Sprite Kit Easing Curves

import SpriteKit

struct ActionTimingFunction {

    private static let pi : Float = 3.14159
    private static let pi2 : Float = pi / 2.0

    static func linear (t : Float) -> Float{
        return t
    }

    static func easeInQuad(t : Float) -> Float{
        return t * t
    }

    static func easeOutQuad(t : Float) -> Float{
        return t * (2.0 - t)
    }

    static func easeInOutQuad(t : Float) -> Float{
        if (t < 0.5) {
            return 2.0 * t * t
        } else {
            let f = t - 1.0
            return 1.0 - 2.0 * f * f
        }
    }

    static func easeInCubic(t : Float) -> Float{
        return t * t * t
    }

    static func easeOutCubic(t : Float) -> Float{
        let f = t - 1.0
        return 1.0 + f * f * f
    }

    static func easeInOutCubic(t : Float) -> Float{
        if (t < 0.5) {
            return 4.0 * t * t * t
        } else {
            let f = t - 1.0
            return 1.0 + 4.0 * f * f * f
        }
    }

    static func easeInQuart(t : Float) -> Float{
        return t * t * t * t
    }

    static func easeOutQuart(t : Float) -> Float{
        let f = t - 1.0
        return 1.0 - f * f * f * f
    }

    static func easeInOutQuart(t : Float) -> Float{
        if (t < 0.5) {
            return 8.0 * t * t * t * t
        } else {
            let f = t - 1.0
            return 1.0 - 8.0 * f * f * f * f
        }
    }

    static func easeInQuint(t : Float) -> Float{
        return t * t * t * t * t
    }

    static func easeOutQuint(t : Float) -> Float{
        let f = t - 1.0
        return 1.0 + f * f * f * f * f
    }

    static func easeInOutQuint(t : Float) -> Float{
        if (t < 0.5) {
            return 16.0 * t * t * t * t * t
        } else {
            let f = t - 1.0
            return 1.0 + 16.0 * f * f * f * f * f
        }
    }

    static func easeInSine(t : Float) -> Float{
        return sinf((t - 1.0) * pi2) + 1.0
    }

    static func easeOutSine(t : Float) -> Float{
        return sinf(t * pi2)
    }

    static func easeInOutSine(t : Float) -> Float{
        return 0.5 * (1.0 - cosf(t * pi))
    }

    static func easeInCirc(t : Float) -> Float{
        return 1.0 - sqrtf(1.0 - t * t)
    }

    static func easeOutCirc(t : Float) -> Float{
        return sqrtf((2.0 - t) * t)
    }

    static func easeInOutCirc(t : Float) -> Float{
        if (t < 0.5) {
            return 0.5 * (1.0 - sqrtf(1.0 - 4.0 * t * t))
        } else {
            return 0.5 * sqrt(-4.0 * t * t + 8.0 * t - 3.0) + 0.5
        }
    }

    static func easeInExpo(t : Float) -> Float{
        return (t == 0.0) ? t : powf(2.0, 10.0 * (t - 1.0))
    }

    static func easeOutExpo(t : Float) -> Float{
        return (t == 1.0) ? t : 1.0 - powf(2.0, -10.0 * t)
    }

    static func easeInOutExpo(t : Float) -> Float{
        if (t == 0.0 || t == 1.0) {
            return t
        } else if (t < 0.5) {
            return 0.5 * powf(2.0, 20.0 * t - 10.0)
        } else {
            return 1.0 - 0.5 * powf(2.0, -20.0 * t + 10.0)
        }
    }

    static func easeInElastic(t : Float) -> Float{
        return sin(13.0 * pi2 * t) * powf(2.0, 10.0 * (t - 1.0))
    }

    static func easeOutElastic(t : Float) -> Float{
        return sinf(-13.0 * pi2 * (t + 1.0)) * powf(2.0, -10.0 * t) + 1.0
    }

    static func easeInOutElastic(t : Float) -> Float{
        if (t < 0.5) {
            return 0.5 * sinf(13.0 * pi * t) * powf(2.0, 20.0 * t - 10.0)
        } else {
            return 0.5 * sinf(-13.0 * pi * t) * powf(2.0, -20.0 * t + 10.0) + 1.0
        }
    }

    private static let s : Float = 1.70158

    static func easeInBack(t : Float) -> Float{
        return ((s + 1.0) * t - s) * t * t
    }

    static func easeOutBack(t : Float) -> Float{
        let f = 1.0 - t
        let a =  1.0 - ((s + 1.0) * f - s) * f * f
        return a
    }

    static func easeInOutBack(t : Float) -> Float{
        if (t < 0.5) {
            let f = 2.0 * t
            return 0.5 * ((s + 1.0) * f - s) * f * f
        } else {
            let f = 2.0 * (1.0 - t)
            return 1.0 - 0.5 * ((s + 1.0) * f - s) * f * f
        }
    }

    static func easeInBackExtreme(t : Float) -> Float{
        return (t * t - sinf(t * pi)) * t
    }

    static func easeOutBackExtreme(t : Float) -> Float{
        let f = 1.0 - t
        return 1.0 - (f * f - sinf(f * pi)) * f
    }

    static func easeInOutBackExtreme(t : Float) -> Float{
        if (t < 0.5) {
            let f = 2.0 * t
            return 0.5 * (f * f - sinf(f * pi)) * f
        } else {
            let f = 2.0 * (1.0 - t)
            return 1.0 - 0.5 * (f * f - sinf(f * pi)) * f
        }
    }

    static func easeInBounce(t : Float) -> Float{
        return 1.0 - easeOutBounce(1.0 - t)
    }

    static func easeOutBounce(t : Float) -> Float{
        let m : Float = 7.5625
        if (t < 1.0 / 2.75) {
            return m * t * t
        } else if (t < 2.0 / 2.75) {
            let t1 = t - 1.5 / 2.75
            return m * t1 * t1 + 0.75
        } else if (t < 2.5 / 2.75) {
            let t1 = t - 2.25 / 2.75
            return m * t1 * t1 + 0.9375
        } else {
            let t1 = t - 2.625 / 2.75
            return m * t1 * t1 + 0.984375
        }
    }

    static func easeInOutBounce(t : Float) -> Float{
        if (t < 0.5) {
            return 0.5 * easeInBounce(t * 2.0)
        } else {
            return 0.5 * easeOutBounce(t * 2.0 - 1.0) + 0.5
        }
    }

    static func smoothStep(t : Float) -> Float{
        return t * t * (3 - 2 * t)
    }
}

Adapted from SKTTimingFunctions.m to work with SKAction.timingFunction

let action = SKAction.scaleTo(2.0, duration:1.0)
action.timingFunction = ActionTimingFunction.easeOutBack

Unfortunately this is broken as actions stop executing as soon as a timing function returns >= 1

Timing functions for SKTEffects. Based on Robert Penner's easing equations
http://robertpenner.com/easing/ and https://github.com/warrenm/AHEasing

Copyright (c) 2013 Razeware LLC

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