Sum of two binaries, represented through strings.

binaryStringsSum.swift

import Foundation

func sum(_ lhs: String,_ rhs: String, carry: Int = 0) -> String {
    
    guard let lhsLast = lhs.last else {
        
        if rhs.count > 0 {
            return rhs
        } else {
            return carry == 1 ? "1" : ""
        }
    }
    
    guard let rhsLast = rhs.last else {
        
        if lhs.count > 0 {
            return lhs
        } else {
            return carry == 1 ? "1" : ""
        }
    }
    
    switch (lhsLast,rhsLast,carry) {
    case ("0","0",0): return sum(String(lhs.dropLast()), String(rhs.dropLast()), carry: 0) + "0"
    case ("0","0",1): return sum(String(lhs.dropLast()), String(rhs.dropLast()), carry: 0) + "1"
    case ("1","0",1),("0","1",1): return sum(String(lhs.dropLast()), String(rhs.dropLast()), carry: 1) + "0"
    case ("1","0",0),("0","1",0): return sum(String(lhs.dropLast()), String(rhs.dropLast()), carry: 0) + "1"
    case ("1","1",0): return sum(String(lhs.dropLast()), String(rhs.dropLast()), carry: 1) + "0"
    case ("1","1",1): return sum(String(lhs.dropLast()), String(rhs.dropLast()), carry: 1) + "1"
    default: return ""
    }
}

let a = "1100"
let b = "1000"

let sumResult = sum(a,b)