Swift - `Scale Number in a Range min - max

edit/update: Xcode 11.5 • Swift 5.2

extension FloatingPoint {
    func converting(from input: ClosedRange<Self>, to output: ClosedRange<Self>) -> Self {
        let x = (output.upperBound - output.lowerBound) * (self - input.lowerBound)
        let y = (input.upperBound - input.lowerBound)
        return x / y + output.lowerBound
    }
}

---

extension BinaryInteger {
    func converting(from input: ClosedRange<Self>, to output: ClosedRange<Self>) -> Self {
        let x = (output.upperBound - output.lowerBound) * (self - input.lowerBound)
        let y = (input.upperBound - input.lowerBound)
        return x / y + output.lowerBound
    }
}

---

let integer = 380
let result = integer.converting(from: 10...750, to: 100...350) // 225

let double = 750.0
let result = double.converting(from: 10.1...750.0, to: 0...350) // 350

Here is the Swift 3 compatible version, which also supports generics (to not bound Rescale just for Doubles).

struct Rescale<Type : BinaryFloatingPoint> {
    typealias RescaleDomain = (lowerBound: Type, upperBound: Type)

    var fromDomain: RescaleDomain
    var toDomain: RescaleDomain

    init(from: RescaleDomain, to: RescaleDomain) {
        self.fromDomain = from
        self.toDomain = to
    }

    func interpolate(_ x: Type ) -> Type {
        return self.toDomain.lowerBound * (1 - x) + self.toDomain.upperBound * x;
    }

    func uninterpolate(_ x: Type) -> Type {
        let b = (self.fromDomain.upperBound - self.fromDomain.lowerBound) != 0 ? self.fromDomain.upperBound - self.fromDomain.lowerBound : 1 / self.fromDomain.upperBound;
        return (x - self.fromDomain.lowerBound) / b
    }

    func rescale(_ x: Type )  -> Type {
        return interpolate( uninterpolate(x) )
    }
}

Example would be:

Rescale(from: (15, 85), to: (0, 100)).rescale(85)

I translated this to Swift from this answer here based on the D3 library.

struct Rescale {
var range0: Double, range1: Double, domain0: Double, domain1: Double

init( domain0: Double, domain1: Double, range0: Double, range1: Double ) {
    self.range0 = range0
    self.range1 = range1
    self.domain0 = domain0
    self.domain1 = domain1
}

func interpolate( x: Double ) -> Double {
    return range0 * (1 - x) + range1 * x;
}

func uninterpolate( x: Double) -> Double {
    let b: Double = (domain1 - domain0) != 0 ? domain1 - domain0 : 1 / domain1;
    return (x - domain0) / b
}

func rescale( x: Double )  -> Double {
    return interpolate( uninterpolate(x) )
}
}

Usage:

let scaleUp = Rescale( domain0: 15, domain1: 85, range0: 0, range1: 100 )
let scaleDown = Rescale( domain0: 0, domain1: 100, range0: 15, range1: 85 )
// scales it to 0 - 100 from the input range 15 - 85
let scaledValue = scaleUp.rescale( 85 ) // = 100
// or the other way...
let scaledValue = scaleDown.rescale( 100 ) // = 85