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Diffstat (limited to 'vespajlib/src/main/java/com/yahoo/collections/BobHash.java')
| -rw-r--r-- | vespajlib/src/main/java/com/yahoo/collections/BobHash.java | 200 |
1 files changed, 200 insertions, 0 deletions
diff --git a/vespajlib/src/main/java/com/yahoo/collections/BobHash.java b/vespajlib/src/main/java/com/yahoo/collections/BobHash.java new file mode 100644 index 00000000000..b942c4e78f0 --- /dev/null +++ b/vespajlib/src/main/java/com/yahoo/collections/BobHash.java @@ -0,0 +1,200 @@ +// Copyright 2016 Yahoo Inc. Licensed under the terms of the Apache 2.0 license. See LICENSE in the project root. +package com.yahoo.collections; + +import com.yahoo.text.Utf8; + +/** + * <p>A Java port of Michael Susag's BobHash in FastLib. This version is + * specifically done to be bit compatible with the one in FastLib, as it + * is used in decoding packets from FastServer.</p> + * + * <p>Hash function based on + * <a href="http://burtleburtle.net/bob/hash/index.html"> + * http://burtleburtle.net/bob/hash/index.html</a> + * by Bob Jenkins, 1996. bob_jenkins@burtleburtle.net. You may use this + * code any way you wish, private, educational, or commercial. It's free.</p> + * + * @author Michael Susag + * @author <a href="mailto:steinar@yahoo-inc.com">Steinar Knutsen</a> + * + * + */ + +public class BobHash { + + /** + * mix -- mix 3 32-bit values reversibly. + * For every delta with one or two bits set, and the deltas of all three + * high bits or all three low bits, whether the original value of a,b,c + * is almost all zero or is uniformly distributed, + * If mix() is run forward or backward, at least 32 bits in a,b,c + * have at least 1/4 probability of changing. + * If mix() is run forward, every bit of c will change between 1/3 and + * 2/3 of the time. (Well, 22/100 and 78/100 for some 2-bit deltas.) + * mix() was built out of 36 single-cycle latency instructions in a + * structure that could supported 2x parallelism, like so: + * + * <pre> + * a -= b; + * a -= c; x = (c>>13); + * b -= c; a ^= x; + * b -= a; x = (a<<8); + * c -= a; b ^= x; + * c -= b; x = (b>>13); + * ... + * </pre> + * + * <p> + * Unfortunately, superscalar Pentiums and Sparcs can't take advantage + * of that parallelism. They've also turned some of those single-cycle + * latency instructions into multi-cycle latency instructions. Still, + * this is the fastest good hash I could find. There were about 2^^68 + * to choose from. I only looked at a billion or so. + */ + private static int[] mix(int a, int b, int c) { + a -= b; a -= c; a ^= (c >>> 13); + b -= c; b -= a; b ^= (a << 8); + c -= a; c -= b; c ^= (b >>> 13); + a -= b; a -= c; a ^= (c >>> 12); + b -= c; b -= a; b ^= (a << 16); + c -= a; c -= b; c ^= (b >>> 5); + a -= b; a -= c; a ^= (c >>> 3); + b -= c; b -= a; b ^= (a << 10); + c -= a; c -= b; c ^= (b >>> 15); + + return new int[]{ a, b, c }; + } + + /** + * Transform a byte to an int viewed as an unsigned byte. + */ + private static int unsign(byte x) { + int y; + + y = 0xFF & x; + return y; + } + + /** + * Hashes a string, by calling hash(byte[] key,int initval) with + * the utf-8 bytes of the string as key and 0 as initval. + * Note: This is copying the string content, change implementation to + * use efficiently on large strings. + * + * <a href="mailto:bratseth@yahoo-inc.com">Jon S Bratseth</a> + */ + public static int hash(String key) { + return hash(Utf8.toBytes(key), 0); + } + + /** + * The hash function + * + * <p> + * hash() -- hash a variable-length key into a 32-bit value<br> + * k : the key (the unaligned variable-length array of bytes)<br> + * len : the length of the key, counting by bytes<br> + * initval : can be any 4-byte value + * + * <p> + * Returns a 32-bit value. Every bit of the key affects every bit of + * the return value. Every 1-bit and 2-bit delta achieves avalanche. + * About 6*len+35 instructions. + * + * <p> + * The best hash table sizes are powers of 2. There is no need to do + * mod a prime (mod is sooo slow!). If you need less than 32 bits, + * use a bitmask. For example, if you need only 10 bits, do + * h = (h & hashmask(10)); + * In which case, the hash table should have hashsize(10) elements. + * + * If you are hashing n strings (ub1 **)k, do it like this: + * for (i=0, h=0; i<n; ++i) h = hash( k[i], len[i], h); + * + * <p> + * By Bob Jenkins, 1996. bob_jenkins@burtleburtle.net. You may use this + * code any way you wish, private, educational, or commercial. It's free. + * + * <p> + * See http://burtleburtle.net/bob/hash/evahash.html + * Use for hash table lookup, or anything where one collision in 2^^32 is + * acceptable. Do NOT use for cryptographic purposes. + * + * @param k the key + * @param initval the previous hash, or an arbitrary value + * @return A 32 bit hash value + */ + @SuppressWarnings("fallthrough") + public static int hash(byte[] k, int initval) { + int a, b, c, len; + int offset = 0; + int[] abcBuffer; + + /* Set up the internal state */ + len = k.length; + a = b = 0x9e3779b9; /* the golden ratio; an arbitrary value */ + c = initval; /* the previous hash value */ + + // handle most of the key + while (len >= 12) { + a += (unsign(k[offset + 0]) + (unsign(k[offset + 1]) << 8) + + (unsign(k[offset + 2]) << 16) + + (unsign(k[offset + 3]) << 24)); + b += (unsign(k[offset + 4]) + (unsign(k[offset + 5]) << 8) + + (unsign(k[offset + 6]) << 16) + + (unsign(k[offset + 7]) << 24)); + c += (unsign(k[offset + 8]) + (unsign(k[offset + 9]) << 8) + + (unsign(k[offset + 10]) << 16) + + (unsign(k[offset + 11]) << 24)); + abcBuffer = mix(a, b, c); + a = abcBuffer[0]; + b = abcBuffer[1]; + c = abcBuffer[2]; + offset += 12; + len -= 12; + } + + // handle the last 11 bytes + c += k.length; + switch (len) { + // all the case statements fall through + case 11: + c += (unsign(k[offset + 10]) << 24); + + case 10: + c += (unsign(k[offset + 9]) << 16); + + case 9: + c += (unsign(k[offset + 8]) << 8); + + /* the first byte of c is reserved for the length */ + case 8: + b += (unsign(k[offset + 7]) << 24); + + case 7: + b += (unsign(k[offset + 6]) << 16); + + case 6: + b += (unsign(k[offset + 5]) << 8); + + case 5: + b += unsign(k[offset + 4]); + + case 4: + a += (unsign(k[offset + 3]) << 24); + + case 3: + a += (unsign(k[offset + 2]) << 16); + + case 2: + a += (unsign(k[offset + 1]) << 8); + + case 1: + a += unsign(k[offset + 0]); + + /* case 0: nothing left to add */ + } + abcBuffer = mix(a, b, c); + return abcBuffer[2]; + } +} |