github.com/code-reading/golang@v0.0.0-20220303082512-ba5bc0e589a3/go/src/math/bits/make_tables.go (about)

     1  // Copyright 2017 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  //go:build ignore
     6  // +build ignore
     7  
     8  // This program generates bits_tables.go.
     9  
    10  package main
    11  
    12  import (
    13  	"bytes"
    14  	"fmt"
    15  	"go/format"
    16  	"io"
    17  	"log"
    18  	"os"
    19  )
    20  
    21  var header = []byte(`// Copyright 2017 The Go Authors. All rights reserved.
    22  // Use of this source code is governed by a BSD-style
    23  // license that can be found in the LICENSE file.
    24  
    25  // Code generated by go run make_tables.go. DO NOT EDIT.
    26  
    27  package bits
    28  
    29  `)
    30  
    31  func main() {
    32  	buf := bytes.NewBuffer(header)
    33  
    34  	gen(buf, "ntz8tab", ntz8)
    35  	gen(buf, "pop8tab", pop8)
    36  	gen(buf, "rev8tab", rev8)
    37  	gen(buf, "len8tab", len8)
    38  
    39  	out, err := format.Source(buf.Bytes())
    40  	if err != nil {
    41  		log.Fatal(err)
    42  	}
    43  
    44  	err = os.WriteFile("bits_tables.go", out, 0666)
    45  	if err != nil {
    46  		log.Fatal(err)
    47  	}
    48  }
    49  
    50  func gen(w io.Writer, name string, f func(uint8) uint8) {
    51  	// Use a const string to allow the compiler to constant-evaluate lookups at constant index.
    52  	fmt.Fprintf(w, "const %s = \"\"+\n\"", name)
    53  	for i := 0; i < 256; i++ {
    54  		fmt.Fprintf(w, "\\x%02x", f(uint8(i)))
    55  		if i%16 == 15 && i != 255 {
    56  			fmt.Fprint(w, "\"+\n\"")
    57  		}
    58  	}
    59  	fmt.Fprint(w, "\"\n\n")
    60  }
    61  
    62  func ntz8(x uint8) (n uint8) {
    63  	for x&1 == 0 && n < 8 {
    64  		x >>= 1
    65  		n++
    66  	}
    67  	return
    68  }
    69  
    70  func pop8(x uint8) (n uint8) {
    71  	for x != 0 {
    72  		x &= x - 1
    73  		n++
    74  	}
    75  	return
    76  }
    77  
    78  func rev8(x uint8) (r uint8) {
    79  	for i := 8; i > 0; i-- {
    80  		r = r<<1 | x&1
    81  		x >>= 1
    82  	}
    83  	return
    84  }
    85  
    86  func len8(x uint8) (n uint8) {
    87  	for x != 0 {
    88  		x >>= 1
    89  		n++
    90  	}
    91  	return
    92  }