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• VIntCoding.java

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com.datastax.driver.core.VIntCoding Maven / Gradle / Ivy

/*
 *      Copyright (C) 2012-2017 DataStax Inc.
 *
 *      This software can be used solely with DataStax Enterprise. Please consult the license at
 *      http://www.datastax.com/terms/datastax-dse-driver-license-terms
 */
package com.datastax.driver.core;

import io.netty.util.concurrent.FastThreadLocal;

import java.io.DataInput;
import java.io.DataOutput;
import java.io.IOException;

/**
 * Variable length encoding inspired from Google
 * varints.
 * 

 * 
Cassandra vints are encoded with the most significant group first. The most significant
 * byte will contains the information about how many extra bytes need to be read as well as
 * the most significant bits of the integer.
 * The number of extra bytes to read is encoded as 1 bits on the left side.
 * For example, if we need to read 3 more bytes the first byte will start with 1110.
 * If the encoded integer is 8 bytes long the vint will be encoded on 9 bytes and the first
 * byte will be: 11111111
 * 

 * 
Signed integer are (like protocol buffer varints) encoded using the ZigZag encoding
 * so that numbers with a small absolute value have a small vint encoded value too.
 */
class VIntCoding {

    private static long readUnsignedVInt(DataInput input) throws IOException {
        int firstByte = input.readByte();

        //Bail out early if this is one byte, necessary or it fails later
        if (firstByte >= 0)
            return firstByte;

        int size = numberOfExtraBytesToRead(firstByte);
        long retval = firstByte & firstByteValueMask(size);
        for (int ii = 0; ii < size; ii++) {
            byte b = input.readByte();
            retval <<= 8;
            retval |= b & 0xff;
        }

        return retval;
    }

    static long readVInt(DataInput input) throws IOException {
        return decodeZigZag64(readUnsignedVInt(input));
    }

    // & this with the first byte to give the value part for a given extraBytesToRead encoded in the byte
    private static int firstByteValueMask(int extraBytesToRead) {
        // by including the known 0bit in the mask, we can use this for encodeExtraBytesToRead
        return 0xff >> extraBytesToRead;
    }

    private static int encodeExtraBytesToRead(int extraBytesToRead) {
        // because we have an extra bit in the value mask, we just need to invert it
        return ~firstByteValueMask(extraBytesToRead);
    }

    private static int numberOfExtraBytesToRead(int firstByte) {
        // we count number of set upper bits; so if we simply invert all of the bits, we're golden
        // this is aided by the fact that we only work with negative numbers, so when upcast to an int all
        // of the new upper bits are also set, so by inverting we set all of them to zero
        return Integer.numberOfLeadingZeros(~firstByte) - 24;
    }

    private static final FastThreadLocal encodingBuffer = new FastThreadLocal() {
        @Override
        public byte[] initialValue() {
            return new byte[9];
        }
    };

    private static void writeUnsignedVInt(long value, DataOutput output) throws IOException {
        int size = VIntCoding.computeUnsignedVIntSize(value);
        if (size == 1) {
            output.write((int) value);
            return;
        }

        output.write(VIntCoding.encodeVInt(value, size), 0, size);
    }

    private static byte[] encodeVInt(long value, int size) {
        byte encodingSpace[] = encodingBuffer.get();
        int extraBytes = size - 1;

        for (int i = extraBytes; i >= 0; --i) {
            encodingSpace[i] = (byte) value;
            value >>= 8;
        }
        encodingSpace[0] |= encodeExtraBytesToRead(extraBytes);
        return encodingSpace;
    }

    static void writeVInt(long value, DataOutput output) throws IOException {
        writeUnsignedVInt(encodeZigZag64(value), output);
    }

    /**
     * Decode a ZigZag-encoded 64-bit value.  ZigZag encodes signed integers
     * into values that can be efficiently encoded with varint.  (Otherwise,
     * negative values must be sign-extended to 64 bits to be varint encoded,
     * thus always taking 10 bytes on the wire.)
     *
     * @param n An unsigned 64-bit integer, stored in a signed int because
     *          Java has no explicit unsigned support.
     * @return A signed 64-bit integer.
     */
    private static long decodeZigZag64(final long n) {
        return (n >>> 1) ^ -(n & 1);
    }

    /**
     * Encode a ZigZag-encoded 64-bit value.  ZigZag encodes signed integers
     * into values that can be efficiently encoded with varint.  (Otherwise,
     * negative values must be sign-extended to 64 bits to be varint encoded,
     * thus always taking 10 bytes on the wire.)
     *
     * @param n A signed 64-bit integer.
     * @return An unsigned 64-bit integer, stored in a signed int because
     * Java has no explicit unsigned support.
     */
    private static long encodeZigZag64(final long n) {
        // Note:  the right-shift must be arithmetic
        return (n << 1) ^ (n >> 63);
    }

    /**
     * Compute the number of bytes that would be needed to encode a varint.
     */
    static int computeVIntSize(final long param) {
        return computeUnsignedVIntSize(encodeZigZag64(param));
    }

    /**
     * Compute the number of bytes that would be needed to encode an unsigned varint.
     */
    private static int computeUnsignedVIntSize(final long value) {
        int magnitude = Long.numberOfLeadingZeros(value | 1); // | with 1 to ensure magntiude <= 63, so (63 - 1) / 7 <= 8
        return (639 - magnitude * 9) >> 6;
    }
}