netty5/codec/src/main/java/io/netty/handler/codec/compression/Bzip2BitWriter.java

/*
 * Copyright 2014 The Netty Project
 *
 * The Netty Project 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 io.netty.handler.codec.compression;

import io.netty.buffer.ByteBuf;

/**
 * A bit writer that allows the writing of single bit booleans, unary numbers, bit strings
 * of arbitrary length (up to 24 bits), and bit aligned 32-bit integers. A single byte at a
 * time is written to the {@link ByteBuf} when sufficient bits have been accumulated.
 */
final class Bzip2BitWriter {
    /**
     * A buffer of bits waiting to be written to the output stream.
     */
    private int bitBuffer;

    /**
     * The number of bits currently buffered in {@link #bitBuffer}.
     */
    private int bitCount;

    /**
     * Writes up to 24 bits to the output {@link ByteBuf}.
     * @param count The number of bits to write (maximum {@code 24}, because the {@link #bitBuffer}
     *              is {@code int} and it can store up to {@code 8} bits before calling)
     * @param value The bits to write
     */
    void writeBits(ByteBuf out, final int count, final int value) {
        if (count < 0 || count > 24) {
            throw new IllegalArgumentException("count: " + count + " (expected: 0-24)");
        }
        int bitCount = this.bitCount;
        int bitBuffer = this.bitBuffer | (value << (32 - count)) >>> bitCount;
        bitCount += count;

        while (bitCount >= 8) {
            out.writeByte(bitBuffer >>> 24);
            bitBuffer <<= 8;
            bitCount -= 8;
        }
        this.bitBuffer = bitBuffer;
        this.bitCount = bitCount;
    }

    /**
     * Writes a single bit to the output {@link ByteBuf}.
     * @param value The bit to write
     */
    void writeBoolean(ByteBuf out, final boolean value) {
        int bitCount = this.bitCount + 1;
        int bitBuffer = this.bitBuffer | (value ? 1 : 0) << (32 - bitCount);

        if (bitCount == 8) {
            out.writeByte(bitBuffer >>> 24);
            bitBuffer = 0;
            bitCount = 0;
        }
        this.bitBuffer = bitBuffer;
        this.bitCount = bitCount;
    }

    /**
     * Writes a zero-terminated unary number to the output {@link ByteBuf}.
     * Example of the output for value = 6: {@code 1111110}
     * @param value The number of {@code 1} to write
     */
    void writeUnary(ByteBuf out, int value) {
        if (value < 0) {
            throw new IllegalArgumentException("value: " + value + " (expected 0 or more)");
        }
        while (value-- > 0) {
            writeBoolean(out, true);
        }
        writeBoolean(out, false);
    }

    /**
     * Writes an integer as 32 bits to the output {@link ByteBuf}.
     * @param value The integer to write
     */
    void writeInt(ByteBuf out, final int value) {
        writeBits(out, 16, (value >>> 16) & 0xffff);
        writeBits(out, 16, value & 0xffff);
    }

    /**
     * Writes any remaining bits to the output {@link ByteBuf},
     * zero padding to a whole byte as required.
     */
    void flush(ByteBuf out) {
        if (bitCount > 0) {
            writeBits(out, 8 - bitCount, 0);
        }
    }
}