Damm check digit package for golang

damm.go

package damm

import "strconv"

// DefaultDamm10 has as its Matrix a 10x10 totally anti-symmetric quasigroup
// retrieved from Damm's doctoral dissertation.
//
// https://en.wikipedia.org/wiki/Damm_algorithm
var DefaultDamm10 = Damm{Matrix: [][]int{
	[]int{0, 3, 1, 7, 5, 9, 8, 6, 4, 2},
	[]int{7, 0, 9, 2, 1, 5, 4, 8, 6, 3},
	[]int{4, 2, 0, 6, 8, 7, 1, 3, 5, 9},
	[]int{1, 7, 5, 0, 9, 8, 3, 4, 2, 6},
	[]int{6, 1, 2, 3, 0, 4, 5, 9, 7, 8},
	[]int{3, 6, 7, 4, 2, 0, 9, 5, 8, 1},
	[]int{5, 8, 6, 9, 7, 2, 0, 1, 3, 4},
	[]int{8, 9, 4, 5, 3, 6, 2, 0, 1, 7},
	[]int{9, 4, 3, 8, 6, 1, 7, 2, 0, 5},
	[]int{2, 5, 8, 1, 4, 3, 6, 7, 9, 0},
}}

// Damm generates and validates digit strings with a final check digit generated by the Damm algorithm.
type Damm struct {
	Matrix [][]int
}

// Create returns the digits string with a Damm check digit appended.
func (damm *Damm) Create(digits string) string {
	return digits + strconv.Itoa(damm.checkDigit(digits))
}

// Check validates the last digit in the digits string is a Damm check digit for the rest of the digits.
func (damm *Damm) Check(digits string) bool {
	return damm.checkDigit(digits) == 0
}

// checkDigit returns the integer Damm check digit for the digits string.
func (damm *Damm) checkDigit(digits string) int {
	interim := 0
	for _, r := range digits {
		digit, err := strconv.Atoi(string(r))
		if err != nil {
			panic(err)
		}

		interim = damm.Matrix[interim][digit]
	}

	return interim
}