github.com/coyove/sdss@v0.0.0-20231129015646-c2ec58cca6a2/contrib/roaring/ctz_compat.go (about)

     1  //go:build !go1.9
     2  // +build !go1.9
     3  
     4  package roaring
     5  
     6  // Reuse of portions of go/src/math/big standard lib code
     7  // under this license:
     8  /*
     9  Copyright (c) 2009 The Go Authors. All rights reserved.
    10  
    11  Redistribution and use in source and binary forms, with or without
    12  modification, are permitted provided that the following conditions are
    13  met:
    14  
    15     * Redistributions of source code must retain the above copyright
    16  notice, this list of conditions and the following disclaimer.
    17     * Redistributions in binary form must reproduce the above
    18  copyright notice, this list of conditions and the following disclaimer
    19  in the documentation and/or other materials provided with the
    20  distribution.
    21     * Neither the name of Google Inc. nor the names of its
    22  contributors may be used to endorse or promote products derived from
    23  this software without specific prior written permission.
    24  
    25  THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
    26  "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
    27  LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
    28  A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
    29  OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
    30  SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
    31  LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
    32  DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
    33  THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
    34  (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
    35  OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
    36  */
    37  
    38  const deBruijn32 = 0x077CB531
    39  
    40  var deBruijn32Lookup = []byte{
    41  	0, 1, 28, 2, 29, 14, 24, 3, 30, 22, 20, 15, 25, 17, 4, 8,
    42  	31, 27, 13, 23, 21, 19, 16, 7, 26, 12, 18, 6, 11, 5, 10, 9,
    43  }
    44  
    45  const deBruijn64 = 0x03f79d71b4ca8b09
    46  
    47  var deBruijn64Lookup = []byte{
    48  	0, 1, 56, 2, 57, 49, 28, 3, 61, 58, 42, 50, 38, 29, 17, 4,
    49  	62, 47, 59, 36, 45, 43, 51, 22, 53, 39, 33, 30, 24, 18, 12, 5,
    50  	63, 55, 48, 27, 60, 41, 37, 16, 46, 35, 44, 21, 52, 32, 23, 11,
    51  	54, 26, 40, 15, 34, 20, 31, 10, 25, 14, 19, 9, 13, 8, 7, 6,
    52  }
    53  
    54  // trailingZeroBits returns the number of consecutive least significant zero
    55  // bits of x.
    56  func countTrailingZeros(x uint64) int {
    57  	// x & -x leaves only the right-most bit set in the word. Let k be the
    58  	// index of that bit. Since only a single bit is set, the value is two
    59  	// to the power of k. Multiplying by a power of two is equivalent to
    60  	// left shifting, in this case by k bits. The de Bruijn constant is
    61  	// such that all six bit, consecutive substrings are distinct.
    62  	// Therefore, if we have a left shifted version of this constant we can
    63  	// find by how many bits it was shifted by looking at which six bit
    64  	// substring ended up at the top of the word.
    65  	// (Knuth, volume 4, section 7.3.1)
    66  	if x == 0 {
    67  		// We have to special case 0; the fomula
    68  		// below doesn't work for 0.
    69  		return 64
    70  	}
    71  	return int(deBruijn64Lookup[((x&-x)*(deBruijn64))>>58])
    72  }