mirror of
https://github.com/golang/go
synced 2024-11-18 14:54:40 -07:00
b7f0150d16
This function accounts for 2% of "godoc -analysis=pointer" and this change makes it twice as fast---and simpler. Added test and benchmark. Change-Id: I8578fa42dce34df057d81f6c522a7b4e0506d09d Reviewed-on: https://go-review.googlesource.com/15211 Run-TryBot: Robert Griesemer <gri@golang.org> Reviewed-by: Ilya Tocar <ilya.tocar@intel.com> Reviewed-by: Robert Griesemer <gri@golang.org>
85 lines
1.5 KiB
Go
85 lines
1.5 KiB
Go
// Copyright 2013 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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package intsets
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// From Hacker's Delight, fig 5.2.
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func popcountHD(x uint32) int {
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x -= (x >> 1) & 0x55555555
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x = (x & 0x33333333) + ((x >> 2) & 0x33333333)
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x = (x + (x >> 4)) & 0x0f0f0f0f
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x = x + (x >> 8)
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x = x + (x >> 16)
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return int(x & 0x0000003f)
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}
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var a [1 << 8]byte
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func init() {
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for i := range a {
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var n byte
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for x := i; x != 0; x >>= 1 {
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if x&1 != 0 {
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n++
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}
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}
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a[i] = n
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}
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}
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func popcountTable(x word) int {
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return int(a[byte(x>>(0*8))] +
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a[byte(x>>(1*8))] +
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a[byte(x>>(2*8))] +
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a[byte(x>>(3*8))] +
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a[byte(x>>(4*8))] +
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a[byte(x>>(5*8))] +
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a[byte(x>>(6*8))] +
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a[byte(x>>(7*8))])
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}
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// nlz returns the number of leading zeros of x.
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// From Hacker's Delight, fig 5.11.
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func nlz(x word) int {
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x |= (x >> 1)
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x |= (x >> 2)
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x |= (x >> 4)
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x |= (x >> 8)
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x |= (x >> 16)
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x |= (x >> 32)
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return popcount(^x)
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}
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// ntz returns the number of trailing zeros of x.
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// From Hacker's Delight, fig 5.13.
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func ntz(x word) int {
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if x == 0 {
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return bitsPerWord
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}
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n := 1
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if bitsPerWord == 64 {
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if (x & 0xffffffff) == 0 {
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n = n + 32
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x = x >> 32
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}
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}
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if (x & 0x0000ffff) == 0 {
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n = n + 16
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x = x >> 16
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}
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if (x & 0x000000ff) == 0 {
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n = n + 8
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x = x >> 8
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}
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if (x & 0x0000000f) == 0 {
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n = n + 4
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x = x >> 4
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}
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if (x & 0x00000003) == 0 {
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n = n + 2
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x = x >> 2
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}
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return n - int(x&1)
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}
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