libgo/go/math/bits/make_tables.go

   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 }