Add recursive chunker

docs/changelog/126866.yaml

@@ -0,0 +1,5 @@
+pr: 126866
+summary: Add recursive chunker
+area: Machine Learning
+type: enhancement
+issues: []

server/src/main/java/org/elasticsearch/inference/ChunkingStrategy.java

@@ -16,6 +16,7 @@
 public enum ChunkingStrategy {
     WORD("word"),
     SENTENCE("sentence"),
+    RECURSIVE("recursive"),
     NONE("none");
 
     private final String chunkingStrategy;

x-pack/plugin/inference/src/main/java/org/elasticsearch/xpack/inference/InferenceNamedWriteablesProvider.java

@@ -27,6 +27,7 @@
 import org.elasticsearch.xpack.core.inference.results.TextEmbeddingFloatResults;
 import org.elasticsearch.xpack.inference.action.task.StreamingTaskManager;
 import org.elasticsearch.xpack.inference.chunking.NoneChunkingSettings;
+import org.elasticsearch.xpack.inference.chunking.RecursiveChunkingSettings;
 import org.elasticsearch.xpack.inference.chunking.SentenceBoundaryChunkingSettings;
 import org.elasticsearch.xpack.inference.chunking.WordBoundaryChunkingSettings;
 import org.elasticsearch.xpack.inference.common.amazon.AwsSecretSettings;
@@ -567,6 +568,9 @@ private static void addChunkingSettingsNamedWriteables(List<NamedWriteableRegist
                 SentenceBoundaryChunkingSettings::new
             )
         );
+        namedWriteables.add(
+            new NamedWriteableRegistry.Entry(ChunkingSettings.class, RecursiveChunkingSettings.NAME, RecursiveChunkingSettings::new)
+        );
     }
 
     private static void addInferenceResultsNamedWriteables(List<NamedWriteableRegistry.Entry> namedWriteables) {

x-pack/plugin/inference/src/main/java/org/elasticsearch/xpack/inference/chunking/ChunkerBuilder.java

@@ -19,6 +19,7 @@ public static Chunker fromChunkingStrategy(ChunkingStrategy chunkingStrategy) {
             case NONE -> NoopChunker.INSTANCE;
             case WORD -> new WordBoundaryChunker();
             case SENTENCE -> new SentenceBoundaryChunker();
+            case RECURSIVE -> new RecursiveChunker();
         };
     }
 }

x-pack/plugin/inference/src/main/java/org/elasticsearch/xpack/inference/chunking/ChunkerUtils.java

@@ -0,0 +1,31 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the Elastic License
+ * 2.0; you may not use this file except in compliance with the Elastic License
+ * 2.0.
+ */
+
+package org.elasticsearch.xpack.inference.chunking;
+
+import com.ibm.icu.text.BreakIterator;
+
+public class ChunkerUtils {
+
+    // setText() should be applied before using this function.
+    static int countWords(int start, int end, BreakIterator wordIterator) {
+        assert start < end;
+        wordIterator.preceding(start); // start of the current word
+
+        int boundary = wordIterator.current();
+        int wordCount = 0;
+        while (boundary != BreakIterator.DONE && boundary <= end) {
+            int wordStatus = wordIterator.getRuleStatus();
+            if (wordStatus != BreakIterator.WORD_NONE) {
+                wordCount++;
+            }
+            boundary = wordIterator.next();
+        }
+
+        return wordCount;
+    }
+}

x-pack/plugin/inference/src/main/java/org/elasticsearch/xpack/inference/chunking/ChunkingSettingsBuilder.java

@@ -48,6 +48,7 @@ public static ChunkingSettings fromMap(Map<String, Object> settings, boolean ret
             case NONE -> NoneChunkingSettings.INSTANCE;
             case WORD -> WordBoundaryChunkingSettings.fromMap(new HashMap<>(settings));
             case SENTENCE -> SentenceBoundaryChunkingSettings.fromMap(new HashMap<>(settings));
+            case RECURSIVE -> RecursiveChunkingSettings.fromMap(new HashMap<>(settings));
         };
     }
 }

x-pack/plugin/inference/src/main/java/org/elasticsearch/xpack/inference/chunking/ChunkingSettingsOptions.java

@@ -11,7 +11,9 @@ public enum ChunkingSettingsOptions {
     STRATEGY("strategy"),
     MAX_CHUNK_SIZE("max_chunk_size"),
     OVERLAP("overlap"),
-    SENTENCE_OVERLAP("sentence_overlap");
+    SENTENCE_OVERLAP("sentence_overlap"),
+    SEPARATOR_SET("separator_set"),
+    SEPARATORS("separators");
 
     private final String chunkingSettingsOption;
 

x-pack/plugin/inference/src/main/java/org/elasticsearch/xpack/inference/chunking/RecursiveChunker.java

@@ -0,0 +1,125 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the Elastic License
+ * 2.0; you may not use this file except in compliance with the Elastic License
+ * 2.0.
+ */
+
+package org.elasticsearch.xpack.inference.chunking;
+
+import com.ibm.icu.text.BreakIterator;
+
+import org.elasticsearch.common.Strings;
+import org.elasticsearch.inference.ChunkingSettings;
+
+import java.util.ArrayList;
+import java.util.List;
+import java.util.regex.Pattern;
+
+/**
+ * Split text into chunks recursively based on a list of separator regex strings.
+ * The maximum chunk size is measured in words and controlled
+ * by {@code maxNumberWordsPerChunk}. For each separator the chunker will go through the following process:
+ * 1. Split the text on each regex match of the separator.
+ * 2. For each chunk after the merge:
+ *     1. Return it if it is within the maximum chunk size.
+ *     2. Repeat the process using the next separator in the list if the chunk exceeds the maximum chunk size.
+ *     If there are no more separators left to try, run the {@code SentenceBoundaryChunker} with the provided
+ *     max chunk size and no overlaps.
+ */
+public class RecursiveChunker implements Chunker {
+    private final BreakIterator wordIterator;
+
+    public RecursiveChunker() {
+        wordIterator = BreakIterator.getWordInstance();
+    }
+
+    @Override
+    public List<ChunkOffset> chunk(String input, ChunkingSettings chunkingSettings) {
+        if (chunkingSettings instanceof RecursiveChunkingSettings recursiveChunkingSettings) {
+            return chunk(
+                input,
+                new ChunkOffset(0, input.length()),
+                recursiveChunkingSettings.getSeparators(),
+                recursiveChunkingSettings.getMaxChunkSize(),
+                0
+            );
+        } else {
+            throw new IllegalArgumentException(
+                Strings.format("RecursiveChunker can't use ChunkingSettings with strategy [%s]", chunkingSettings.getChunkingStrategy())
+            );
+        }
+    }
+
+    private List<ChunkOffset> chunk(String input, ChunkOffset offset, List<String> separators, int maxChunkSize, int separatorIndex) {
+        if (offset.start() == offset.end() || isChunkWithinMaxSize(buildChunkOffsetAndCount(input, offset), maxChunkSize)) {
+            return List.of(offset);
+        }
+
+        if (separatorIndex > separators.size() - 1) {
+            return chunkWithBackupChunker(input, offset, maxChunkSize);
+        }
+
+        var potentialChunks = splitTextBySeparatorRegex(input, offset, separators.get(separatorIndex));
+        var actualChunks = new ArrayList<ChunkOffset>();
+        for (var potentialChunk : potentialChunks) {
+            if (isChunkWithinMaxSize(potentialChunk, maxChunkSize)) {
+                actualChunks.add(potentialChunk.chunkOffset());
+            } else {
+                actualChunks.addAll(chunk(input, potentialChunk.chunkOffset(), separators, maxChunkSize, separatorIndex + 1));
+            }
+        }
+
+        return actualChunks;
+    }
+
+    private boolean isChunkWithinMaxSize(ChunkOffsetAndCount chunkOffsetAndCount, int maxChunkSize) {
+        return chunkOffsetAndCount.wordCount <= maxChunkSize;
+    }
+
+    private ChunkOffsetAndCount buildChunkOffsetAndCount(String fullText, ChunkOffset offset) {
+        wordIterator.setText(fullText);
+        return new ChunkOffsetAndCount(offset, ChunkerUtils.countWords(offset.start(), offset.end(), wordIterator));
+    }
+
+    private List<ChunkOffsetAndCount> splitTextBySeparatorRegex(String input, ChunkOffset offset, String separatorRegex) {
+        var pattern = Pattern.compile(separatorRegex, Pattern.MULTILINE);
+        var matcher = pattern.matcher(input);
+
+        var chunkOffsets = new ArrayList<ChunkOffsetAndCount>();
+        int chunkStart = offset.start();
+        int searchStart = offset.start();
+        while (matcher.find(searchStart)) {
+            var chunkEnd = matcher.start();
+            if (chunkEnd >= offset.end()) {
+                break; // No more matches within the chunk offset
+            }
+
+            if (chunkStart < chunkEnd) {
+                chunkOffsets.add(buildChunkOffsetAndCount(input, new ChunkOffset(chunkStart, chunkEnd)));
+            }
+            chunkStart = chunkEnd;
+            searchStart = matcher.end();
+        }
+
+        if (chunkStart < offset.end()) {
+            chunkOffsets.add(buildChunkOffsetAndCount(input, new ChunkOffset(chunkStart, offset.end())));
+        }
+
+        return chunkOffsets;
+    }
+
+    private List<ChunkOffset> chunkWithBackupChunker(String input, ChunkOffset offset, int maxChunkSize) {
+        var chunks = new SentenceBoundaryChunker().chunk(
+            input.substring(offset.start(), offset.end()),
+            new SentenceBoundaryChunkingSettings(maxChunkSize, 0)
+        );
+        var chunksWithOffsets = new ArrayList<ChunkOffset>();
+        for (var chunk : chunks) {
+            chunksWithOffsets.add(new ChunkOffset(chunk.start() + offset.start(), chunk.end() + offset.start()));
+        }
+        return chunksWithOffsets;
+    }
+
+    private record ChunkOffsetAndCount(ChunkOffset chunkOffset, int wordCount) {}
+}

x-pack/plugin/inference/src/main/java/org/elasticsearch/xpack/inference/chunking/RecursiveChunkingSettings.java

@@ -0,0 +1,173 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the Elastic License
+ * 2.0; you may not use this file except in compliance with the Elastic License
+ * 2.0.
+ */
+
+package org.elasticsearch.xpack.inference.chunking;
+
+import org.elasticsearch.TransportVersion;
+import org.elasticsearch.common.Strings;
+import org.elasticsearch.common.ValidationException;
+import org.elasticsearch.common.io.stream.StreamInput;
+import org.elasticsearch.common.io.stream.StreamOutput;
+import org.elasticsearch.inference.ChunkingSettings;
+import org.elasticsearch.inference.ChunkingStrategy;
+import org.elasticsearch.inference.ModelConfigurations;
+import org.elasticsearch.xcontent.XContentBuilder;
+import org.elasticsearch.xpack.inference.services.ServiceUtils;
+
+import java.io.IOException;
+import java.util.Arrays;
+import java.util.EnumSet;
+import java.util.List;
+import java.util.Locale;
+import java.util.Map;
+import java.util.Objects;
+import java.util.Set;
+
+public class RecursiveChunkingSettings implements ChunkingSettings {
+    public static final String NAME = "RecursiveChunkingSettings";
+    private static final ChunkingStrategy STRATEGY = ChunkingStrategy.RECURSIVE;
+    private static final int MAX_CHUNK_SIZE_LOWER_LIMIT = 10;
+    private static final int MAX_CHUNK_SIZE_UPPER_LIMIT = 300;
+
+    private static final Set<String> VALID_KEYS = Set.of(
+        ChunkingSettingsOptions.STRATEGY.toString(),
+        ChunkingSettingsOptions.MAX_CHUNK_SIZE.toString(),
+        ChunkingSettingsOptions.SEPARATOR_SET.toString(),
+        ChunkingSettingsOptions.SEPARATORS.toString()
+    );
+
+    private final int maxChunkSize;
+    private final List<String> separators;
+
+    public RecursiveChunkingSettings(int maxChunkSize, List<String> separators) {
+        this.maxChunkSize = maxChunkSize;
+        this.separators = separators == null ? SeparatorSet.PLAINTEXT.getSeparators() : separators;
+    }
+
+    public RecursiveChunkingSettings(StreamInput in) throws IOException {
+        maxChunkSize = in.readInt();
+        separators = in.readCollectionAsList(StreamInput::readString);
+    }
+
+    public static RecursiveChunkingSettings fromMap(Map<String, Object> map) {
+        ValidationException validationException = new ValidationException();
+
+        var invalidSettings = map.keySet().stream().filter(key -> VALID_KEYS.contains(key) == false).toArray();
+        if (invalidSettings.length > 0) {
+            validationException.addValidationError(
+                Strings.format("Recursive chunking settings can not have the following settings: %s", Arrays.toString(invalidSettings))
+            );
+        }
+
+        Integer maxChunkSize = ServiceUtils.extractRequiredPositiveIntegerBetween(
+            map,
+            ChunkingSettingsOptions.MAX_CHUNK_SIZE.toString(),
+            MAX_CHUNK_SIZE_LOWER_LIMIT,
+            MAX_CHUNK_SIZE_UPPER_LIMIT,
+            ModelConfigurations.CHUNKING_SETTINGS,
+            validationException
+        );
+
+        SeparatorSet separatorSet = ServiceUtils.extractOptionalEnum(
+            map,
+            ChunkingSettingsOptions.SEPARATOR_SET.toString(),
+            ModelConfigurations.CHUNKING_SETTINGS,
+            SeparatorSet::fromString,
+            EnumSet.allOf(SeparatorSet.class),
+            validationException
+        );
+
+        List<String> separators = ServiceUtils.extractOptionalList(
+            map,
+            ChunkingSettingsOptions.SEPARATORS.toString(),
+            String.class,
+            validationException
+        );
+
+        if (separators != null && separatorSet != null) {
+            validationException.addValidationError("Recursive chunking settings can not have both separators and separator_set");
+        }
+
+        if (separatorSet != null) {
+            separators = separatorSet.getSeparators();
+        } else if (separators != null && separators.isEmpty()) {
+            validationException.addValidationError("Recursive chunking settings can not have an empty list of separators");
+        }
+
+        if (validationException.validationErrors().isEmpty() == false) {
+            throw validationException;
+        }
+
+        return new RecursiveChunkingSettings(maxChunkSize, separators);
+    }
+
+    public int getMaxChunkSize() {
+        return maxChunkSize;
+    }
+
+    public List<String> getSeparators() {
+        return separators;
+    }
+
+    @Override
+    public ChunkingStrategy getChunkingStrategy() {
+        return STRATEGY;
+    }
+
+    @Override
+    public Map<String, Object> asMap() {
+        return Map.of(
+            ChunkingSettingsOptions.STRATEGY.toString(),
+            STRATEGY.toString().toLowerCase(Locale.ROOT),
+            ChunkingSettingsOptions.MAX_CHUNK_SIZE.toString(),
+            maxChunkSize,
+            ChunkingSettingsOptions.SEPARATORS.toString(),
+            separators
+        );
+    }
+
+    @Override
+    public String getWriteableName() {
+        return NAME;
+    }
+
+    @Override
+    public TransportVersion getMinimalSupportedVersion() {
+        return null; // TODO: Add transport version
+    }
+
+    @Override
+    public void writeTo(StreamOutput out) throws IOException {
+        out.writeInt(maxChunkSize);
+        out.writeCollection(separators, StreamOutput::writeString);
+    }
+
+    @Override
+    public XContentBuilder toXContent(XContentBuilder builder, Params params) throws IOException {
+        builder.startObject();
+        {
+            builder.field(ChunkingSettingsOptions.STRATEGY.toString(), STRATEGY);
+            builder.field(ChunkingSettingsOptions.MAX_CHUNK_SIZE.toString(), maxChunkSize);
+            builder.field(ChunkingSettingsOptions.SEPARATORS.toString(), separators);
+        }
+        builder.endObject();
+        return builder;
+    }
+
+    @Override
+    public boolean equals(Object o) {
+        if (this == o) return true;
+        if (o == null || getClass() != o.getClass()) return false;
+        RecursiveChunkingSettings that = (RecursiveChunkingSettings) o;
+        return Objects.equals(maxChunkSize, that.maxChunkSize) && Objects.equals(separators, that.separators);
+    }
+
+    @Override
+    public int hashCode() {
+        return Objects.hash(maxChunkSize, separators);
+    }
+}