final char[] output = "xxxx".toCharArray();
+
+            final int produced = PatchCommandEncoder.applyTo("abc", "Ic", WordTraversalDirection.BACKWARD, output, 0,
+                    output.length - 1);
+
+            assertAll(() -> assertEquals(PatchCommandEncoder.APPLY_INSUFFICIENT_CAPACITY, produced),
+                    () -> assertArrayEquals("xxxx".toCharArray(), output));
+        }
+
+        /**
+         * Verifies exception parity with string-returning application.
+         */
+        @Test
+        @DisplayName("applyTo throws for unsupported opcode and NOOP argument")
+        void shouldApplyToThrowForUnsupportedOpcodeAndNoopArgument() {
+            final char[] output = new char[8];
+
+            assertAll(() -> {
+                final IllegalArgumentException exception = assertThrows(IllegalArgumentException.class,
+                        () -> PatchCommandEncoder.applyTo("abc", "Xa", WordTraversalDirection.BACKWARD, output, 0,
+                                output.length));
+                assertEquals("Unsupported patch opcode: X", exception.getMessage());
+            }, () -> {
+                final IllegalArgumentException exception = assertThrows(IllegalArgumentException.class,
+                        () -> PatchCommandEncoder.applyTo("abc", "Nb", WordTraversalDirection.BACKWARD, output, 0,
+                                output.length));
+                assertEquals("Unsupported NOOP patch argument: b", exception.getMessage());
+            });
+        }
+
+        /**
+         * Verifies malformed compatibility behavior for buffer application.
+         *
+         * @param caseId         numeric case identifier
+         * @param source         original source
+         * @param malformedPatch malformed patch
+         */
+        @ParameterizedTest(name = "[{index}] case {0}: malformed applyTo patch {2} preserves {1}")
+        @MethodSource("org.egothor.stemmer.PatchCommandEncoderTest#provideMalformedPatchCases")
+        @DisplayName("applyTo preserves source for malformed or index-invalid patch commands")
+        void shouldApplyToPreserveSourceForMalformedOrIndexInvalidPatchCommands(int caseId, String source,
+                String malformedPatch) {
+            final char[] output = new char[Math.max(source.length(), 1)];
+
+            final int produced = PatchCommandEncoder.applyTo(source, malformedPatch, WordTraversalDirection.BACKWARD,
+                    output, 0, output.length);
+
+            assertAll(() -> assertEquals(source.length(), produced, () -> "Case " + caseId + " produced wrong length."),
+                    () -> assertEquals(source, new String(output, 0, produced)));
+        }
+
+        /**
+         * Verifies explicit traversal direction for buffer application.
+         */
+        @Test
+        @DisplayName("applyTo follows explicit forward traversal direction")
+        void shouldApplyToWithForwardTraversalDirection() {
+            final PatchCommandEncoder encoder = PatchCommandEncoder.builder()
+                    .traversalDirection(WordTraversalDirection.FORWARD)
+                    .build();
+            final String patch = encoder.encode("cities", "city");
+
+            assertEquals(PatchCommandEncoder.apply("cities", patch, WordTraversalDirection.FORWARD),
+                    applyToString("cities", patch, WordTraversalDirection.FORWARD));
+        }
+
+        /**
+         * Verifies overlapping source/output slices are rejected.
+         */
+        @Test
+        @DisplayName("applyTo rejects overlapping char-array source and output ranges")
+        void shouldRejectOverlappingSourceAndOutputRanges() {
+            final char[] buffer = "teacher....".toCharArray();
+
+            final IllegalArgumentException exception = assertThrows(IllegalArgumentException.class,
+                    () -> PatchCommandEncoder.applyTo(buffer, 0, 7, "Db", WordTraversalDirection.BACKWARD, buffer, 2, 5));
+
+            assertEquals("source and output ranges must not overlap.", exception.getMessage());
+        }
     }
 
     /**
@@ -598,6 +953,9 @@ class PatchCommandEncoderTest {
     @Nested
     @DisplayName("stemming-oriented scenarios")
     @Tag("regression")
+    @Tag("unit")
+    @Tag("stemmer")
+    @Tag("patch")
     class StemmingScenarioTests {
 
         /**
@@ -659,6 +1017,9 @@ class PatchCommandEncoderTest {
     @Nested
     @DisplayName("reversed-word processing")
     @Tag("normalization")
+    @Tag("unit")
+    @Tag("stemmer")
+    @Tag("patch")
     class ReversedWordProcessingTests {
 
         /**
@@ -743,6 +1104,7 @@ class PatchCommandEncoderTest {
      */
     @ParameterizedTest(name = "[{index}] case {0}: mirrored consistency for {1} -> {2}")
     @MethodSource("org.egothor.stemmer.PatchCommandEncoderTest#provideReversedRoundTripPairs")
+    @Tag("normalization")
     @DisplayName("preserves correctness under mirrored input orientation")
     void shouldPreserveCorrectnessUnderMirroredInputOrientation(int caseId, String source, String target) {
         PatchCommandEncoder encoder = PatchCommandEncoder.builder().build();

src/test/java/org/egothor/stemmer/trie/CompiledNodeAndNodeDataTest.java

@@ -46,9 +46,39 @@ import org.junit.jupiter.api.Test;
  */
 @Tag("unit")
 @Tag("trie")
+@Tag("lookup")
 @DisplayName("CompiledNode and NodeData")
 class CompiledNodeAndNodeDataTest {
 
+    /**
+     * Creates a typed child array for compiled-node tests.
+     *
+     * @param length requested array length
+     * @return typed child array
+     */
+    @SuppressWarnings("unchecked")
+    private static CompiledNode<String>[] children(final int length) {
+        return new CompiledNode[length];
+    }
+
+    /**
+     * Creates an empty child array for leaf compiled-node tests.
+     *
+     * @return empty typed child array
+     */
+    private static CompiledNode<String>[] noChildren() {
+        return children(0);
+    }
+
+    /**
+     * Creates a leaf node used as a child in lookup tests.
+     *
+     * @return leaf node
+     */
+    private static CompiledNode<String> leaf() {
+        return new CompiledNode<>(new char[0], noChildren(), new String[0], new int[0]);
+    }
+
     /**
      * Verifies that {@link NodeData} rejects mismatched edge-related array lengths.
      */
@@ -99,8 +129,7 @@ class CompiledNodeAndNodeDataTest {
     @Test
     @DisplayName("CompiledNode rejects mismatched edge and child arrays")
     void compiledNodeShouldRejectMismatchedEdgeAndChildArrays() {
-        @SuppressWarnings("unchecked")
+        final CompiledNode<String>[] children = noChildren();
-        final CompiledNode<String>[] children = new CompiledNode[0];
 
         final IllegalArgumentException exception = assertThrows(IllegalArgumentException.class,
                 () -> new CompiledNode<String>(new char[] { 'a' }, children, new String[0], new int[0]));
@@ -114,8 +143,7 @@ class CompiledNodeAndNodeDataTest {
     @Test
     @DisplayName("CompiledNode rejects mismatched value arrays")
     void compiledNodeShouldRejectMismatchedValueArrays() {
-        @SuppressWarnings("unchecked")
+        final CompiledNode<String>[] children = noChildren();
-        final CompiledNode<String>[] children = new CompiledNode[0];
 
         final IllegalArgumentException exception = assertThrows(IllegalArgumentException.class,
                 () -> new CompiledNode<String>(new char[0], children, new String[] { "stem" }, new int[0]));
@@ -131,8 +159,7 @@ class CompiledNodeAndNodeDataTest {
     @DisplayName("CompiledNode accessors expose documented backing arrays")
     void compiledNodeAccessorsShouldExposeDocumentedBackingArrays() {
         final char[] edgeLabels = new char[] { 'a' };
-        @SuppressWarnings("unchecked")
+        final CompiledNode<String>[] children = children(1);
-        final CompiledNode<String>[] children = new CompiledNode[1];
         final String[] orderedValues = new String[] { "stem" };
         final int[] orderedCounts = new int[] { 5 };
         final CompiledNode<String> node = new CompiledNode<>(edgeLabels, children, orderedValues, orderedCounts);
@@ -149,12 +176,11 @@ class CompiledNodeAndNodeDataTest {
     @Test
     @DisplayName("CompiledNode can resolve child via dense lookup table")
     void compiledNodeUsesDenseLookupForCompactIntervals() {
-        @SuppressWarnings("unchecked")
+        final CompiledNode<String>[] children = children(4);
-        final CompiledNode<String>[] children = new CompiledNode[4];
+        children[0] = leaf();
-        children[0] = new CompiledNode<>(new char[0], new CompiledNode[0], new String[0], new int[0]);
+        children[1] = leaf();
-        children[1] = new CompiledNode<>(new char[0], new CompiledNode[0], new String[0], new int[0]);
+        children[2] = leaf();
-        children[2] = new CompiledNode<>(new char[0], new CompiledNode[0], new String[0], new int[0]);
+        children[3] = leaf();
-        children[3] = new CompiledNode<>(new char[0], new CompiledNode[0], new String[0], new int[0]);
 
         final CompiledNode<String> node = new CompiledNode<>(new char[] { 'a', 'b', 'c', 'd' }, children,
                 new String[] { "1", "2", "3", "4" }, new int[] { 1, 1, 1, 1 });
@@ -172,12 +198,11 @@ class CompiledNodeAndNodeDataTest {
     @Test
     @DisplayName("CompiledNode resolves child by linear scan for small degree")
     void compiledNodeUsesLinearScanForSmallDegree() {
-        @SuppressWarnings("unchecked")
+        final CompiledNode<String>[] children = children(4);
-        final CompiledNode<String>[] children = new CompiledNode[4];
+        final CompiledNode<String> childA = leaf();
-        final CompiledNode<String> childA = new CompiledNode<>(new char[0], new CompiledNode[0], new String[0], new int[0]);
+        final CompiledNode<String> childB = leaf();
-        final CompiledNode<String> childB = new CompiledNode<>(new char[0], new CompiledNode[0], new String[0], new int[0]);
+        final CompiledNode<String> childC = leaf();
-        final CompiledNode<String> childC = new CompiledNode<>(new char[0], new CompiledNode[0], new String[0], new int[0]);
+        final CompiledNode<String> childD = leaf();
-        final CompiledNode<String> childD = new CompiledNode<>(new char[0], new CompiledNode[0], new String[0], new int[0]);
         children[0] = childA;
         children[1] = childB;
         children[2] = childC;
@@ -200,13 +225,12 @@ class CompiledNodeAndNodeDataTest {
     @Test
     @DisplayName("CompiledNode resolves child by binary search for large degree")
     void compiledNodeUsesBinarySearchForLargeDegree() {
-        @SuppressWarnings("unchecked")
+        final CompiledNode<String>[] children = children(5);
-        final CompiledNode<String>[] children = new CompiledNode[5];
+        final CompiledNode<String> childA = leaf();
-        final CompiledNode<String> childA = new CompiledNode<>(new char[0], new CompiledNode[0], new String[0], new int[0]);
+        final CompiledNode<String> childB = leaf();
-        final CompiledNode<String> childB = new CompiledNode<>(new char[0], new CompiledNode[0], new String[0], new int[0]);
+        final CompiledNode<String> childC = leaf();
-        final CompiledNode<String> childC = new CompiledNode<>(new char[0], new CompiledNode[0], new String[0], new int[0]);
+        final CompiledNode<String> childD = leaf();
-        final CompiledNode<String> childD = new CompiledNode<>(new char[0], new CompiledNode[0], new String[0], new int[0]);
+        final CompiledNode<String> childE = leaf();
-        final CompiledNode<String> childE = new CompiledNode<>(new char[0], new CompiledNode[0], new String[0], new int[0]);
         children[0] = childA;
         children[1] = childB;
         children[2] = childC;
@@ -230,8 +254,7 @@ class CompiledNodeAndNodeDataTest {
     @Test
     @DisplayName("CompiledNode reports leaf, value and edge presence state")
     void compiledNodeReportsNodeStateHelpers() {
-        @SuppressWarnings("unchecked")
+        final CompiledNode<String>[] childless = noChildren();
-        final CompiledNode<String>[] childless = new CompiledNode[0];
         final CompiledNode<String> leaf = new CompiledNode<>(new char[0], childless, new String[0], new int[0]);
 
         assertTrue(leaf.isLeaf());
@@ -239,11 +262,10 @@ class CompiledNodeAndNodeDataTest {
         assertFalse(leaf.hasValues());
         assertFalse(leaf.hasEdge('a'));
 
-        @SuppressWarnings("unchecked")
+        final CompiledNode<String>[] child = children(1);
-        final CompiledNode<String>[] child = new CompiledNode[1];
         final String[] orderedValues = new String[] { "leaf" };
         final int[] orderedCounts = new int[] { 1 };
-        child[0] = new CompiledNode<>(new char[0], new CompiledNode[0], orderedValues, orderedCounts);
+        child[0] = new CompiledNode<>(new char[0], noChildren(), orderedValues, orderedCounts);
         final CompiledNode<String> node = new CompiledNode<>(new char[] { 'a' }, child, orderedValues, orderedCounts);
 
         assertFalse(node.isLeaf());
@@ -260,9 +282,8 @@ class CompiledNodeAndNodeDataTest {
     @Test
     @DisplayName("CompiledNode equals and hashCode align for identical structure")
     void compiledNodeEqualsAndHashCodeAlignForIdenticalStructure() {
-        @SuppressWarnings("unchecked")
+        final CompiledNode<String>[] child = children(1);
-        final CompiledNode<String>[] child = new CompiledNode[1];
+        final CompiledNode<String> leaf = new CompiledNode<>(new char[0], noChildren(), new String[] { "v" },
-        final CompiledNode<String> leaf = new CompiledNode<>(new char[0], new CompiledNode[0], new String[] { "v" },
                 new int[] { 1 });
         child[0] = leaf;