script/print-crc32-magic-numbers.go

// Copyright 2018 The Wuffs Authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//    https://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

//go:build ignore
// +build ignore

package main

// print-crc32-magic-numbers.go prints the std/crc32 magic number tables.
//
// Usage: go run print-crc32-magic-numbers.go

import (
	"flag"
	"fmt"
	"hash/crc32"
	"os"
)

var (
	reverse = flag.Bool("reverse", false, "whether to reverse the bits")
)

func main() {
	if err := main1(); err != nil {
		os.Stderr.WriteString(err.Error() + "\n")
		os.Exit(1)
	}
}

func main1() error {
	flag.Parse()
	tables := [16]crc32.Table{}
	tables[0] = *crc32.MakeTable(crc32.IEEE)

	// See "Multi-Byte Lookup Tables" in std/crc32/README.md for more detail on
	// the slicing-by-M algorithm. We use an M of 16.
	for i := 0; i < 256; i++ {
		crc := tables[0][i]
		for j := 1; j < 16; j++ {
			crc = tables[0][crc&0xFF] ^ (crc >> 8)
			tables[j][i] = crc
		}
	}

	for i, t := range tables {
		if *reverse {
			reverseTable(&t)
		}
		if i != 0 {
			fmt.Println("],[")
		}
		for j, x := range t {
			fmt.Printf("0x%08X,", x)
			if j&7 == 7 {
				fmt.Println()
			}
		}
	}
	return nil
}

// reverseTable reverses the bits in the keys and values of a crc32.Table
// (which is a [256]uint32).
func reverseTable(t *crc32.Table) {
	revT := crc32.Table{}
	for i, v := range *t {
		revI := reverse8[i]
		revV := uint32(0)
		for j := 0; j < 32; j++ {
			if (v & (1 << j)) != 0 {
				revV |= 1 << (31 - j)
			}
		}
		revT[revI] = revV
	}
	*t = revT
}

var reverse8 = [256]uint8{
	0x00, 0x80, 0x40, 0xC0, 0x20, 0xA0, 0x60, 0xE0, // 0x00 - 0x07
	0x10, 0x90, 0x50, 0xD0, 0x30, 0xB0, 0x70, 0xF0, // 0x08 - 0x0F
	0x08, 0x88, 0x48, 0xC8, 0x28, 0xA8, 0x68, 0xE8, // 0x10 - 0x17
	0x18, 0x98, 0x58, 0xD8, 0x38, 0xB8, 0x78, 0xF8, // 0x18 - 0x1F
	0x04, 0x84, 0x44, 0xC4, 0x24, 0xA4, 0x64, 0xE4, // 0x20 - 0x27
	0x14, 0x94, 0x54, 0xD4, 0x34, 0xB4, 0x74, 0xF4, // 0x28 - 0x2F
	0x0C, 0x8C, 0x4C, 0xCC, 0x2C, 0xAC, 0x6C, 0xEC, // 0x30 - 0x37
	0x1C, 0x9C, 0x5C, 0xDC, 0x3C, 0xBC, 0x7C, 0xFC, // 0x38 - 0x3F
	0x02, 0x82, 0x42, 0xC2, 0x22, 0xA2, 0x62, 0xE2, // 0x40 - 0x47
	0x12, 0x92, 0x52, 0xD2, 0x32, 0xB2, 0x72, 0xF2, // 0x48 - 0x4F
	0x0A, 0x8A, 0x4A, 0xCA, 0x2A, 0xAA, 0x6A, 0xEA, // 0x50 - 0x57
	0x1A, 0x9A, 0x5A, 0xDA, 0x3A, 0xBA, 0x7A, 0xFA, // 0x58 - 0x5F
	0x06, 0x86, 0x46, 0xC6, 0x26, 0xA6, 0x66, 0xE6, // 0x60 - 0x67
	0x16, 0x96, 0x56, 0xD6, 0x36, 0xB6, 0x76, 0xF6, // 0x68 - 0x6F
	0x0E, 0x8E, 0x4E, 0xCE, 0x2E, 0xAE, 0x6E, 0xEE, // 0x70 - 0x77
	0x1E, 0x9E, 0x5E, 0xDE, 0x3E, 0xBE, 0x7E, 0xFE, // 0x78 - 0x7F
	0x01, 0x81, 0x41, 0xC1, 0x21, 0xA1, 0x61, 0xE1, // 0x80 - 0x87
	0x11, 0x91, 0x51, 0xD1, 0x31, 0xB1, 0x71, 0xF1, // 0x88 - 0x8F
	0x09, 0x89, 0x49, 0xC9, 0x29, 0xA9, 0x69, 0xE9, // 0x90 - 0x97
	0x19, 0x99, 0x59, 0xD9, 0x39, 0xB9, 0x79, 0xF9, // 0x98 - 0x9F
	0x05, 0x85, 0x45, 0xC5, 0x25, 0xA5, 0x65, 0xE5, // 0xA0 - 0xA7
	0x15, 0x95, 0x55, 0xD5, 0x35, 0xB5, 0x75, 0xF5, // 0xA8 - 0xAF
	0x0D, 0x8D, 0x4D, 0xCD, 0x2D, 0xAD, 0x6D, 0xED, // 0xB0 - 0xB7
	0x1D, 0x9D, 0x5D, 0xDD, 0x3D, 0xBD, 0x7D, 0xFD, // 0xB8 - 0xBF
	0x03, 0x83, 0x43, 0xC3, 0x23, 0xA3, 0x63, 0xE3, // 0xC0 - 0xC7
	0x13, 0x93, 0x53, 0xD3, 0x33, 0xB3, 0x73, 0xF3, // 0xC8 - 0xCF
	0x0B, 0x8B, 0x4B, 0xCB, 0x2B, 0xAB, 0x6B, 0xEB, // 0xD0 - 0xD7
	0x1B, 0x9B, 0x5B, 0xDB, 0x3B, 0xBB, 0x7B, 0xFB, // 0xD8 - 0xDF
	0x07, 0x87, 0x47, 0xC7, 0x27, 0xA7, 0x67, 0xE7, // 0xE0 - 0xE7
	0x17, 0x97, 0x57, 0xD7, 0x37, 0xB7, 0x77, 0xF7, // 0xE8 - 0xEF
	0x0F, 0x8F, 0x4F, 0xCF, 0x2F, 0xAF, 0x6F, 0xEF, // 0xF0 - 0xF7
	0x1F, 0x9F, 0x5F, 0xDF, 0x3F, 0xBF, 0x7F, 0xFF, // 0xF8 - 0xFF
}