zstd/hash.go

// Copyright 2019+ Klaus Post. All rights reserved.
// License information can be found in the LICENSE file.
// Based on work by Yann Collet, released under BSD License.

package zstd

const (
	prime3bytes = 506832829
	prime4bytes = 2654435761
	prime5bytes = 889523592379
	prime6bytes = 227718039650203
	prime7bytes = 58295818150454627
	prime8bytes = 0xcf1bbcdcb7a56463
)

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