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package org.apache.lucene.util.encoding;

import java.io.IOException;

/**
 * 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.
 */

/**
 * An {@link IntEncoder} which implements variable length encoding. A number is
 * encoded as follows:
 * 
 * If it is less than 127 and non-negative, i.e. uses only 7 bits, it is
 * encoded as a single byte: 0bbbbbbb.
 * 
If it occupies more than 7 bits, it is represented as a series of bytes,
 * each byte carrying 7 bits. All but the last byte have the MSB set, the last
 * one has it unset.
 * 
 * Example:
 * 
 * n = 117 = 01110101: This has less than 8 significant bits, therefore is
 * encoded as 01110101 = 0x75.
 * 
n = 100000 = (binary) 11000011010100000. This has 17 significant bits,
 * thus needs three Vint8 bytes. Pad it to a multiple of 7 bits, then split it
 * into chunks of 7 and add an MSB, 0 for the last byte, 1 for the others:
 * 1|0000110 1|0001101 0|0100000 = 0x86 0x8D 0x20.
 * 
 * NOTE: although this encoder is not limited to values ≥ 0, it is not
 * recommended for use with negative values, as their encoding will result in 5
 * bytes written to the output stream, rather than 4. For such values, either
 * use {@link SimpleIntEncoder} or write your own version of variable length
 * encoding, which can better handle negative values.
 * 
 * @lucene.experimental
 */
public class VInt8IntEncoder extends IntEncoder {

  @Override
  public void encode(int value) throws IOException {
    if ((value & ~0x7F) == 0) {
      out.write(value);
    } else if ((value & ~0x3FFF) == 0) {
      out.write(0x80 | (value >> 7));
      out.write(0x7F & value);
    } else if ((value & ~0x1FFFFF) == 0) {
      out.write(0x80 | (value >> 14));
      out.write(0x80 | (value >> 7));
      out.write(0x7F & value);
    } else if ((value & ~0xFFFFFFF) == 0) {
      out.write(0x80 | (value >> 21));
      out.write(0x80 | (value >> 14));
      out.write(0x80 | (value >> 7));
      out.write(0x7F & value);
    } else {
      out.write(0x80 | (value >> 28));
      out.write(0x80 | (value >> 21));
      out.write(0x80 | (value >> 14));
      out.write(0x80 | (value >> 7));
      out.write(0x7F & value);
    }
  }

  @Override
  public IntDecoder createMatchingDecoder() {
    return new VInt8IntDecoder();
  }

  @Override
  public String toString() {
    return "VInt8";
  }

}