Roman_Numerals.swift

//: Playground - noun: a place where people can play

import UIKit

/*
 * Description: a String extension to convert roman numerals to their respective integer values
 * Synopsis:
 *   Any of the letters representing numbers in the Roman numerical system: 
 *
 *      I = 1, V = 5, X = 10, L = 50, C = 100, D = 500, M = 1,000.
 *
 *   In this system, a letter placed after another of greater value adds (thus XVI or xvi is 16),
 *   whereas a letter placed before another of greater value subtracts (thus XC or xc is 90).
 */

extension String {
    
    func romanToInt() -> Int {
        
        let romainNumerals: [String: Int] = ["I": 1,  "V": 5, "X": 10, "L": 50, "C": 100, "D": 500, "M": 1000]
        
        func intFromNumerals(numeral1: String?, numeral2: String?) -> Int {
            
            let n1 = romainNumerals[numeral1 ?? "nil"] ?? 0
            let n2 = romainNumerals[numeral2 ?? "nil"] ?? 0
            
            if n1 < n2 {
                return n2 - n1
            }
            else if n1 >= n2 {
                return n1 + n2
            }
            
            return 0
        }

        let numeral = self
        let maxIndex = numeral.characters.count
        var total: Int = 0
        var index: Int = 0
        
        while index < maxIndex {
            let c1 = String(numeral[numeral.index(numeral.startIndex, offsetBy: index)])
            var c2: String? = nil
            let next = index + 1

            if next > maxIndex {
                break
            }
            else if next < maxIndex {
                c2 = String(numeral[numeral.index(numeral.startIndex, offsetBy: next)])
            }

            total += intFromNumerals(numeral1: c1, numeral2: c2)
            index += 2
        }

        return total
    }
}

print("\("DCLXVI".romanToInt())")
print("\("V".romanToInt())")
print("\("XIV".romanToInt())")
print("\("XXXIV".romanToInt())")