BigInt2.swift

BigInt2.swift

struct BigInt {
    let string: String

    func add(_ value: BigInt) -> BigInt {
        if self.string == "0" { return value }
        if value.string == "0" { return self }

        var lhs = self.string
        var rhs = value.string

        if lhs.count < rhs.count {
            lhs = String(repeating: "0", count: rhs.count - lhs.count) + lhs
        } else if lhs.count > rhs.count {
            rhs = String(repeating: "0", count: lhs.count - rhs.count) + rhs
        }

        var movedToNext = 0
        var acc = ""
        for pair in zip(lhs.reversed(), rhs.reversed()) {
            let lhsDigit = Int(String(pair.0))!
            let rhsDigit = Int(String(pair.1))!
            let sum = (lhsDigit + rhsDigit + movedToNext)
            let lastDigit = sum % 10
            movedToNext = sum / 10
            acc = "\(lastDigit)" + acc
        }

        let string = movedToNext == 0
        ? acc
        : "\(movedToNext)" + acc

        return BigInt(string: string)
    }

    func multiply(by value: BigInt) -> BigInt {
        let lhs = Array(String(self.string.reversed()))
        let rhs = Array(String(value.string.reversed()))

        var totalAcc = "0"
        for (index, l) in lhs.enumerated() {
            var movedToNext = 0
            var acc = String(repeating: "0", count: index)
            for r in rhs {
                let sum = Int(String(r))! * Int(String(l))! + movedToNext
                let lastDigit = sum % 10
                movedToNext = sum / 10
                acc = "\(lastDigit)" + acc
            }

            acc = movedToNext == 0 ? acc : "\(movedToNext)" + acc

            totalAcc = BigInt(string: totalAcc).add(BigInt(string: acc)).string
        }

        return BigInt(string: totalAcc)
    }
}