interpolation.swift

import CoreGraphics

public struct Interpolation<Value> {
    private let interpolateFunc: (from: Value, to: Value, at: CGFloat) -> Value

    public init(interpolate interpolateFunc: (from: Value, to: Value, at: CGFloat) -> Value) {
        self.interpolateFunc = interpolateFunc
    }

    public func interpolate(from: Value, to: Value, at: CGFloat) -> Value {
        return interpolateFunc(from: from, to: to, at: at)
    }
}

public protocol Interpolatable {
    typealias ValueType

    func interpolation() -> Interpolation<ValueType>
}

extension Interpolation: Interpolatable {
    public func interpolation() -> Interpolation<Value> {
        return self
    }
}

extension CGFloat: Interpolatable {
    public func interpolation() -> Interpolation<CGFloat> {
        return Interpolation { from, to, at in
            return (to - from) * at + from
        }
    }
}

public struct Keyframe<Value> {
    public var time: CGFloat
    public var value: Value
    public var interpolation: Interpolation<Value>

    public init<I: Interpolatable where I.ValueType == Value>(time: CGFloat, value: Value, interpolation: I) {
        self.time = time
        self.value = value
        self.interpolation = interpolation.interpolation()
    }
}

public extension Keyframe {
    public init(time: CGFloat, value: Value, interpolate: (from: Value, to: Value, at: CGFloat) -> Value) {
        self.init(time: time, value: value, interpolation: Interpolation(interpolate: interpolate))
    }
}

public extension Keyframe where Value: Interpolatable, Value.ValueType == Value {
    public init(time: CGFloat, value: Value) {
        self.init(time: time, value: value, interpolation: value)
    }
}

public func inbetween<Value>(start start: Keyframe<Value>, end: Keyframe<Value>) -> (at: CGFloat) -> Value {
    return { at in
        return end.interpolation.interpolate(start.value, to: end.value, at: at)
    }
}

do {
    let start = Keyframe<CGFloat>(time: 0, value: 0)
    let end = Keyframe<CGFloat>(time: 1, value: 1)

    inbetween(start: start, end: end)(at: 0.5) // 0.5
}

do {
    let start = Keyframe<CGFloat>(time: 0, value: 0)
    let end = Keyframe<CGFloat>(time: 1, value: 1) { from, to, at in
        return (to - from) * at / 2 + from
    }

    inbetween(start: start, end: end)(at: 0.5) // 0.25
}