mirror of
https://github.com/golang/go
synced 2024-10-01 10:28:31 -06:00
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.
|
|
// Use of this source code is governed by a BSD-style
|
|
// license that can be found in the LICENSE file.
|
|
|
|
package intsets
|
|
|
|
// From Hacker's Delight, fig 5.2.
|
|
func popcountHD(x uint32) int {
|
|
x -= (x >> 1) & 0x55555555
|
|
x = (x & 0x33333333) + ((x >> 2) & 0x33333333)
|
|
x = (x + (x >> 4)) & 0x0f0f0f0f
|
|
x = x + (x >> 8)
|
|
x = x + (x >> 16)
|
|
return int(x & 0x0000003f)
|
|
}
|
|
|
|
var a [1 << 8]byte
|
|
|
|
func init() {
|
|
for i := range a {
|
|
var n byte
|
|
for x := i; x != 0; x >>= 1 {
|
|
if x&1 != 0 {
|
|
n++
|
|
}
|
|
}
|
|
a[i] = n
|
|
}
|
|
}
|
|
|
|
func popcountTable(x word) int {
|
|
return int(a[byte(x>>(0*8))] +
|
|
a[byte(x>>(1*8))] +
|
|
a[byte(x>>(2*8))] +
|
|
a[byte(x>>(3*8))] +
|
|
a[byte(x>>(4*8))] +
|
|
a[byte(x>>(5*8))] +
|
|
a[byte(x>>(6*8))] +
|
|
a[byte(x>>(7*8))])
|
|
}
|
|
|
|
// nlz returns the number of leading zeros of x.
|
|
// From Hacker's Delight, fig 5.11.
|
|
func nlz(x word) int {
|
|
x |= (x >> 1)
|
|
x |= (x >> 2)
|
|
x |= (x >> 4)
|
|
x |= (x >> 8)
|
|
x |= (x >> 16)
|
|
x |= (x >> 32)
|
|
return popcount(^x)
|
|
}
|
|
|
|
// ntz returns the number of trailing zeros of x.
|
|
// From Hacker's Delight, fig 5.13.
|
|
func ntz(x word) int {
|
|
if x == 0 {
|
|
return bitsPerWord
|
|
}
|
|
n := 1
|
|
if bitsPerWord == 64 {
|
|
if (x & 0xffffffff) == 0 {
|
|
n = n + 32
|
|
x = x >> 32
|
|
}
|
|
}
|
|
if (x & 0x0000ffff) == 0 {
|
|
n = n + 16
|
|
x = x >> 16
|
|
}
|
|
if (x & 0x000000ff) == 0 {
|
|
n = n + 8
|
|
x = x >> 8
|
|
}
|
|
if (x & 0x0000000f) == 0 {
|
|
n = n + 4
|
|
x = x >> 4
|
|
}
|
|
if (x & 0x00000003) == 0 {
|
|
n = n + 2
|
|
x = x >> 2
|
|
}
|
|
return n - int(x&1)
|
|
}
|