go lang go lan

advance_letter.go

func advanceLetter(s string, n int) string {
	var x []rune
	// add a known rune to the slice in case we need to advance it
	x = append(x, rune(1337))
	// Create a slice of runes
	for _, r := range s {
		x = append(x, r)
	}
	// Need to reverse index
	l := len(x) - 1
	// indicate a carried character place
	var carry bool
	for i := 0; i < l+1; i++ {
		// get our next rune working backwards
		r := x[l-i]
		// only use the original increment on the first pass
		if i > 0 {
			n = 0
		}
		// increment a carried place by one
		if carry {
			n = 1
			// did we reach our dummy?
			if r == rune(1337) {
				// set it to one less than A
				r = 64
			}
		} else {
			if r == rune(1337) {
				// we can stop here
				break
			}
		}
		carry = false
		// increment
		r = r + rune(n)
		// loop from Z(90) to A(65)
		if r > 90 {
			// carry our place
			carry = true
			// 64 is the "zero value"
			r = r - 90 + 64
		}
		// save our character
		x[l-i] = r
	}
	// make a new string to return
	var a string
	// work forwards again converting our runes to characters and appending
	for _, r := range x {
		if r != rune(1337) {
			a = a + fmt.Sprintf("%c", r)
		}
	}
	return a
}