30802fac73
zstd is a compression algorithm that has a very fast decoder, while providing also good compression ratios. The fast decoder makes it suitable for container images, as decompressing the tarballs is a very expensive operation. https://github.com/opencontainers/image-spec/pull/788 added support for zstd to the OCI image specs. Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>
78 lines
2.6 KiB
Go
78 lines
2.6 KiB
Go
// Copyright 2019+ Klaus Post. All rights reserved.
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// License information can be found in the LICENSE file.
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// Based on work by Yann Collet, released under BSD License.
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package zstd
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const (
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prime3bytes = 506832829
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prime4bytes = 2654435761
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prime5bytes = 889523592379
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prime6bytes = 227718039650203
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prime7bytes = 58295818150454627
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prime8bytes = 0xcf1bbcdcb7a56463
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)
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// hashLen returns a hash of the lowest l bytes of u for a size size of h bytes.
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// l must be >=4 and <=8. Any other value will return hash for 4 bytes.
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// h should always be <32.
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// Preferably h and l should be a constant.
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// FIXME: This does NOT get resolved, if 'mls' is constant,
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// so this cannot be used.
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func hashLen(u uint64, hashLog, mls uint8) uint32 {
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switch mls {
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case 5:
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return hash5(u, hashLog)
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case 6:
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return hash6(u, hashLog)
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case 7:
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return hash7(u, hashLog)
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case 8:
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return hash8(u, hashLog)
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default:
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return hash4x64(u, hashLog)
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}
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}
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// hash3 returns the hash of the lower 3 bytes of u to fit in a hash table with h bits.
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// Preferably h should be a constant and should always be <32.
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func hash3(u uint32, h uint8) uint32 {
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return ((u << (32 - 24)) * prime3bytes) >> ((32 - h) & 31)
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}
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// hash4 returns the hash of u to fit in a hash table with h bits.
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// Preferably h should be a constant and should always be <32.
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func hash4(u uint32, h uint8) uint32 {
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return (u * prime4bytes) >> ((32 - h) & 31)
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}
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// hash4x64 returns the hash of the lowest 4 bytes of u to fit in a hash table with h bits.
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// Preferably h should be a constant and should always be <32.
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func hash4x64(u uint64, h uint8) uint32 {
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return (uint32(u) * prime4bytes) >> ((32 - h) & 31)
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}
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// hash5 returns the hash of the lowest 5 bytes of u to fit in a hash table with h bits.
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// Preferably h should be a constant and should always be <64.
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func hash5(u uint64, h uint8) uint32 {
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return uint32(((u << (64 - 40)) * prime5bytes) >> ((64 - h) & 63))
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}
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// hash6 returns the hash of the lowest 6 bytes of u to fit in a hash table with h bits.
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// Preferably h should be a constant and should always be <64.
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func hash6(u uint64, h uint8) uint32 {
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return uint32(((u << (64 - 48)) * prime6bytes) >> ((64 - h) & 63))
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}
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// hash7 returns the hash of the lowest 7 bytes of u to fit in a hash table with h bits.
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// Preferably h should be a constant and should always be <64.
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func hash7(u uint64, h uint8) uint32 {
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return uint32(((u << (64 - 56)) * prime7bytes) >> ((64 - h) & 63))
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}
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// hash8 returns the hash of u to fit in a hash table with h bits.
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// Preferably h should be a constant and should always be <64.
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func hash8(u uint64, h uint8) uint32 {
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return uint32((u * prime8bytes) >> ((64 - h) & 63))
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}
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