base36 or base58 decoder

BaseUnitValue.swift

import Foundation

extension String {
	
	func baseUnitValue(letters: String) -> Int? {
		let base = countElements(letters)
		func getNum(s: String) -> Int? {
			if let idx = letters.rangeOfString(s) {
				return distance(letters.startIndex, idx.startIndex)
			} else {
				return nil
			}
		}
		
		var ret = 0
		var m = 1
		var i = self.endIndex
		do {
			let s = self.substringWithRange(Range<String.Index>(start: advance(i, -1), end: i))
			if let num = getNum(s) {
				ret += num * m
			} else {
				return nil
			}
			m *= base
			i = advance(i, -1)
		} while (distance(self.startIndex, i) > 0)
		return ret
	}
	
	var base58Value: Int? {
		return baseUnitValue("123456789abcdefghijkmnopqrstuvwxyzABCDEFGHJKLMNPQRSTUVWXYZ")
	}

	var base36Value: Int? {
		return baseUnitValue("0123456789abcdefghijklmnopqrstuvwxyz")
	}
}

For Base 2~36 we now have this built-in in the Swift Standard Library:

Encode: https://developer.apple.com/documentation/swift/string/2997127-init
Decode: https://developer.apple.com/documentation/swift/int/2924481-init

It doens't work for 58, though.