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/*
* Copyright (c) 2010-2017 Richard Braun.
*
* This program is free software: you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation, either version 3 of the License, or
* (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program. If not, see <http://www.gnu.org/licenses/>.
*
* Upstream site with license notes :
* http://git.sceen.net/rbraun/librbraun.git/
*
*
* Hash functions for integers and strings.
*
* Integer hashing follows Thomas Wang's paper about his 32/64-bits mix
* functions :
* - https://gist.github.com/badboy/6267743
*
* String hashing uses a variant of the djb2 algorithm with k=31, as in
* the implementation of the hashCode() method of the Java String class :
* - http://www.javamex.com/tutorials/collections/hash_function_technical.shtml
*
* Note that this algorithm isn't suitable to obtain usable 64-bits hashes
* and is expected to only serve as an array index producer.
*
* These functions all have a bits parameter that indicates the number of
* relevant bits the caller is interested in. When returning a hash, its
* value must be truncated so that it can fit in the requested bit size.
* It can be used by the implementation to select high or low bits, depending
* on their relative randomness. To get complete, unmasked hashes, use the
* HASH_ALLBITS macro.
*/
#ifndef KERN_HASH_H
#define KERN_HASH_H
#include <assert.h>
#include <stdbool.h>
#include <stdint.h>
#ifdef __LP64__
#define HASH_ALLBITS 64
#define hash_long(n, bits) hash_int64(n, bits)
#else /* __LP64__ */
static_assert(sizeof(long) == 4, "unsupported data model");
#define HASH_ALLBITS 32
#define hash_long(n, bits) hash_int32(n, bits)
#endif
static inline bool
hash_bits_valid(unsigned int bits)
{
return (bits != 0) && (bits <= HASH_ALLBITS);
}
static inline uint32_t
hash_int32(uint32_t n, unsigned int bits)
{
uint32_t hash;
assert(hash_bits_valid(bits));
hash = n;
hash = ~hash + (hash << 15);
hash ^= (hash >> 12);
hash += (hash << 2);
hash ^= (hash >> 4);
hash += (hash << 3) + (hash << 11);
hash ^= (hash >> 16);
return hash >> (32 - bits);
}
static inline uint64_t
hash_int64(uint64_t n, unsigned int bits)
{
uint64_t hash;
assert(hash_bits_valid(bits));
hash = n;
hash = ~hash + (hash << 21);
hash ^= (hash >> 24);
hash += (hash << 3) + (hash << 8);
hash ^= (hash >> 14);
hash += (hash << 2) + (hash << 4);
hash ^= (hash >> 28);
hash += (hash << 31);
return hash >> (64 - bits);
}
static inline uintptr_t
hash_ptr(const void *ptr, unsigned int bits)
{
if (sizeof(uintptr_t) == 8) {
return hash_int64((uintptr_t)ptr, bits);
} else {
return hash_int32((uintptr_t)ptr, bits);
}
}
static inline unsigned long
hash_str(const char *str, unsigned int bits)
{
unsigned long hash, mask;
char c;
assert(hash_bits_valid(bits));
for (hash = 0; /* no condition */; str++) {
c = *str;
if (c == '\0') {
break;
}
hash = ((hash << 5) - hash) + c;
}
/*
* This mask construction avoids the undefined behavior that would
* result from directly shifting by the number of bits, if that number
* is equal to the width of the hash.
*/
mask = (~0UL >> (HASH_ALLBITS - bits));
return hash & mask;
}
#endif /* KERN_HASH_H */
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