make_tables.go

Documentation: math/bits

     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 string, 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
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