lucene/core/src/java/org/apache/lucene/index/PrefixCodedTerms.java

/*
 * 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.index;

import java.io.IOException;
import java.util.Objects;

import org.apache.lucene.store.IndexInput;
import org.apache.lucene.store.RAMFile;
import org.apache.lucene.store.RAMInputStream;
import org.apache.lucene.store.RAMOutputStream;
import org.apache.lucene.util.Accountable;
import org.apache.lucene.util.BytesRef;
import org.apache.lucene.util.BytesRefBuilder;
import org.apache.lucene.util.StringHelper;

/**
 * Prefix codes term instances (prefixes are shared). This is expected to be
 * faster to build than a FST and might also be more compact if there are no
 * common suffixes.
 * @lucene.internal
 */
public class PrefixCodedTerms implements Accountable {
  final RAMFile buffer;
  private final long size;
  private long delGen;

  private PrefixCodedTerms(RAMFile buffer, long size) {
    this.buffer = Objects.requireNonNull(buffer);
    this.size = size;
  }

  @Override
  public long ramBytesUsed() {
    return buffer.ramBytesUsed() + 2 * Long.BYTES;
  }

  /** Records del gen for this packet. */
  public void setDelGen(long delGen) {
    this.delGen = delGen;
  }
  
  /** Builds a PrefixCodedTerms: call add repeatedly, then finish. */
  public static class Builder {
    private RAMFile buffer = new RAMFile();
    private RAMOutputStream output = new RAMOutputStream(buffer, false);
    private Term lastTerm = new Term("");
    private BytesRefBuilder lastTermBytes = new BytesRefBuilder();
    private long size;

    /** Sole constructor. */
    public Builder() {}

    /** add a term */
    public void add(Term term) {
      add(term.field(), term.bytes());
    }

    /** add a term.  This fully consumes in the incoming {@link BytesRef}. */
    public void add(String field, BytesRef bytes) {
      assert lastTerm.equals(new Term("")) || new Term(field, bytes).compareTo(lastTerm) > 0;

      try {
        final int prefix;
        if (size > 0 && field.equals(lastTerm.field)) {
          // same field as the last term
          prefix = StringHelper.bytesDifference(lastTerm.bytes, bytes);
          output.writeVInt(prefix << 1);
        } else {
          // field change
          prefix = 0;
          output.writeVInt(1);
          output.writeString(field);
        }

        int suffix = bytes.length - prefix;
        output.writeVInt(suffix);
        output.writeBytes(bytes.bytes, bytes.offset + prefix, suffix);
        lastTermBytes.copyBytes(bytes);
        lastTerm.bytes = lastTermBytes.get();
        lastTerm.field = field;
        size += 1;
      } catch (IOException e) {
        throw new RuntimeException(e);
      }
    }
    
    /** return finalized form */
    public PrefixCodedTerms finish() {
      try {
        output.close();
        return new PrefixCodedTerms(buffer, size);
      } catch (IOException e) {
        throw new RuntimeException(e);
      }
    }
  }

  /** An iterator over the list of terms stored in a {@link PrefixCodedTerms}. */
  public static class TermIterator extends FieldTermIterator {
    final IndexInput input;
    final BytesRefBuilder builder = new BytesRefBuilder();
    final BytesRef bytes = builder.get();
    final long end;
    final long delGen;
    String field = "";

    private TermIterator(long delGen, RAMFile buffer) {
      try {
        input = new RAMInputStream("PrefixCodedTermsIterator", buffer);
      } catch (IOException e) {
        throw new RuntimeException(e);
      }
      end = input.length();
      this.delGen = delGen;
    }

    @Override
    public BytesRef next() {
      if (input.getFilePointer() < end) {
        try {
          int code = input.readVInt();
          boolean newField = (code & 1) != 0;
          if (newField) {
            field = input.readString();
          }
          int prefix = code >>> 1;
          int suffix = input.readVInt();
          readTermBytes(prefix, suffix);
          return bytes;
        } catch (IOException e) {
          throw new RuntimeException(e);
        }
      } else {
        field = null;
        return null;
      }
    }

    // TODO: maybe we should freeze to FST or automaton instead?
    private void readTermBytes(int prefix, int suffix) throws IOException {
      builder.grow(prefix + suffix);
      input.readBytes(builder.bytes(), prefix, suffix);
      builder.setLength(prefix + suffix);
    }

    @Override
    public String field() {
      return field;
    }

    @Override
    public long delGen() {
      return delGen;
    }
  }

  /** Return an iterator over the terms stored in this {@link PrefixCodedTerms}. */
  public TermIterator iterator() {
    return new TermIterator(delGen, buffer);
  }

  /** Return the number of terms stored in this {@link PrefixCodedTerms}. */
  public long size() {
    return size;
  }

  @Override
  public int hashCode() {
    int h = buffer.hashCode();
    h = 31 * h + (int) (delGen ^ (delGen >>> 32));
    return h;
  }

  @Override
  public boolean equals(Object obj) {
    if (this == obj) return true;
    if (obj == null) return false;
    if (getClass() != obj.getClass()) return false;
    PrefixCodedTerms other = (PrefixCodedTerms) obj;
    return buffer.equals(other.buffer) && delGen == other.delGen;
  }
}