diff options
Diffstat (limited to 'cpukit/libfs/src/rfs/rtems-rfs-dir-hash.c')
| -rw-r--r-- | cpukit/libfs/src/rfs/rtems-rfs-dir-hash.c | 341 |
1 files changed, 341 insertions, 0 deletions
diff --git a/cpukit/libfs/src/rfs/rtems-rfs-dir-hash.c b/cpukit/libfs/src/rfs/rtems-rfs-dir-hash.c new file mode 100644 index 0000000000..0aaf405213 --- /dev/null +++ b/cpukit/libfs/src/rfs/rtems-rfs-dir-hash.c @@ -0,0 +1,341 @@ +/* + * $Id$ + */ +/** + * @file + * + * @ingroup rtems-rfs + * + * RTEMS File Systems Directory Hash function. + */ + +#if HAVE_CONFIG_H +#include "config.h" +#endif + +#include <rtems/rfs/rtems-rfs-dir-hash.h> + +#ifdef __rtems__ +# include <machine/endian.h> /* attempt to define endianness */ +#endif +#ifdef linux +# include <endian.h> /* attempt to define endianness */ +#endif + +/* + * My best guess at if you are big-endian or little-endian. This may + * need adjustment. + */ +#if (defined(__BYTE_ORDER) && defined(__LITTLE_ENDIAN) && \ + __BYTE_ORDER == __LITTLE_ENDIAN) || \ + (defined(i386) || defined(__i386__) || defined(__i486__) || \ + defined(__i586__) || defined(__i686__) || defined(vax) || defined(MIPSEL)) +# define HASH_LITTLE_ENDIAN 1 +# define HASH_BIG_ENDIAN 0 +#elif (defined(__BYTE_ORDER) && defined(__BIG_ENDIAN) && \ + __BYTE_ORDER == __BIG_ENDIAN) || \ + (defined(sparc) || defined(POWERPC) || defined(mc68000) || defined(sel)) +# define HASH_LITTLE_ENDIAN 0 +# define HASH_BIG_ENDIAN 1 +#else +# define HASH_LITTLE_ENDIAN 0 +# define HASH_BIG_ENDIAN 0 +#endif + +#define hashsize(n) ((uint32_t)1<<(n)) +#define hashmask(n) (hashsize(n)-1) +#define rot(x,k) (((x)<<(k)) | ((x)>>(32-(k)))) + +/* + ------------------------------------------------------------------------------- + mix -- mix 3 32-bit values reversibly. + + This is reversible, so any information in (a,b,c) before mix() is still in + (a,b,c) after mix(). + + If four pairs of (a,b,c) inputs are run through mix(), or through mix() in + reverse, there are at least 32 bits of the output that are sometimes the same + for one pair and different for another pair. This was tested for: + + * pairs that differed by one bit, by two bits, in any combination of top bits + of (a,b,c), or in any combination of bottom bits of (a,b,c). + + * "differ" is defined as +, -, ^, or ~^. For + and -, I transformed the + output delta to a Gray code (a^(a>>1)) so a string of 1's (as is commonly + produced by subtraction) look like a single 1-bit difference. + + * the base values were pseudorandom, all zero but one bit set, or all zero + plus a counter that starts at zero. + + Some k values for my "a-=c; a^=rot(c,k); c+=b;" arrangement that satisfy this + are: + + 4 6 8 16 19 4 + 9 15 3 18 27 15 + 14 9 3 7 17 3 + + Well, "9 15 3 18 27 15" didn't quite get 32 bits diffing for "differ" defined + as + with a one-bit base and a two-bit delta. I used + http://burtleburtle.net/bob/hash/avalanche.html to choose the operations, + constants, and arrangements of the variables. + + This does not achieve avalanche. There are input bits of (a,b,c) that fail + to affect some output bits of (a,b,c), especially of a. The most thoroughly + mixed value is c, but it doesn't really even achieve avalanche in c. + + This allows some parallelism. Read-after-writes are good at doubling the + number of bits affected, so the goal of mixing pulls in the opposite + direction as the goal of parallelism. I did what I could. Rotates seem to + cost as much as shifts on every machine I could lay my hands on, and rotates + are much kinder to the top and bottom bits, so I used rotates. + ------------------------------------------------------------------------------- +*/ +#define mix(a,b,c) \ + { \ + a -= c; a ^= rot(c, 4); c += b; \ + b -= a; b ^= rot(a, 6); a += c; \ + c -= b; c ^= rot(b, 8); b += a; \ + a -= c; a ^= rot(c,16); c += b; \ + b -= a; b ^= rot(a,19); a += c; \ + c -= b; c ^= rot(b, 4); b += a; \ + } + +/* + ------------------------------------------------------------------------------- + final -- final mixing of 3 32-bit values (a,b,c) into c + + Pairs of (a,b,c) values differing in only a few bits will usually produce + values of c that look totally different. This was tested for + + * pairs that differed by one bit, by two bits, in any combination of top bits + of (a,b,c), or in any combination of bottom bits of (a,b,c). + + * "differ" is defined as +, -, ^, or ~^. For + and -, I transformed the + output delta to a Gray code (a^(a>>1)) so a string of 1's (as is commonly + produced by subtraction) look like a single 1-bit difference. * the base + values were pseudorandom, all zero but one bit set, or all zero plus a + counter that starts at zero. + + These constants passed: + 14 11 25 16 4 14 24 + 12 14 25 16 4 14 24 + and these came close: + 4 8 15 26 3 22 24 + 10 8 15 26 3 22 24 + 11 8 15 26 3 22 24 + ------------------------------------------------------------------------------- +*/ +#define final(a,b,c) \ + { \ + c ^= b; c -= rot(b,14); \ + a ^= c; a -= rot(c,11); \ + b ^= a; b -= rot(a,25); \ + c ^= b; c -= rot(b,16); \ + a ^= c; a -= rot(c,4); \ + b ^= a; b -= rot(a,14); \ + c ^= b; c -= rot(b,24); \ + } + +/** + * The follow is the documentation from Bob Jenkin's hash function: + * + * http://burtleburtle.net/bob/hash/doobs.html + * + * The function hashlittle() has been renamed. + * + * hashlittle() -- hash a variable-length key into a 32-bit value + * + * k : the key (the unaligned variable-length array of bytes) + * length : the length of the key, counting by bytes + * initval : can be any 4-byte value + * + * Returns a 32-bit value. Every bit of the key affects every bit of the + * return value. Two keys differing by one or two bits will have totally + * different hash values. + * + * 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 (uint8_t **)k, do it like this: for (i=0, h=0; + * i<n; ++i) h = hashlittle( k[i], len[i], h); + * + * By Bob Jenkins, 2006. bob_jenkins@burtleburtle.net. You may use this + * code any way you wish, private, educational, or commercial. It's free. + * + * Use for hash table lookup, or anything where one collision in 2^^32 is + * acceptable. Do NOT use for cryptographic purposes. +*/ + +#define initval (20010928) +uint32_t +rtems_rfs_dir_hash (const void *key, size_t length) +{ + uint32_t a,b,c; /* internal state */ + union { const void *ptr; size_t i; } u; /* needed for Mac Powerbook G4 */ + + /* Set up the internal state */ + a = b = c = 0xdeadbeef + ((uint32_t)length) + initval; + + u.ptr = key; + if (HASH_LITTLE_ENDIAN && ((u.i & 0x3) == 0)) { + const uint32_t *k = (const uint32_t *)key; /* read 32-bit chunks */ + /*const uint8_t *k8;*/ + + /*------ all but last block: aligned reads and affect 32 bits of (a,b,c) */ + while (length > 12) + { + a += k[0]; + b += k[1]; + c += k[2]; + mix(a,b,c); + length -= 12; + k += 3; + } + + /*----------------------------- handle the last (probably partial) block */ + /* + * "k[2]&0xffffff" actually reads beyond the end of the string, but + * then masks off the part it's not allowed to read. Because the + * string is aligned, the masked-off tail is in the same word as the + * rest of the string. Every machine with memory protection I've seen + * does it on word boundaries, so is OK with this. But VALGRIND will + * still catch it and complain. The masking trick does make the hash + * noticably faster for short strings (like English words). + */ +#ifndef VALGRIND + + switch(length) + { + case 12: c+=k[2]; b+=k[1]; a+=k[0]; break; + case 11: c+=k[2]&0xffffff; b+=k[1]; a+=k[0]; break; + case 10: c+=k[2]&0xffff; b+=k[1]; a+=k[0]; break; + case 9 : c+=k[2]&0xff; b+=k[1]; a+=k[0]; break; + case 8 : b+=k[1]; a+=k[0]; break; + case 7 : b+=k[1]&0xffffff; a+=k[0]; break; + case 6 : b+=k[1]&0xffff; a+=k[0]; break; + case 5 : b+=k[1]&0xff; a+=k[0]; break; + case 4 : a+=k[0]; break; + case 3 : a+=k[0]&0xffffff; break; + case 2 : a+=k[0]&0xffff; break; + case 1 : a+=k[0]&0xff; break; + case 0 : return c; /* zero length strings require no mixing */ + } + +#else /* make valgrind happy */ + + k8 = (const uint8_t *)k; + switch(length) + { + case 12: c+=k[2]; b+=k[1]; a+=k[0]; break; + case 11: c+=((uint32_t)k8[10])<<16; /* fall through */ + case 10: c+=((uint32_t)k8[9])<<8; /* fall through */ + case 9 : c+=k8[8]; /* fall through */ + case 8 : b+=k[1]; a+=k[0]; break; + case 7 : b+=((uint32_t)k8[6])<<16; /* fall through */ + case 6 : b+=((uint32_t)k8[5])<<8; /* fall through */ + case 5 : b+=k8[4]; /* fall through */ + case 4 : a+=k[0]; break; + case 3 : a+=((uint32_t)k8[2])<<16; /* fall through */ + case 2 : a+=((uint32_t)k8[1])<<8; /* fall through */ + case 1 : a+=k8[0]; break; + case 0 : return c; + } + +#endif /* !valgrind */ + + } else if (HASH_LITTLE_ENDIAN && ((u.i & 0x1) == 0)) { + const uint16_t *k = (const uint16_t *)key; /* read 16-bit chunks */ + const uint8_t *k8; + + /*--------------- all but last block: aligned reads and different mixing */ + while (length > 12) + { + a += k[0] + (((uint32_t)k[1])<<16); + b += k[2] + (((uint32_t)k[3])<<16); + c += k[4] + (((uint32_t)k[5])<<16); + mix(a,b,c); + length -= 12; + k += 6; + } + + /*----------------------------- handle the last (probably partial) block */ + k8 = (const uint8_t *)k; + switch(length) + { + case 12: c+=k[4]+(((uint32_t)k[5])<<16); + b+=k[2]+(((uint32_t)k[3])<<16); + a+=k[0]+(((uint32_t)k[1])<<16); + break; + case 11: c+=((uint32_t)k8[10])<<16; /* fall through */ + case 10: c+=k[4]; + b+=k[2]+(((uint32_t)k[3])<<16); + a+=k[0]+(((uint32_t)k[1])<<16); + break; + case 9 : c+=k8[8]; /* fall through */ + case 8 : b+=k[2]+(((uint32_t)k[3])<<16); + a+=k[0]+(((uint32_t)k[1])<<16); + break; + case 7 : b+=((uint32_t)k8[6])<<16; /* fall through */ + case 6 : b+=k[2]; + a+=k[0]+(((uint32_t)k[1])<<16); + break; + case 5 : b+=k8[4]; /* fall through */ + case 4 : a+=k[0]+(((uint32_t)k[1])<<16); + break; + case 3 : a+=((uint32_t)k8[2])<<16; /* fall through */ + case 2 : a+=k[0]; + break; + case 1 : a+=k8[0]; + break; + case 0 : return c; /* zero length requires no mixing */ + } + + } else { /* need to read the key one byte at a time */ + const uint8_t *k = (const uint8_t *)key; + + /*--------------- all but the last block: affect some 32 bits of (a,b,c) */ + while (length > 12) + { + a += k[0]; + a += ((uint32_t)k[1])<<8; + a += ((uint32_t)k[2])<<16; + a += ((uint32_t)k[3])<<24; + b += k[4]; + b += ((uint32_t)k[5])<<8; + b += ((uint32_t)k[6])<<16; + b += ((uint32_t)k[7])<<24; + c += k[8]; + c += ((uint32_t)k[9])<<8; + c += ((uint32_t)k[10])<<16; + c += ((uint32_t)k[11])<<24; + mix(a,b,c); + length -= 12; + k += 12; + } + + /*-------------------------------- last block: affect all 32 bits of (c) */ + switch(length) /* all the case statements fall through */ + { + case 12: c+=((uint32_t)k[11])<<24; + case 11: c+=((uint32_t)k[10])<<16; + case 10: c+=((uint32_t)k[9])<<8; + case 9 : c+=k[8]; + case 8 : b+=((uint32_t)k[7])<<24; + case 7 : b+=((uint32_t)k[6])<<16; + case 6 : b+=((uint32_t)k[5])<<8; + case 5 : b+=k[4]; + case 4 : a+=((uint32_t)k[3])<<24; + case 3 : a+=((uint32_t)k[2])<<16; + case 2 : a+=((uint32_t)k[1])<<8; + case 1 : a+=k[0]; + break; + case 0 : return c; + } + } + + final(a,b,c); + return c; +} + |