Fraction extension of https://gist.github.com/takatoshi/29e1c50919e62ac19886 by @takatoshi

Fraction.swift

class Fraction: ExpressibleByStringLiteral {
    private(set) var n: Int
    private(set) var d: Int
    
    init(numerator: Int, denominator:Int) {
        self.n = numerator
        self.d = denominator
    }
    
    required public convenience init(stringLiteral string: String) {
        if string.contains("/"), string.split(separator: "/").count == 2{
            let parts = string.split(separator: "/")
            self.init(numerator: Int(parts[0]) ?? 1, denominator: Int(parts[1]) ?? 1)
        } else {
            self.init(numerator: Int(string) ?? 1, denominator: 1)
        }
    }
    
    var value: Double {
        get { return Double(n) / Double(d) }
    }
    
    var isImproper: Bool {
        get { return n >= d }
    }
    
    var wholePart: Int {
        get { return n / d }
    }
    
    var mixedPart: Int {
        get { return n % d }
    }
}

extension Fraction: CustomStringConvertible {
    var description: String {
        if isImproper {
            return mixedPart > 0 ? "\(wholePart) \(mixedPart)/\(d)" : "\(wholePart)"
        }
        return d == 1 ? "\(n)" : "\(n)/\(d)"
    }
}

// 最大公約数
func greatestCommonDenominator(_ first: Int, _ second: Int) -> Int {
    return second == 0 ? first : greatestCommonDenominator(second, first % second)
}

func * (left: Fraction, right: Fraction) -> Fraction {
    var n = left.n * right.n
    var d = left.d * right.d
    let g = greatestCommonDenominator(n, d)
    n /= g
    d /= g
    return Fraction(numerator: n, denominator: d)
}

func * (left: Int, right: Fraction) -> Fraction {
    return Fraction(numerator: left, denominator: 1) * right
}

func * (left: Fraction, right: Int) -> Fraction {
    return left * Fraction(numerator: right, denominator: 1)
}

func / (left: Fraction, right: Fraction) -> Fraction {
    var n = left.n * right.d
    var d = left.d * right.n
    let g = greatestCommonDenominator(n, d)
    n /= g
    d /= g
    return Fraction(numerator: n, denominator: d)
}

func / (left: Int, right: Fraction) -> Fraction {
    return Fraction(numerator: left, denominator: 1) / right
}

func / (left: Fraction, right: Int) -> Fraction {
    return left / Fraction(numerator: right, denominator: 1)
}

func + (left: Fraction, right: Fraction) -> Fraction {
    var n = left.n * right.d + right.n * left.d
    var d = left.d * right.d
    let g = greatestCommonDenominator(n, d)
    n /= g
    d /= g
    return Fraction(numerator: n, denominator: d)
}

func + (left: Int, right: Fraction) -> Fraction {
    return Fraction(numerator: left, denominator: 1) + right
}

func + (left: Fraction, right: Int) -> Fraction {
    return left + Fraction(numerator: right, denominator: 1)
}

func - (left: Fraction, right: Fraction) -> Fraction {
    var n = left.n * right.d - right.n * left.d
    var d = left.d * right.d
    let g = greatestCommonDenominator(n, d)
    n /= g
    d /= g
    return Fraction(numerator: n, denominator: d)
}

func - (left: Int, right: Fraction) -> Fraction {
    return Fraction(numerator: left, denominator: 1) - right
}

func - (left: Fraction, right: Int) -> Fraction {
    return left - Fraction(numerator: right, denominator: 1)
}


let f1: Fraction = "3/4"
let f2 = Fraction(numerator: 1, denominator: 4)
print(f1 + f2)