/*
 * 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 static com.google.common.base.Preconditions.checkPositionIndexes;
import static com.google.common.hash.LittleEndianByteArray.load32;
import static com.google.common.hash.LittleEndianByteArray.load64;
import static java.lang.Long.rotateRight;

import com.google.common.annotations.VisibleForTesting;

Implementation of FarmHash Fingerprint64, an open-source fingerprinting algorithm for strings.

Its speed is comparable to CityHash64, and its quality of hashing is at least as good.

Note to maintainers: This implementation relies on signed arithmetic being bit-wise equivalent to unsigned arithmetic in all cases except:

  • comparisons (signed values can be negative)
  • division (avoided here)
  • shifting (right shift must be unsigned)
Author:Kyle Maddison, Geoff Pike
/** * Implementation of FarmHash Fingerprint64, an open-source fingerprinting algorithm for strings. * * <p>Its speed is comparable to CityHash64, and its quality of hashing is at least as good. * * <p>Note to maintainers: This implementation relies on signed arithmetic being bit-wise equivalent * to unsigned arithmetic in all cases except: * * <ul> * <li>comparisons (signed values can be negative) * <li>division (avoided here) * <li>shifting (right shift must be unsigned) * </ul> * * @author Kyle Maddison * @author Geoff Pike */
final class FarmHashFingerprint64 extends AbstractNonStreamingHashFunction { static final HashFunction FARMHASH_FINGERPRINT_64 = new FarmHashFingerprint64(); // Some primes between 2^63 and 2^64 for various uses. private static final long K0 = 0xc3a5c85c97cb3127L; private static final long K1 = 0xb492b66fbe98f273L; private static final long K2 = 0x9ae16a3b2f90404fL; @Override public HashCode hashBytes(byte[] input, int off, int len) { checkPositionIndexes(off, off + len, input.length); return HashCode.fromLong(fingerprint(input, off, len)); } @Override public int bits() { return 64; } @Override public String toString() { return "Hashing.farmHashFingerprint64()"; } // End of public functions. @VisibleForTesting static long fingerprint(byte[] bytes, int offset, int length) { if (length <= 32) { if (length <= 16) { return hashLength0to16(bytes, offset, length); } else { return hashLength17to32(bytes, offset, length); } } else if (length <= 64) { return hashLength33To64(bytes, offset, length); } else { return hashLength65Plus(bytes, offset, length); } } private static long shiftMix(long val) { return val ^ (val >>> 47); } private static long hashLength16(long u, long v, long mul) { long a = (u ^ v) * mul; a ^= (a >>> 47); long b = (v ^ a) * mul; b ^= (b >>> 47); b *= mul; return b; }
Computes intermediate hash of 32 bytes of byte array from the given offset. Results are returned in the output array because when we last measured, this was 12% faster than allocating new arrays every time.
/** * Computes intermediate hash of 32 bytes of byte array from the given offset. Results are * returned in the output array because when we last measured, this was 12% faster than allocating * new arrays every time. */
private static void weakHashLength32WithSeeds( byte[] bytes, int offset, long seedA, long seedB, long[] output) { long part1 = load64(bytes, offset); long part2 = load64(bytes, offset + 8); long part3 = load64(bytes, offset + 16); long part4 = load64(bytes, offset + 24); seedA += part1; seedB = rotateRight(seedB + seedA + part4, 21); long c = seedA; seedA += part2; seedA += part3; seedB += rotateRight(seedA, 44); output[0] = seedA + part4; output[1] = seedB + c; } private static long hashLength0to16(byte[] bytes, int offset, int length) { if (length >= 8) { long mul = K2 + length * 2; long a = load64(bytes, offset) + K2; long b = load64(bytes, offset + length - 8); long c = rotateRight(b, 37) * mul + a; long d = (rotateRight(a, 25) + b) * mul; return hashLength16(c, d, mul); } if (length >= 4) { long mul = K2 + length * 2; long a = load32(bytes, offset) & 0xFFFFFFFFL; return hashLength16(length + (a << 3), load32(bytes, offset + length - 4) & 0xFFFFFFFFL, mul); } if (length > 0) { byte a = bytes[offset]; byte b = bytes[offset + (length >> 1)]; byte c = bytes[offset + (length - 1)]; int y = (a & 0xFF) + ((b & 0xFF) << 8); int z = length + ((c & 0xFF) << 2); return shiftMix(y * K2 ^ z * K0) * K2; } return K2; } private static long hashLength17to32(byte[] bytes, int offset, int length) { long mul = K2 + length * 2; long a = load64(bytes, offset) * K1; long b = load64(bytes, offset + 8); long c = load64(bytes, offset + length - 8) * mul; long d = load64(bytes, offset + length - 16) * K2; return hashLength16( rotateRight(a + b, 43) + rotateRight(c, 30) + d, a + rotateRight(b + K2, 18) + c, mul); } private static long hashLength33To64(byte[] bytes, int offset, int length) { long mul = K2 + length * 2; long a = load64(bytes, offset) * K2; long b = load64(bytes, offset + 8); long c = load64(bytes, offset + length - 8) * mul; long d = load64(bytes, offset + length - 16) * K2; long y = rotateRight(a + b, 43) + rotateRight(c, 30) + d; long z = hashLength16(y, a + rotateRight(b + K2, 18) + c, mul); long e = load64(bytes, offset + 16) * mul; long f = load64(bytes, offset + 24); long g = (y + load64(bytes, offset + length - 32)) * mul; long h = (z + load64(bytes, offset + length - 24)) * mul; return hashLength16( rotateRight(e + f, 43) + rotateRight(g, 30) + h, e + rotateRight(f + a, 18) + g, mul); } /* * Compute an 8-byte hash of a byte array of length greater than 64 bytes. */ private static long hashLength65Plus(byte[] bytes, int offset, int length) { final int seed = 81; // For strings over 64 bytes we loop. Internal state consists of 56 bytes: v, w, x, y, and z. long x = seed; @SuppressWarnings("ConstantOverflow") long y = seed * K1 + 113; long z = shiftMix(y * K2 + 113) * K2; long[] v = new long[2]; long[] w = new long[2]; x = x * K2 + load64(bytes, offset); // Set end so that after the loop we have 1 to 64 bytes left to process. int end = offset + ((length - 1) / 64) * 64; int last64offset = end + ((length - 1) & 63) - 63; do { x = rotateRight(x + y + v[0] + load64(bytes, offset + 8), 37) * K1; y = rotateRight(y + v[1] + load64(bytes, offset + 48), 42) * K1; x ^= w[1]; y += v[0] + load64(bytes, offset + 40); z = rotateRight(z + w[0], 33) * K1; weakHashLength32WithSeeds(bytes, offset, v[1] * K1, x + w[0], v); weakHashLength32WithSeeds(bytes, offset + 32, z + w[1], y + load64(bytes, offset + 16), w); long tmp = x; x = z; z = tmp; offset += 64; } while (offset != end); long mul = K1 + ((z & 0xFF) << 1); // Operate on the last 64 bytes of input. offset = last64offset; w[0] += ((length - 1) & 63); v[0] += w[0]; w[0] += v[0]; x = rotateRight(x + y + v[0] + load64(bytes, offset + 8), 37) * mul; y = rotateRight(y + v[1] + load64(bytes, offset + 48), 42) * mul; x ^= w[1] * 9; y += v[0] * 9 + load64(bytes, offset + 40); z = rotateRight(z + w[0], 33) * mul; weakHashLength32WithSeeds(bytes, offset, v[1] * mul, x + w[0], v); weakHashLength32WithSeeds(bytes, offset + 32, z + w[1], y + load64(bytes, offset + 16), w); return hashLength16( hashLength16(v[0], w[0], mul) + shiftMix(y) * K0 + x, hashLength16(v[1], w[1], mul) + z, mul); } }