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.
*/
/**
* A {@link ChunksIntEncoder} which encodes values in chunks of 4. Every group
* starts with a single byte (called indicator) which represents 4 - 2 bit
* flags, where the values:
* <ul>
* <li>1, 2 or 3 mean the encoded value is '1', '2' or '3' respectively.
* <li>0 means the value is encoded using {@link VInt8IntEncoder}, and the
* encoded bytes follow the indicator.<br>
* Since value 0 is illegal, and 1-3 are encoded in the indicator, the actual
* value that is encoded is <code>value-4</code>, which saves some more bits.
* </ul>
* Encoding example:
* <ul>
* <li>Original values: 6, 16, 5, 9, 7, 1, 11
* <li>After sorting: 1, 5, 6, 7, 9, 11, 16
* <li>D-Gap computing: 1, 4, 1, 1, 2, 5 (so far - done by
* {@link DGapIntEncoder})
* <li>Encoding: 1,0,1,1 as the first indicator, followed by 0 (4-4), than
* 2,0,0,0 as the second indicator, followed by 1 (5-4) encoded with.
* <li>Binary encode: <u>01 | 01 | 00 | 01</u> 00000000 <u>00 | 00 | 00 | 10</u>
* 00000001 (indicators are <u>underlined</u>).<br>
* <b>NOTE:</b> the order of the values in the indicator is lsb ⇒ msb,
* which allows for more efficient decoding.
* </ul>
*
* @lucene.experimental
*/
public class FourFlagsIntEncoder extends ChunksIntEncoder {
/**
* Holds all combinations of <i>indicator</i> flags for fast encoding (saves
* time on bit manipulation @ encode time)
*/
private static byte[][] encodeTable = new byte[][] {
new byte[] { 0x00, 0x00, 0x00, 0x00 },
new byte[] { 0x01, 0x04, 0x10, 0x40 },
new byte[] { 0x02, 0x08, 0x20, (byte) 0x80 },
new byte[] { 0x03, 0x0C, 0x30, (byte) 0xC0 },
};
public FourFlagsIntEncoder() {
super(4);
}
/**
* Small values (<=3) are stored in the <code>indicator</code> while larger
* values are saved for later encoding in the {@link #encodeQueue}. Since
* Vint8 will only encode values larger or equal to 4, the values saves for
* encoded are transformed to (value - 4).<br>
* When a chunk is ready (got 4 values), the {@link #encodeChunk()}
* takes control.
*/
@Override
public void encode(int data) throws IOException {
if (data <= 3) {
indicator |= encodeTable[data][ordinal];
} else {
encodeQueue[encodeQueueSize++] = data - 4;
}
++ordinal;
// If 4 values were encoded thus far, 'flush' them including the indicator.
if ((ordinal & 0x3) == 0) {
encodeChunk();
}
}
@Override
public IntDecoder createMatchingDecoder() {
return new FourFlagsIntDecoder();
}
@Override
public String toString() {
return "FourFlags (" + encoder.toString() + ")";
}
}