Turn a decimal number into a fractional approximation rounded to a maximum denominator

fractions.swift

import Foundation
import UIKit

// Turn a decimal number into a fractional approximation rounded to a maximum denominator
// https://github.com/jadengeller/fractional?
// https://stackoverflow.com/questions/35895154/decimal-to-fraction-conversion-in-swift#35895607

typealias Rational = (whole: Int, numerator: Int, denominator: Int)

func decimalToRational(of value: Double, withDenominator denominator: Int) -> Rational {
    let wholePortion = floor(value)
    let decimalPortion = value - wholePortion
    let numerator: Int = Int(round(decimalPortion * Double(denominator)))

    guard numerator != 0 else { return (Int(wholePortion), 0, 0) }
    guard numerator != denominator else { return (Int(wholePortion + 1), 0, 0) }

    return (Int(wholePortion), numerator, denominator)
}

func gcd(_ m: Int, _ n: Int) -> Int {
    let r: Int = m % n
    if r != 0 {
        return gcd(n, r)
    } else {
        return n
    }
}

func reduce(rational: Rational) -> Rational {
    guard rational.denominator != 0 else { return rational }

    var divisor = gcd(rational.numerator, rational.denominator)
    if divisor < 0 { divisor *= -1 }
    guard divisor != 0 else { return (rational.whole, rational.numerator, 0) }
    return (rational.whole, rational.numerator / divisor, rational.denominator / divisor)
}

// Quick Test
for index in 1 ... 150 {
    let measurementCm = Double(index)
    let measurementInchDecimal = measurementCm / 2.54
    let measurementInch = reduce(rational: decimalToRational(of: measurementInchDecimal, withDenominator: 8))
    let measurementInchDecimalTruncated = String(format: "%.3f", measurementInchDecimal)

    print("\(measurementCm) cm = \(measurementInchDecimalTruncated) in ≈ \(measurementInch.whole) \(measurementInch.numerator)/\(measurementInch.denominator) in")
}