github.com/dolthub/swiss@v0.2.2-0.20240312182618-f4b2babd2bc1/bits.go (about)

     1  // Copyright 2023 Dolthub, Inc.
     2  //
     3  // Licensed under the Apache License, Version 2.0 (the "License");
     4  // you may not use this file except in compliance with the License.
     5  // You may obtain a copy of the License at
     6  //
     7  //     http://www.apache.org/licenses/LICENSE-2.0
     8  //
     9  // Unless required by applicable law or agreed to in writing, software
    10  // distributed under the License is distributed on an "AS IS" BASIS,
    11  // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    12  // See the License for the specific language governing permissions and
    13  // limitations under the License.
    14  
    15  //go:build !amd64 || nosimd
    16  
    17  package swiss
    18  
    19  import (
    20  	"math/bits"
    21  	"unsafe"
    22  )
    23  
    24  const (
    25  	groupSize       = 8
    26  	maxAvgGroupLoad = 7
    27  
    28  	loBits uint64 = 0x0101010101010101
    29  	hiBits uint64 = 0x8080808080808080
    30  )
    31  
    32  type bitset uint64
    33  
    34  func metaMatchH2(m *metadata, h h2) bitset {
    35  	// https://graphics.stanford.edu/~seander/bithacks.html##ValueInWord
    36  	return hasZeroByte(castUint64(m) ^ (loBits * uint64(h)))
    37  }
    38  
    39  func metaMatchEmpty(m *metadata) bitset {
    40  	return hasZeroByte(castUint64(m) ^ hiBits)
    41  }
    42  
    43  func nextMatch(b *bitset) uint32 {
    44  	s := uint32(bits.TrailingZeros64(uint64(*b)))
    45  	*b &= ^(1 << s) // clear bit |s|
    46  	return s >> 3   // div by 8
    47  }
    48  
    49  func hasZeroByte(x uint64) bitset {
    50  	return bitset(((x - loBits) & ^(x)) & hiBits)
    51  }
    52  
    53  func castUint64(m *metadata) uint64 {
    54  	return *(*uint64)((unsafe.Pointer)(m))
    55  }