54 lines
1.8 KiB
Go
54 lines
1.8 KiB
Go
// Copyright 2018 Google Inc. All rights reserved.
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// http://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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// +build !go1.9
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package s2
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// This file is for the bit manipulation code pre-Go 1.9.
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// findMSBSetNonZero64 returns the index (between 0 and 63) of the most
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// significant set bit. Passing zero to this function returns zero.
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func findMSBSetNonZero64(x uint64) int {
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val := []uint64{0x2, 0xC, 0xF0, 0xFF00, 0xFFFF0000, 0xFFFFFFFF00000000}
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shift := []uint64{1, 2, 4, 8, 16, 32}
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var msbPos uint64
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for i := 5; i >= 0; i-- {
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if x&val[i] != 0 {
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x >>= shift[i]
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msbPos |= shift[i]
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}
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}
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return int(msbPos)
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}
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const deBruijn64 = 0x03f79d71b4ca8b09
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const digitMask = uint64(1<<64 - 1)
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var deBruijn64Lookup = []byte{
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0, 1, 56, 2, 57, 49, 28, 3, 61, 58, 42, 50, 38, 29, 17, 4,
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62, 47, 59, 36, 45, 43, 51, 22, 53, 39, 33, 30, 24, 18, 12, 5,
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63, 55, 48, 27, 60, 41, 37, 16, 46, 35, 44, 21, 52, 32, 23, 11,
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54, 26, 40, 15, 34, 20, 31, 10, 25, 14, 19, 9, 13, 8, 7, 6,
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}
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// findLSBSetNonZero64 returns the index (between 0 and 63) of the least
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// significant set bit. Passing zero to this function returns zero.
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//
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// This code comes from trailingZeroBits in https://golang.org/src/math/big/nat.go
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// which references (Knuth, volume 4, section 7.3.1).
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func findLSBSetNonZero64(x uint64) int {
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return int(deBruijn64Lookup[((x&-x)*(deBruijn64&digitMask))>>58])
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}
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