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/*
* Copyright (C) 2015 The Guava Authors
*
* 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.
*/
package com.google.common.hash;
import com.google.common.primitives.Longs;
/**
* Utility functions for loading and storing values from a byte array.
*
* @author Kevin Damm
* @author Kyle Maddison
*/
final class LittleEndianByteArray {
/** The instance that actually does the work; delegates to Unsafe or a pure-Java fallback. */
private static final LittleEndianBytes byteArray;
/**
* Load 8 bytes into long in a little endian manner, from the substring between position and
* position + 8. The array must have at least 8 bytes from offset (inclusive).
*
* @param input the input bytes
* @param offset the offset into the array at which to start
* @return a long of a concatenated 8 bytes
*/
static long load64(byte[] input, int offset) {
// We don't want this in production code as this is the most critical part of the loop.
assert input.length >= offset + 8;
// Delegates to the fast (unsafe) version or the fallback.
return byteArray.getLongLittleEndian(input, offset);
}
/**
* Similar to load64, but allows offset + 8 > input.length, padding the result with zeroes. This
* has to explicitly reverse the order of the bytes as it packs them into the result which makes
* it slower than the native version.
*
* @param input the input bytes
* @param offset the offset into the array at which to start reading
* @param length the number of bytes from the input to read
* @return a long of a concatenated 8 bytes
*/
static long load64Safely(byte[] input, int offset, int length) {
long result = 0;
// Due to the way we shift, we can stop iterating once we've run out of data, the rest
// of the result already being filled with zeros.
// This loop is critical to performance, so please check HashBenchmark if altering it.
int limit = Math.min(length, 8);
for (int i = 0; i < limit; i++) {
// Shift value left while iterating logically through the array.
result |= (input[offset + i] & 0xFFL) << (i * 8);
}
return result;
}
/**
* Store 8 bytes into the provided array at the indicated offset, using the value provided.
*
* @param sink the output byte array
* @param offset the offset into the array at which to start writing
* @param value the value to write
*/
static void store64(byte[] sink, int offset, long value) {
// We don't want to assert in production code.
assert offset >= 0 && offset + 8 <= sink.length;
// Delegates to the fast (unsafe)version or the fallback.
byteArray.putLongLittleEndian(sink, offset, value);
}
/**
* Load 4 bytes from the provided array at the indicated offset.
*
* @param source the input bytes
* @param offset the offset into the array at which to start
* @return the value found in the array in the form of a long
*/
static int load32(byte[] source, int offset) {
// TODO(user): Measure the benefit of delegating this to LittleEndianBytes also.
return (source[offset] & 0xFF)
| ((source[offset + 1] & 0xFF) << 8)
| ((source[offset + 2] & 0xFF) << 16)
| ((source[offset + 3] & 0xFF) << 24);
}
/**
* Common interface for retrieving a 64-bit long from a little-endian byte array.
*
* This abstraction allows us to use single-instruction load and put when available, or fall
* back on the slower approach of using Longs.fromBytes(byte...).
*/
private interface LittleEndianBytes {
long getLongLittleEndian(byte[] array, int offset);
void putLongLittleEndian(byte[] array, int offset, long value);
}
/** Fallback implementation for when Unsafe is not available in our current environment. */
private enum JavaLittleEndianBytes implements LittleEndianBytes {
INSTANCE {
@Override
public long getLongLittleEndian(byte[] source, int offset) {
return Longs.fromBytes(
source[offset + 7],
source[offset + 6],
source[offset + 5],
source[offset + 4],
source[offset + 3],
source[offset + 2],
source[offset + 1],
source[offset]);
}
@Override
public void putLongLittleEndian(byte[] sink, int offset, long value) {
long mask = 0xFFL;
for (int i = 0; i < 8; mask <<= 8, i++) {
sink[offset + i] = (byte) ((value & mask) >> (i * 8));
}
}
};
}
static {
LittleEndianBytes theGetter = JavaLittleEndianBytes.INSTANCE;
byteArray = theGetter;
}
/** Deter instantiation of this class. */
private LittleEndianByteArray() {}
}