gentables.go

//go:build ignore
// +build ignore

package main

import (
	"fmt"
)

var logTable = [fieldSize]int16{
	-1, 0, 1, 25, 2, 50, 26, 198,
	3, 223, 51, 238, 27, 104, 199, 75,
	4, 100, 224, 14, 52, 141, 239, 129,
	28, 193, 105, 248, 200, 8, 76, 113,
	5, 138, 101, 47, 225, 36, 15, 33,
	53, 147, 142, 218, 240, 18, 130, 69,
	29, 181, 194, 125, 106, 39, 249, 185,
	201, 154, 9, 120, 77, 228, 114, 166,
	6, 191, 139, 98, 102, 221, 48, 253,
	226, 152, 37, 179, 16, 145, 34, 136,
	54, 208, 148, 206, 143, 150, 219, 189,
	241, 210, 19, 92, 131, 56, 70, 64,
	30, 66, 182, 163, 195, 72, 126, 110,
	107, 58, 40, 84, 250, 133, 186, 61,
	202, 94, 155, 159, 10, 21, 121, 43,
	78, 212, 229, 172, 115, 243, 167, 87,
	7, 112, 192, 247, 140, 128, 99, 13,
	103, 74, 222, 237, 49, 197, 254, 24,
	227, 165, 153, 119, 38, 184, 180, 124,
	17, 68, 146, 217, 35, 32, 137, 46,
	55, 63, 209, 91, 149, 188, 207, 205,
	144, 135, 151, 178, 220, 252, 190, 97,
	242, 86, 211, 171, 20, 42, 93, 158,
	132, 60, 57, 83, 71, 109, 65, 162,
	31, 45, 67, 216, 183, 123, 164, 118,
	196, 23, 73, 236, 127, 12, 111, 246,
	108, 161, 59, 82, 41, 157, 85, 170,
	251, 96, 134, 177, 187, 204, 62, 90,
	203, 89, 95, 176, 156, 169, 160, 81,
	11, 245, 22, 235, 122, 117, 44, 215,
	79, 174, 213, 233, 230, 231, 173, 232,
	116, 214, 244, 234, 168, 80, 88, 175,
}

const (
	// The number of elements in the field.
	fieldSize = 256

	// The polynomial used to generate the logarithm table.
	//
	// There are a number of polynomials that work to generate
	// a Galois field of 256 elements.  The choice is arbitrary,
	// and we just use the first one.
	//
	// The possibilities are: 29, 43, 45, 77, 95, 99, 101, 105,
	//* 113, 135, 141, 169, 195, 207, 231, and 245.
	generatingPolynomial = 29
)

func main() {
	t := generateExpTable()
	fmt.Printf("var expTable = %#v\n", t)
	//t2 := generateMulTableSplit(t)
	//fmt.Printf("var mulTable = %#v\n", t2)
	low, high := generateMulTableHalf(t)
	fmt.Printf("var mulTableLow = %#v\n", low)
	fmt.Printf("var mulTableHigh = %#v\n", high)
}

/**
 * Generates the inverse log table.
 */
func generateExpTable() []byte {
	result := make([]byte, fieldSize*2-2)
	for i := 1; i < fieldSize; i++ {
		log := logTable[i]
		result[log] = byte(i)
		result[log+fieldSize-1] = byte(i)
	}
	return result
}

func generateMulTable(expTable []byte) []byte {
	result := make([]byte, 256*256)
	for v := range result {
		a := byte(v & 0xff)
		b := byte(v >> 8)
		if a == 0 || b == 0 {
			result[v] = 0
			continue
		}
		logA := int(logTable[a])
		logB := int(logTable[b])
		result[v] = expTable[logA+logB]
	}
	return result
}

func generateMulTableSplit(expTable []byte) [256][256]byte {
	var result [256][256]byte
	for a := range result {
		for b := range result[a] {
			if a == 0 || b == 0 {
				result[a][b] = 0
				continue
			}
			logA := int(logTable[a])
			logB := int(logTable[b])
			result[a][b] = expTable[logA+logB]
		}
	}
	return result
}

func generateMulTableHalf(expTable []byte) (low [256][16]byte, high [256][16]byte) {
	for a := range low {
		for b := range low {
			result := 0
			if !(a == 0 || b == 0) {
				logA := int(logTable[a])
				logB := int(logTable[b])
				result = int(expTable[logA+logB])
			}
			if (b & 0xf) == b {
				low[a][b] = byte(result)
			}
			if (b & 0xf0) == b {
				high[a][b>>4] = byte(result)
			}
		}
	}
	return
}