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42 lines
1.3 KiB
42 lines
1.3 KiB
3 years ago
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// 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 mls bytes of with length output bits.
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// mls must be >=3 and <=8. Any other value will return hash for 4 bytes.
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// length should always be < 32.
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// Preferably length and mls should be a constant for inlining.
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func hashLen(u uint64, length, mls uint8) uint32 {
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switch mls {
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case 3:
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return (uint32(u<<8) * prime3bytes) >> (32 - length)
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case 5:
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return uint32(((u << (64 - 40)) * prime5bytes) >> (64 - length))
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case 6:
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return uint32(((u << (64 - 48)) * prime6bytes) >> (64 - length))
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case 7:
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return uint32(((u << (64 - 56)) * prime7bytes) >> (64 - length))
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case 8:
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return uint32((u * prime8bytes) >> (64 - length))
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default:
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return (uint32(u) * prime4bytes) >> (32 - length)
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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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