bits_go18.go (1879B)
1 // Copyright 2018 Google Inc. All rights reserved. 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 // +build !go1.9 16 17 package s2 18 19 // This file is for the bit manipulation code pre-Go 1.9. 20 21 // findMSBSetNonZero64 returns the index (between 0 and 63) of the most 22 // significant set bit. Passing zero to this function returns zero. 23 func findMSBSetNonZero64(x uint64) int { 24 val := []uint64{0x2, 0xC, 0xF0, 0xFF00, 0xFFFF0000, 0xFFFFFFFF00000000} 25 shift := []uint64{1, 2, 4, 8, 16, 32} 26 var msbPos uint64 27 for i := 5; i >= 0; i-- { 28 if x&val[i] != 0 { 29 x >>= shift[i] 30 msbPos |= shift[i] 31 } 32 } 33 return int(msbPos) 34 } 35 36 const deBruijn64 = 0x03f79d71b4ca8b09 37 const digitMask = uint64(1<<64 - 1) 38 39 var deBruijn64Lookup = []byte{ 40 0, 1, 56, 2, 57, 49, 28, 3, 61, 58, 42, 50, 38, 29, 17, 4, 41 62, 47, 59, 36, 45, 43, 51, 22, 53, 39, 33, 30, 24, 18, 12, 5, 42 63, 55, 48, 27, 60, 41, 37, 16, 46, 35, 44, 21, 52, 32, 23, 11, 43 54, 26, 40, 15, 34, 20, 31, 10, 25, 14, 19, 9, 13, 8, 7, 6, 44 } 45 46 // findLSBSetNonZero64 returns the index (between 0 and 63) of the least 47 // significant set bit. Passing zero to this function returns zero. 48 // 49 // This code comes from trailingZeroBits in https://golang.org/src/math/big/nat.go 50 // which references (Knuth, volume 4, section 7.3.1). 51 func findLSBSetNonZero64(x uint64) int { 52 return int(deBruijn64Lookup[((x&-x)*(deBruijn64&digitMask))>>58]) 53 }