make_tables.mx raw

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