hash.go raw

   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