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Apache Lucene (module: highlighter)
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/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package org.apache.lucene.search.matchhighlight;

import java.io.IOException;
import java.util.ArrayList;
import java.util.List;
import org.apache.lucene.analysis.Analyzer;
import org.apache.lucene.analysis.TokenStream;
import org.apache.lucene.analysis.tokenattributes.OffsetAttribute;
import org.apache.lucene.analysis.tokenattributes.PositionIncrementAttribute;
import org.apache.lucene.search.MatchesIterator;

/**
 * This strategy applies to fields with stored positions but no offsets. We re-analyze the field's
 * value to find out offsets of match positions.
 *
 * Note that this may fail if index data (positions stored in the index) is out of sync with the
 * field values or the analyzer. This strategy assumes it'll never happen.
 */
public final class OffsetsFromPositions implements OffsetsRetrievalStrategy {
  private final String field;
  private final Analyzer analyzer;

  public OffsetsFromPositions(String field, Analyzer analyzer) {
    this.field = field;
    this.analyzer = analyzer;
  }

  @Override
  public List get(
      MatchesIterator matchesIterator, MatchRegionRetriever.FieldValueProvider doc)
      throws IOException {
    ArrayList positionRanges = new ArrayList<>();
    while (matchesIterator.next()) {
      int from = matchesIterator.startPosition();
      int to = matchesIterator.endPosition();
      if (from < 0 || to < 0) {
        throw new IOException("Matches API returned negative positions for field: " + field);
      }
      positionRanges.add(new OffsetRange(from, to));
    }

    // Convert from positions to offsets.
    return convertPositionsToOffsets(positionRanges, doc.getValues(field));
  }

  List convertPositionsToOffsets(
      ArrayList positionRanges, List values) throws IOException {

    if (positionRanges.isEmpty()) {
      return positionRanges;
    }

    class PositionSpan extends OffsetRange {
      int leftOffset = Integer.MAX_VALUE;
      int rightOffset = Integer.MIN_VALUE;

      PositionSpan(int from, int to) {
        super(from, to);
      }

      @Override
      public String toString() {
        return "[from=" + from + ", to=" + to + ", L: " + leftOffset + ", R: " + rightOffset + ']';
      }
    }

    ArrayList spans = new ArrayList<>();
    int minPosition = Integer.MAX_VALUE;
    int maxPosition = Integer.MIN_VALUE;
    for (OffsetRange range : positionRanges) {
      spans.add(new PositionSpan(range.from, range.to));
      minPosition = Math.min(minPosition, range.from);
      maxPosition = Math.max(maxPosition, range.to);
    }

    PositionSpan[] spansTable = spans.toArray(PositionSpan[]::new);
    int spanCount = spansTable.length;
    int position = -1;
    int valueOffset = 0;
    for (int valueIndex = 0, max = values.size(); valueIndex < max; valueIndex++) {
      final String value = values.get(valueIndex).toString();
      final boolean lastValue = valueIndex + 1 == max;

      TokenStream ts = analyzer.tokenStream(field, value);
      OffsetAttribute offsetAttr = ts.getAttribute(OffsetAttribute.class);
      PositionIncrementAttribute posAttr = ts.getAttribute(PositionIncrementAttribute.class);
      ts.reset();
      while (ts.incrementToken()) {
        position += posAttr.getPositionIncrement();

        if (position >= minPosition) {
          // Correct left and right offsets for each span this position applies to.
          int startOffset = valueOffset + offsetAttr.startOffset();
          int endOffset = valueOffset + offsetAttr.endOffset();

          int j = 0;
          for (int i = 0; i < spanCount; i++) {
            PositionSpan span = spansTable[j] = spansTable[i];
            if (position >= span.from) {
              if (position <= span.to) {
                span.leftOffset = Math.min(span.leftOffset, startOffset);
                span.rightOffset = Math.max(span.rightOffset, endOffset);
              } else {
                // this span can't intersect with any following position
                // so omit it by skipping j++.
                continue;
              }
            }
            j++;
          }
          spanCount = j;

          // Only short-circuit if we're on the last value (which should be the common
          // case since most fields would only have a single value anyway). We need
          // to make sure of this because otherwise offsetAttr would have incorrect value.
          if (position > maxPosition && lastValue) {
            break;
          }
        }
      }
      ts.end();
      position += posAttr.getPositionIncrement() + analyzer.getPositionIncrementGap(field);
      valueOffset += offsetAttr.endOffset() + analyzer.getOffsetGap(field);
      ts.close();
    }

    ArrayList converted = new ArrayList<>(spans.size());
    for (PositionSpan span : spans) {
      if (span.leftOffset == Integer.MAX_VALUE || span.rightOffset == Integer.MIN_VALUE) {
        throw new RuntimeException("One of the offsets missing for position range: " + span);
      }
      converted.add(new OffsetRange(span.leftOffset, span.rightOffset));
    }
    return converted;
  }
}