1// Copyright 2019+ Klaus Post. All rights reserved.
2// License information can be found in the LICENSE file.
3// Based on work by Yann Collet, released under BSD License.
4
5package zstd
6
7const (
8	prime3bytes = 506832829
9	prime4bytes = 2654435761
10	prime5bytes = 889523592379
11	prime6bytes = 227718039650203
12	prime7bytes = 58295818150454627
13	prime8bytes = 0xcf1bbcdcb7a56463
14)
15
16// hashLen returns a hash of the lowest l bytes of u for a size size of h bytes.
17// l must be >=4 and <=8. Any other value will return hash for 4 bytes.
18// h should always be <32.
19// Preferably h and l should be a constant.
20// FIXME: This does NOT get resolved, if 'mls' is constant,
21//  so this cannot be used.
22func hashLen(u uint64, hashLog, mls uint8) uint32 {
23	switch mls {
24	case 5:
25		return hash5(u, hashLog)
26	case 6:
27		return hash6(u, hashLog)
28	case 7:
29		return hash7(u, hashLog)
30	case 8:
31		return hash8(u, hashLog)
32	default:
33		return hash4x64(u, hashLog)
34	}
35}
36
37// hash3 returns the hash of the lower 3 bytes of u to fit in a hash table with h bits.
38// Preferably h should be a constant and should always be <32.
39func hash3(u uint32, h uint8) uint32 {
40	return ((u << (32 - 24)) * prime3bytes) >> ((32 - h) & 31)
41}
42
43// hash4 returns the hash of u to fit in a hash table with h bits.
44// Preferably h should be a constant and should always be <32.
45func hash4(u uint32, h uint8) uint32 {
46	return (u * prime4bytes) >> ((32 - h) & 31)
47}
48
49// hash4x64 returns the hash of the lowest 4 bytes of u to fit in a hash table with h bits.
50// Preferably h should be a constant and should always be <32.
51func hash4x64(u uint64, h uint8) uint32 {
52	return (uint32(u) * prime4bytes) >> ((32 - h) & 31)
53}
54
55// hash5 returns the hash of the lowest 5 bytes of u to fit in a hash table with h bits.
56// Preferably h should be a constant and should always be <64.
57func hash5(u uint64, h uint8) uint32 {
58	return uint32(((u << (64 - 40)) * prime5bytes) >> ((64 - h) & 63))
59}
60
61// hash6 returns the hash of the lowest 6 bytes of u to fit in a hash table with h bits.
62// Preferably h should be a constant and should always be <64.
63func hash6(u uint64, h uint8) uint32 {
64	return uint32(((u << (64 - 48)) * prime6bytes) >> ((64 - h) & 63))
65}
66
67// hash7 returns the hash of the lowest 7 bytes of u to fit in a hash table with h bits.
68// Preferably h should be a constant and should always be <64.
69func hash7(u uint64, h uint8) uint32 {
70	return uint32(((u << (64 - 56)) * prime7bytes) >> ((64 - h) & 63))
71}
72
73// hash8 returns the hash of u to fit in a hash table with h bits.
74// Preferably h should be a constant and should always be <64.
75func hash8(u uint64, h uint8) uint32 {
76	return uint32((u * prime8bytes) >> ((64 - h) & 63))
77}
78