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71 lines
2.9 KiB
Go
71 lines
2.9 KiB
Go
// +build !go1.9
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package roaring
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// Reuse of portions of go/src/math/big standard lib code
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// under this license:
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/*
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Copyright (c) 2009 The Go Authors. All rights reserved.
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Redistribution and use in source and binary forms, with or without
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modification, are permitted provided that the following conditions are
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met:
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* Redistributions of source code must retain the above copyright
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notice, this list of conditions and the following disclaimer.
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* Redistributions in binary form must reproduce the above
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copyright notice, this list of conditions and the following disclaimer
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in the documentation and/or other materials provided with the
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distribution.
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* Neither the name of Google Inc. nor the names of its
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contributors may be used to endorse or promote products derived from
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this software without specific prior written permission.
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THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
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"AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
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LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
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A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
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OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
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SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
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LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
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THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
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(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
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OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
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*/
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const deBruijn32 = 0x077CB531
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var deBruijn32Lookup = []byte{
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0, 1, 28, 2, 29, 14, 24, 3, 30, 22, 20, 15, 25, 17, 4, 8,
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31, 27, 13, 23, 21, 19, 16, 7, 26, 12, 18, 6, 11, 5, 10, 9,
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}
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const deBruijn64 = 0x03f79d71b4ca8b09
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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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// trailingZeroBits returns the number of consecutive least significant zero
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// bits of x.
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func countTrailingZeros(x uint64) int {
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// x & -x leaves only the right-most bit set in the word. Let k be the
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// index of that bit. Since only a single bit is set, the value is two
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// to the power of k. Multiplying by a power of two is equivalent to
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// left shifting, in this case by k bits. The de Bruijn constant is
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// such that all six bit, consecutive substrings are distinct.
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// Therefore, if we have a left shifted version of this constant we can
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// find by how many bits it was shifted by looking at which six bit
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// substring ended up at the top of the word.
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// (Knuth, volume 4, section 7.3.1)
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if x == 0 {
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// We have to special case 0; the fomula
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// below doesn't work for 0.
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return 64
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}
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return int(deBruijn64Lookup[((x&-x)*(deBruijn64))>>58])
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}
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