class Solution {
    func addBinary(_ a: String, _ b: String) -> String {
        let zeroChar: Character = "0"
        let oneChar: Character = "1"
        let aReversed = String(a.reversed())
        let bReversed = String(b.reversed())

        let bigger: String
        let smaller: String
        if a.count > b.count {
            bigger = aReversed
            smaller = bReversed
        }
        else {
            bigger = bReversed
            smaller = aReversed
        }

        let finalIndex = bigger.count - 1

        var outputChars = [Character]()
        var digitsToCarry = 0
        for i in 0...finalIndex {
            let stringIndex = bigger.index(bigger.startIndex, offsetBy: i)
            let char1 = bigger[stringIndex]

            let char2: Character
            if i < smaller.count {
                char2 = smaller[stringIndex]
            }
            else if digitsToCarry > 0 {
                char2 = oneChar
                digitsToCarry -= 1
            }
            else {
                char2 = zeroChar
            }

            var digit = 0
            digit += (char1 == zeroChar) ? 0 : 1
            digit += (char2 == zeroChar) ? 0 : 1
            if digitsToCarry > 0 {
                digit += 1
                digitsToCarry -= 1
            }

            if digit == 0 {
                outputChars.append(zeroChar)
            }
            else if digit == 1 {
                outputChars.append(oneChar)
            }
            else {
                let char = (digit % 2 == 0) ? zeroChar : oneChar
                outputChars.append(char)
                digitsToCarry += 1
            }
        }

        while digitsToCarry > 0 {
            outputChars.append(oneChar)
            digitsToCarry -= 1
        }

        return String(outputChars.reversed())
    }
}

/*
Leetcode results:
-----------------------------------------------------------------------------------
Runtime: 20 ms, faster than 41.96% of Swift online submissions for Add Binary.
Memory Usage: 21.4 MB, less than 16.67% of Swift online submissions for Add Binary.
-----------------------------------------------------------------------------------
*/


///* SOLUTION ANALYSIS: */
//let solution = Solution()
//struct Problem {
//    let input: (String, String)
//    let expectedAnswer: String
//}
//let problems: [Problem] = [
//    Problem(input: ("11","1"), expectedAnswer: "100"),
//    Problem(input: ("1010","101"), expectedAnswer: "1111"),
//    Problem(input: ("11111","1"), expectedAnswer: "100000"),
//    Problem(input: ("1010","1011"), expectedAnswer: "10101"),
//    Problem(input: ("1011","101"), expectedAnswer: "10000"),
//    Problem(input: ("1111", "1111"), expectedAnswer: "11110")
//]
//
//let answers = problems.map { (problem) -> String in
//    let answer = solution.addBinary(problem.input.0, problem.input.1)
//    let gotIt = (answer == problem.expectedAnswer) ? "CORRECT" : "WRONG"
//    return "\(gotIt)! Input: \(problem.input), Expected: \(problem.expectedAnswer), Got: \(answer)."
//}
//
//let joinedAnswers = answers.joined(separator: "\n")
//print(joinedAnswers)