From b012cde1ed34805ba6996933bee437a00d253c0c Mon Sep 17 00:00:00 2001 From: Wayne Davison Date: Tue, 13 Sep 2022 22:37:39 -0700 Subject: [PATCH] Add hashlittle2() and ensure the hash is never 0 It's probably time for a faster hash algorithm, but this gives us the free 64-bit hashing that things like the xattr code can use. --- hashtable.c | 172 ++++++++++++++++++++++++++++++++++++++++++++++++++-- 1 file changed, 168 insertions(+), 4 deletions(-) diff --git a/hashtable.c b/hashtable.c index e272f439..04543f60 100644 --- a/hashtable.c +++ b/hashtable.c @@ -350,6 +350,9 @@ void *hashtable_find(struct hashtable *tbl, int64 key, void *data_when_new) ------------------------------------------------------------------------------- */ +#define NON_ZERO_32(x) ((x) ? (x) : (uint32_t)1) +#define NON_ZERO_64(x, y) ((x) || (y) ? ((int64)(x) << 32) | (y) : (int64)1) + uint32_t hashlittle(const void *key, size_t length) { uint32_t a,b,c; /* internal state */ @@ -390,7 +393,7 @@ uint32_t hashlittle(const void *key, size_t length) 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; + case 0 : return NON_ZERO_32(c); } } else if (HASH_LITTLE_ENDIAN && ((u.i & 0x1) == 0)) { const uint16_t *k = (const uint16_t *)key; /* read 16-bit chunks */ @@ -436,7 +439,7 @@ uint32_t hashlittle(const void *key, size_t length) break; case 1 : a+=k8[0]; break; - case 0 : return c; /* zero length requires no mixing */ + case 0 : return NON_ZERO_32(c); /* zero length requires no mixing */ } } else { /* need to read the key one byte at a time */ @@ -489,10 +492,171 @@ uint32_t hashlittle(const void *key, size_t length) /* FALLTHROUGH */ case 1 : a+=k[0]; break; - case 0 : return c; + case 0 : return NON_ZERO_32(c); } } final(a,b,c); - return c; + return NON_ZERO_32(c); } + +#if SIZEOF_INT64 >= 8 +/* + * hashlittle2: return 2 32-bit hash values joined into an int64. + * + * This is identical to hashlittle(), except it returns two 32-bit hash + * values instead of just one. This is good enough for hash table + * lookup with 2^^64 buckets, or if you want a second hash if you're not + * happy with the first, or if you want a probably-unique 64-bit ID for + * the key. *pc is better mixed than *pb, so use *pc first. If you want + * a 64-bit value do something like "*pc + (((uint64_t)*pb)<<32)". + */ +int64 hashlittle2(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); + + 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 */ + 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 NON_ZERO_64(b, c); + } + } 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 NON_ZERO_64(b, c); /* zero length strings require 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; + /* FALLTHROUGH */ + case 11: c+=((uint32_t)k[10])<<16; + /* FALLTHROUGH */ + case 10: c+=((uint32_t)k[9])<<8; + /* FALLTHROUGH */ + case 9 : c+=k[8]; + /* FALLTHROUGH */ + case 8 : b+=((uint32_t)k[7])<<24; + /* FALLTHROUGH */ + case 7 : b+=((uint32_t)k[6])<<16; + /* FALLTHROUGH */ + case 6 : b+=((uint32_t)k[5])<<8; + /* FALLTHROUGH */ + case 5 : b+=k[4]; + /* FALLTHROUGH */ + case 4 : a+=((uint32_t)k[3])<<24; + /* FALLTHROUGH */ + case 3 : a+=((uint32_t)k[2])<<16; + /* FALLTHROUGH */ + case 2 : a+=((uint32_t)k[1])<<8; + /* FALLTHROUGH */ + case 1 : a+=k[0]; + break; + case 0 : return NON_ZERO_64(b, c); + } + } + + final(a,b,c); + return NON_ZERO_64(b, c); +} +#else +#define hashlittle2(key, len) hashlittle(key, len) +#endif -- 2.47.2