rotxiii.go

package rotxiii

func Decode(src string) string {
	SHIFT_LENGTH := 13
	ALPHABET_LENGTH := 26
	SKIP_SEQUENCES := []byte{' ', '.'}

	b := []byte(src)
	r := make([]byte, len(src))
	for i, _ := range r {
		hitSkipSeq := false
		for _, ss := range SKIP_SEQUENCES {
			if b[i] == ss {
				hitSkipSeq = true
				r[i] = ss
				break
			}
		}
		if hitSkipSeq {
			continue
		}

		var limitSeq []byte
		if 'A' <= b[i] && b[i] <= 'Z' {
			limitSeq = []byte{'A', 'Z'}
		} else {
			limitSeq = []byte{'a', 'z'}
		}

		decc := b[i] - byte(SHIFT_LENGTH)

		if !(limitSeq[0] <= decc && decc <= limitSeq[1]) {
			decc += byte(ALPHABET_LENGTH)
		}

		r[i] = decc
	}

	return string(r)
}