kesavand | c71914f | 2022-03-25 11:19:03 +0530 | [diff] [blame] | 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 | |
| 5 | package zstd |
| 6 | |
| 7 | const ( |
| 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 mls bytes of with length output bits. |
| 17 | // mls must be >=3 and <=8. Any other value will return hash for 4 bytes. |
| 18 | // length should always be < 32. |
| 19 | // Preferably length and mls should be a constant for inlining. |
| 20 | func hashLen(u uint64, length, mls uint8) uint32 { |
| 21 | switch mls { |
| 22 | case 3: |
| 23 | return (uint32(u<<8) * prime3bytes) >> (32 - length) |
| 24 | case 5: |
| 25 | return uint32(((u << (64 - 40)) * prime5bytes) >> (64 - length)) |
| 26 | case 6: |
| 27 | return uint32(((u << (64 - 48)) * prime6bytes) >> (64 - length)) |
| 28 | case 7: |
| 29 | return uint32(((u << (64 - 56)) * prime7bytes) >> (64 - length)) |
| 30 | case 8: |
| 31 | return uint32((u * prime8bytes) >> (64 - length)) |
| 32 | default: |
| 33 | return (uint32(u) * prime4bytes) >> (32 - length) |
| 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. |
| 39 | func hash3(u uint32, h uint8) uint32 { |
| 40 | return ((u << (32 - 24)) * prime3bytes) >> ((32 - h) & 31) |
| 41 | } |