go make_tables 源码
golang make_tables 代码
文件路径：/src/math/bits/make_tables.go
// Copyright 2017 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.
//go:build ignore
// +build ignore
// This program generates bits_tables.go.
package main
import (
"bytes"
"fmt"
"go/format"
"io"
"log"
"os"
)
var header = []byte(`// Copyright 2017 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.
// Code generated by go run make_tables.go. DO NOT EDIT.
package bits
`)
func main() {
buf := bytes.NewBuffer(header)
gen(buf, "ntz8tab", ntz8)
gen(buf, "pop8tab", pop8)
gen(buf, "rev8tab", rev8)
gen(buf, "len8tab", len8)
out, err := format.Source(buf.Bytes())
if err != nil {
log.Fatal(err)
}
err = os.WriteFile("bits_tables.go", out, 0666)
if err != nil {
log.Fatal(err)
}
}
func gen(w io.Writer, name string, f func(uint8) uint8) {
// Use a const string to allow the compiler to constant-evaluate lookups at constant index.
fmt.Fprintf(w, "const %s = \"\"+\n\"", name)
for i := 0; i < 256; i++ {
fmt.Fprintf(w, "\\x%02x", f(uint8(i)))
if i%16 == 15 && i != 255 {
fmt.Fprint(w, "\"+\n\"")
}
}
fmt.Fprint(w, "\"\n\n")
}
func ntz8(x uint8) (n uint8) {
for x&1 == 0 && n < 8 {
x >>= 1
n++
}
return
}
func pop8(x uint8) (n uint8) {
for x != 0 {
x &= x - 1
n++
}
return
}
func rev8(x uint8) (r uint8) {
for i := 8; i > 0; i-- {
r = r<<1 | x&1
x >>= 1
}
return
}
func len8(x uint8) (n uint8) {
for x != 0 {
x >>= 1
n++
}
return
}
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