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
9package main
10
11import (
12	"bytes"
13	"fmt"
14	"go/format"
15	"io"
16	"log"
17	"os"
18)
19
20var 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
26package bits
27
28`)
29
30func 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
49func 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
61func ntz8(x uint8) (n uint8) {
62	for x&1 == 0 && n < 8 {
63		x >>= 1
64		n++
65	}
66	return
67}
68
69func pop8(x uint8) (n uint8) {
70	for x != 0 {
71		x &= x - 1
72		n++
73	}
74	return
75}
76
77func 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
85func len8(x uint8) (n uint8) {
86	for x != 0 {
87		x >>= 1
88		n++
89	}
90	return
91}
92