/*
* Copyright (C) 2012-2015 DataStax Inc.
*
* Licensed 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.
*/
// Protocol Buffers - Google's data interchange format
// Copyright 2008 Google Inc. All rights reserved.
// https://developers.google.com/protocol-buffers/
//
// Redistribution and use in source and binary forms, with or without
// modification, are permitted provided that the following conditions are
// met:
//
// * Redistributions of source code must retain the above copyright
// notice, this list of conditions and the following disclaimer.
// * Redistributions in binary form must reproduce the above
// copyright notice, this list of conditions and the following disclaimer
// in the documentation and/or other materials provided with the
// distribution.
// * Neither the name of Google Inc. nor the names of its
// contributors may be used to endorse or promote products derived from
// this software without specific prior written permission.
//
// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
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
* <a href='https://developers.google.com/protocol-buffers/docs/encoding#varints'>varints</a>.
* <p>
* <p>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</p>
* <p>
* <p>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.</p>
*/
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<byte[]> encodingBuffer = new FastThreadLocal<byte[]>() {
@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;
}
}