Easing Functions for Unity3D

EasingFunctions.cs

/*
 * Created by C.J. Kimberlin
 * 
 * The MIT License (MIT)
 * 
 * Copyright (c) 2019
 * 
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 * 
 * The above copyright notice and this permission notice shall be included in all
 * copies or substantial portions of the Software.
 * 
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 * SOFTWARE.
 * 
 * 
 * TERMS OF USE - EASING EQUATIONS
 * Open source under the BSD License.
 * Copyright (c)2001 Robert Penner
 * All rights reserved.
 * Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:
 * Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer.
 * Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution.
 * Neither the name of the author nor the names of contributors may be used to endorse or promote products derived from this software without specific prior written permission.
 * 
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, 
 * THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE 
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; 
 * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT 
 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 *
 *
 * ============= Description =============
 *
 * Below is an example of how to use the easing functions in the file. There is a getting function that will return the function
 * from an enum. This is useful since the enum can be exposed in the editor and then the function queried during Start().
 * 
 * EasingFunction.Ease ease = EasingFunction.Ease.EaseInOutQuad;
 * EasingFunction.EasingFunc func = GetEasingFunction(ease;
 * 
 * float value = func(0, 10, 0.67f);
 * 
 * EasingFunction.EaseingFunc derivativeFunc = GetEasingFunctionDerivative(ease);
 * 
 * float derivativeValue = derivativeFunc(0, 10, 0.67f);
 */

using UnityEngine;

public static class EasingFunction
{
    public enum Ease
    {
        EaseInQuad = 0,
        EaseOutQuad,
        EaseInOutQuad,
        EaseInCubic,
        EaseOutCubic,
        EaseInOutCubic,
        EaseInQuart,
        EaseOutQuart,
        EaseInOutQuart,
        EaseInQuint,
        EaseOutQuint,
        EaseInOutQuint,
        EaseInSine,
        EaseOutSine,
        EaseInOutSine,
        EaseInExpo,
        EaseOutExpo,
        EaseInOutExpo,
        EaseInCirc,
        EaseOutCirc,
        EaseInOutCirc,
        Linear,
        Spring,
        EaseInBounce,
        EaseOutBounce,
        EaseInOutBounce,
        EaseInBack,
        EaseOutBack,
        EaseInOutBack,
        EaseInElastic,
        EaseOutElastic,
        EaseInOutElastic,
    }

    private const float NATURAL_LOG_OF_2 = 0.693147181f;

    //
    // Easing functions
    //

    public static float Linear(float start, float end, float value)
    {
        return Mathf.Lerp(start, end, value);
    }

    public static float Spring(float start, float end, float value)
    {
        value = Mathf.Clamp01(value);
        value = (Mathf.Sin(value * Mathf.PI * (0.2f + 2.5f * value * value * value)) * Mathf.Pow(1f - value, 2.2f) + value) * (1f + (1.2f * (1f - value)));
        return start + (end - start) * value;
    }

    public static float EaseInQuad(float start, float end, float value)
    {
        end -= start;
        return end * value * value + start;
    }

    public static float EaseOutQuad(float start, float end, float value)
    {
        end -= start;
        return -end * value * (value - 2) + start;
    }

    public static float EaseInOutQuad(float start, float end, float value)
    {
        value /= .5f;
        end -= start;
        if (value < 1) return end * 0.5f * value * value + start;
        value--;
        return -end * 0.5f * (value * (value - 2) - 1) + start;
    }

    public static float EaseInCubic(float start, float end, float value)
    {
        end -= start;
        return end * value * value * value + start;
    }

    public static float EaseOutCubic(float start, float end, float value)
    {
        value--;
        end -= start;
        return end * (value * value * value + 1) + start;
    }

    public static float EaseInOutCubic(float start, float end, float value)
    {
        value /= .5f;
        end -= start;
        if (value < 1) return end * 0.5f * value * value * value + start;
        value -= 2;
        return end * 0.5f * (value * value * value + 2) + start;
    }

    public static float EaseInQuart(float start, float end, float value)
    {
        end -= start;
        return end * value * value * value * value + start;
    }

    public static float EaseOutQuart(float start, float end, float value)
    {
        value--;
        end -= start;
        return -end * (value * value * value * value - 1) + start;
    }

    public static float EaseInOutQuart(float start, float end, float value)
    {
        value /= .5f;
        end -= start;
        if (value < 1) return end * 0.5f * value * value * value * value + start;
        value -= 2;
        return -end * 0.5f * (value * value * value * value - 2) + start;
    }

    public static float EaseInQuint(float start, float end, float value)
    {
        end -= start;
        return end * value * value * value * value * value + start;
    }

    public static float EaseOutQuint(float start, float end, float value)
    {
        value--;
        end -= start;
        return end * (value * value * value * value * value + 1) + start;
    }

    public static float EaseInOutQuint(float start, float end, float value)
    {
        value /= .5f;
        end -= start;
        if (value < 1) return end * 0.5f * value * value * value * value * value + start;
        value -= 2;
        return end * 0.5f * (value * value * value * value * value + 2) + start;
    }

    public static float EaseInSine(float start, float end, float value)
    {
        end -= start;
        return -end * Mathf.Cos(value * (Mathf.PI * 0.5f)) + end + start;
    }

    public static float EaseOutSine(float start, float end, float value)
    {
        end -= start;
        return end * Mathf.Sin(value * (Mathf.PI * 0.5f)) + start;
    }

    public static float EaseInOutSine(float start, float end, float value)
    {
        end -= start;
        return -end * 0.5f * (Mathf.Cos(Mathf.PI * value) - 1) + start;
    }

    public static float EaseInExpo(float start, float end, float value)
    {
        end -= start;
        return end * Mathf.Pow(2, 10 * (value - 1)) + start;
    }

    public static float EaseOutExpo(float start, float end, float value)
    {
        end -= start;
        return end * (-Mathf.Pow(2, -10 * value) + 1) + start;
    }

    public static float EaseInOutExpo(float start, float end, float value)
    {
        value /= .5f;
        end -= start;
        if (value < 1) return end * 0.5f * Mathf.Pow(2, 10 * (value - 1)) + start;
        value--;
        return end * 0.5f * (-Mathf.Pow(2, -10 * value) + 2) + start;
    }

    public static float EaseInCirc(float start, float end, float value)
    {
        end -= start;
        return -end * (Mathf.Sqrt(1 - value * value) - 1) + start;
    }

    public static float EaseOutCirc(float start, float end, float value)
    {
        value--;
        end -= start;
        return end * Mathf.Sqrt(1 - value * value) + start;
    }

    public static float EaseInOutCirc(float start, float end, float value)
    {
        value /= .5f;
        end -= start;
        if (value < 1) return -end * 0.5f * (Mathf.Sqrt(1 - value * value) - 1) + start;
        value -= 2;
        return end * 0.5f * (Mathf.Sqrt(1 - value * value) + 1) + start;
    }

    public static float EaseInBounce(float start, float end, float value)
    {
        end -= start;
        float d = 1f;
        return end - EaseOutBounce(0, end, d - value) + start;
    }

    public static float EaseOutBounce(float start, float end, float value)
    {
        value /= 1f;
        end -= start;
        if (value < (1 / 2.75f))
        {
            return end * (7.5625f * value * value) + start;
        }
        else if (value < (2 / 2.75f))
        {
            value -= (1.5f / 2.75f);
            return end * (7.5625f * (value) * value + .75f) + start;
        }
        else if (value < (2.5 / 2.75))
        {
            value -= (2.25f / 2.75f);
            return end * (7.5625f * (value) * value + .9375f) + start;
        }
        else
        {
            value -= (2.625f / 2.75f);
            return end * (7.5625f * (value) * value + .984375f) + start;
        }
    }

    public static float EaseInOutBounce(float start, float end, float value)
    {
        end -= start;
        float d = 1f;
        if (value < d * 0.5f) return EaseInBounce(0, end, value * 2) * 0.5f + start;
        else return EaseOutBounce(0, end, value * 2 - d) * 0.5f + end * 0.5f + start;
    }

    public static float EaseInBack(float start, float end, float value)
    {
        end -= start;
        value /= 1;
        float s = 1.70158f;
        return end * (value) * value * ((s + 1) * value - s) + start;
    }

    public static float EaseOutBack(float start, float end, float value)
    {
        float s = 1.70158f;
        end -= start;
        value = (value) - 1;
        return end * ((value) * value * ((s + 1) * value + s) + 1) + start;
    }

    public static float EaseInOutBack(float start, float end, float value)
    {
        float s = 1.70158f;
        end -= start;
        value /= .5f;
        if ((value) < 1)
        {
            s *= (1.525f);
            return end * 0.5f * (value * value * (((s) + 1) * value - s)) + start;
        }
        value -= 2;
        s *= (1.525f);
        return end * 0.5f * ((value) * value * (((s) + 1) * value + s) + 2) + start;
    }

    public static float EaseInElastic(float start, float end, float value)
    {
        end -= start;

        float d = 1f;
        float p = d * .3f;
        float s;
        float a = 0;

        if (value == 0) return start;

        if ((value /= d) == 1) return start + end;

        if (a == 0f || a < Mathf.Abs(end))
        {
            a = end;
            s = p / 4;
        }
        else
        {
            s = p / (2 * Mathf.PI) * Mathf.Asin(end / a);
        }

        return -(a * Mathf.Pow(2, 10 * (value -= 1)) * Mathf.Sin((value * d - s) * (2 * Mathf.PI) / p)) + start;
    }

    public static float EaseOutElastic(float start, float end, float value)
    {
        end -= start;

        float d = 1f;
        float p = d * .3f;
        float s;
        float a = 0;

        if (value == 0) return start;

        if ((value /= d) == 1) return start + end;

        if (a == 0f || a < Mathf.Abs(end))
        {
            a = end;
            s = p * 0.25f;
        }
        else
        {
            s = p / (2 * Mathf.PI) * Mathf.Asin(end / a);
        }

        return (a * Mathf.Pow(2, -10 * value) * Mathf.Sin((value * d - s) * (2 * Mathf.PI) / p) + end + start);
    }

    public static float EaseInOutElastic(float start, float end, float value)
    {
        end -= start;

        float d = 1f;
        float p = d * .3f;
        float s;
        float a = 0;

        if (value == 0) return start;

        if ((value /= d * 0.5f) == 2) return start + end;

        if (a == 0f || a < Mathf.Abs(end))
        {
            a = end;
            s = p / 4;
        }
        else
        {
            s = p / (2 * Mathf.PI) * Mathf.Asin(end / a);
        }

        if (value < 1) return -0.5f * (a * Mathf.Pow(2, 10 * (value -= 1)) * Mathf.Sin((value * d - s) * (2 * Mathf.PI) / p)) + start;
        return a * Mathf.Pow(2, -10 * (value -= 1)) * Mathf.Sin((value * d - s) * (2 * Mathf.PI) / p) * 0.5f + end + start;
    }

    //
    // These are derived functions that the motor can use to get the speed at a specific time.
    //
    // The easing functions all work with a normalized time (0 to 1) and the returned value here
    // reflects that. Values returned here should be divided by the actual time.
    //
    // TODO: These functions have not had the testing they deserve. If there is odd behavior around
    //       dash speeds then this would be the first place I'd look.

    public static float LinearD(float start, float end, float value)
    {
        return end - start;
    }

    public static float EaseInQuadD(float start, float end, float value)
    {
        return 2f * (end - start) * value;
    }

    public static float EaseOutQuadD(float start, float end, float value)
    {
        end -= start;
        return -end * value - end * (value - 2);
    }

    public static float EaseInOutQuadD(float start, float end, float value)
    {
        value /= .5f;
        end -= start;

        if (value < 1)
        {
            return end * value;
        }

        value--;

        return end * (1 - value);
    }

    public static float EaseInCubicD(float start, float end, float value)
    {
        return 3f * (end - start) * value * value;
    }

    public static float EaseOutCubicD(float start, float end, float value)
    {
        value--;
        end -= start;
        return 3f * end * value * value;
    }

    public static float EaseInOutCubicD(float start, float end, float value)
    {
        value /= .5f;
        end -= start;

        if (value < 1)
        {
            return (3f / 2f) * end * value * value;
        }

        value -= 2;

        return (3f / 2f) * end * value * value;
    }

    public static float EaseInQuartD(float start, float end, float value)
    {
        return 4f * (end - start) * value * value * value;
    }

    public static float EaseOutQuartD(float start, float end, float value)
    {
        value--;
        end -= start;
        return -4f * end * value * value * value;
    }

    public static float EaseInOutQuartD(float start, float end, float value)
    {
        value /= .5f;
        end -= start;

        if (value < 1)
        {
            return 2f * end * value * value * value;
        }

        value -= 2;

        return -2f * end * value * value * value;
    }

    public static float EaseInQuintD(float start, float end, float value)
    {
        return 5f * (end - start) * value * value * value * value;
    }

    public static float EaseOutQuintD(float start, float end, float value)
    {
        value--;
        end -= start;
        return 5f * end * value * value * value * value;
    }

    public static float EaseInOutQuintD(float start, float end, float value)
    {
        value /= .5f;
        end -= start;

        if (value < 1)
        {
            return (5f / 2f) * end * value * value * value * value;
        }

        value -= 2;

        return (5f / 2f) * end * value * value * value * value;
    }

    public static float EaseInSineD(float start, float end, float value)
    {
        return (end - start) * 0.5f * Mathf.PI * Mathf.Sin(0.5f * Mathf.PI * value);
    }

    public static float EaseOutSineD(float start, float end, float value)
    {
        end -= start;
        return (Mathf.PI * 0.5f) * end * Mathf.Cos(value * (Mathf.PI * 0.5f));
    }

    public static float EaseInOutSineD(float start, float end, float value)
    {
        end -= start;
        return end * 0.5f * Mathf.PI * Mathf.Sin(Mathf.PI * value);
    }
    public static float EaseInExpoD(float start, float end, float value)
    {
        return (10f * NATURAL_LOG_OF_2 * (end - start) * Mathf.Pow(2f, 10f * (value - 1)));
    }

    public static float EaseOutExpoD(float start, float end, float value)
    {
        end -= start;
        return 5f * NATURAL_LOG_OF_2 * end * Mathf.Pow(2f, 1f - 10f * value);
    }

    public static float EaseInOutExpoD(float start, float end, float value)
    {
        value /= .5f;
        end -= start;

        if (value < 1)
        {
            return 5f * NATURAL_LOG_OF_2 * end * Mathf.Pow(2f, 10f * (value - 1));
        }

        value--;

        return (5f * NATURAL_LOG_OF_2 * end) / (Mathf.Pow(2f, 10f * value));
    }

    public static float EaseInCircD(float start, float end, float value)
    {
        return ((end - start) * value) / Mathf.Sqrt(1f - value * value);
    }

    public static float EaseOutCircD(float start, float end, float value)
    {
        value--;
        end -= start;
        return (-end * value) / Mathf.Sqrt(1f - value * value);
    }

    public static float EaseInOutCircD(float start, float end, float value)
    {
        value /= .5f;
        end -= start;

        if (value < 1)
        {
            return (end * value) / (2f * Mathf.Sqrt(1f - value * value));
        }

        value -= 2;

        return (-end * value) / (2f * Mathf.Sqrt(1f - value * value));
    }

    public static float EaseInBounceD(float start, float end, float value)
    {
        end -= start;
        float d = 1f;

        return EaseOutBounceD(0, end, d - value);
    }

    public static float EaseOutBounceD(float start, float end, float value)
    {
        value /= 1f;
        end -= start;

        if (value < (1 / 2.75f))
        {
            return 2f * end * 7.5625f * value;
        }
        else if (value < (2 / 2.75f))
        {
            value -= (1.5f / 2.75f);
            return 2f * end * 7.5625f * value;
        }
        else if (value < (2.5 / 2.75))
        {
            value -= (2.25f / 2.75f);
            return 2f * end * 7.5625f * value;
        }
        else
        {
            value -= (2.625f / 2.75f);
            return 2f * end * 7.5625f * value;
        }
    }

    public static float EaseInOutBounceD(float start, float end, float value)
    {
        end -= start;
        float d = 1f;

        if (value < d * 0.5f)
        {
            return EaseInBounceD(0, end, value * 2) * 0.5f;
        }
        else
        {
            return EaseOutBounceD(0, end, value * 2 - d) * 0.5f;
        }
    }

    public static float EaseInBackD(float start, float end, float value)
    {
        float s = 1.70158f;

        return 3f * (s + 1f) * (end - start) * value * value - 2f * s * (end - start) * value;
    }

    public static float EaseOutBackD(float start, float end, float value)
    {
        float s = 1.70158f;
        end -= start;
        value = (value) - 1;

        return end * ((s + 1f) * value * value + 2f * value * ((s + 1f) * value + s));
    }

    public static float EaseInOutBackD(float start, float end, float value)
    {
        float s = 1.70158f;
        end -= start;
        value /= .5f;

        if ((value) < 1)
        {
            s *= (1.525f);
            return 0.5f * end * (s + 1) * value * value + end * value * ((s + 1f) * value - s);
        }

        value -= 2;
        s *= (1.525f);
        return 0.5f * end * ((s + 1) * value * value + 2f * value * ((s + 1f) * value + s));
    }

    public static float EaseInElasticD(float start, float end, float value)
    {
        return EaseOutElasticD(start, end, 1f - value);
    }

    public static float EaseOutElasticD(float start, float end, float value)
    {
        end -= start;

        float d = 1f;
        float p = d * .3f;
        float s;
        float a = 0;

        if (a == 0f || a < Mathf.Abs(end))
        {
            a = end;
            s = p * 0.25f;
        }
        else
        {
            s = p / (2 * Mathf.PI) * Mathf.Asin(end / a);
        }

        return (a * Mathf.PI * d * Mathf.Pow(2f, 1f - 10f * value) *
            Mathf.Cos((2f * Mathf.PI * (d * value - s)) / p)) / p - 5f * NATURAL_LOG_OF_2 * a *
            Mathf.Pow(2f, 1f - 10f * value) * Mathf.Sin((2f * Mathf.PI * (d * value - s)) / p);
    }

    public static float EaseInOutElasticD(float start, float end, float value)
    {
        end -= start;

        float d = 1f;
        float p = d * .3f;
        float s;
        float a = 0;

        if (a == 0f || a < Mathf.Abs(end))
        {
            a = end;
            s = p / 4;
        }
        else
        {
            s = p / (2 * Mathf.PI) * Mathf.Asin(end / a);
        }

        if (value < 1)
        {
            value -= 1;

            return -5f * NATURAL_LOG_OF_2 * a * Mathf.Pow(2f, 10f * value) * Mathf.Sin(2 * Mathf.PI * (d * value - 2f) / p) -
                a * Mathf.PI * d * Mathf.Pow(2f, 10f * value) * Mathf.Cos(2 * Mathf.PI * (d * value - s) / p) / p;
        }

        value -= 1;

        return a * Mathf.PI * d * Mathf.Cos(2f * Mathf.PI * (d * value - s) / p) / (p * Mathf.Pow(2f, 10f * value)) -
            5f * NATURAL_LOG_OF_2 * a * Mathf.Sin(2f * Mathf.PI * (d * value - s) / p) / (Mathf.Pow(2f, 10f * value));
    }

    public static float SpringD(float start, float end, float value)
    {
        value = Mathf.Clamp01(value);
        end -= start;

        // Damn... Thanks http://www.derivative-calculator.net/
        // TODO: And it's a little bit wrong
        return end * (6f * (1f - value) / 5f + 1f) * (-2.2f * Mathf.Pow(1f - value, 1.2f) *
            Mathf.Sin(Mathf.PI * value * (2.5f * value * value * value + 0.2f)) + Mathf.Pow(1f - value, 2.2f) *
            (Mathf.PI * (2.5f * value * value * value + 0.2f) + 7.5f * Mathf.PI * value * value * value) *
            Mathf.Cos(Mathf.PI * value * (2.5f * value * value * value + 0.2f)) + 1f) -
            6f * end * (Mathf.Pow(1 - value, 2.2f) * Mathf.Sin(Mathf.PI * value * (2.5f * value * value * value + 0.2f)) + value
            / 5f);

    }

    public delegate float Function(float s, float e, float v);

    /// <summary>
    /// Returns the function associated to the easingFunction enum. This value returned should be cached as it allocates memory
    /// to return.
    /// </summary>
    /// <param name="easingFunction">The enum associated with the easing function.</param>
    /// <returns>The easing function</returns>
    public static Function GetEasingFunction(Ease easingFunction)
    {
        if (easingFunction == Ease.EaseInQuad)
        {
            return EaseInQuad;
        }

        if (easingFunction == Ease.EaseOutQuad)
        {
            return EaseOutQuad;
        }

        if (easingFunction == Ease.EaseInOutQuad)
        {
            return EaseInOutQuad;
        }

        if (easingFunction == Ease.EaseInCubic)
        {
            return EaseInCubic;
        }

        if (easingFunction == Ease.EaseOutCubic)
        {
            return EaseOutCubic;
        }

        if (easingFunction == Ease.EaseInOutCubic)
        {
            return EaseInOutCubic;
        }

        if (easingFunction == Ease.EaseInQuart)
        {
            return EaseInQuart;
        }

        if (easingFunction == Ease.EaseOutQuart)
        {
            return EaseOutQuart;
        }

        if (easingFunction == Ease.EaseInOutQuart)
        {
            return EaseInOutQuart;
        }

        if (easingFunction == Ease.EaseInQuint)
        {
            return EaseInQuint;
        }

        if (easingFunction == Ease.EaseOutQuint)
        {
            return EaseOutQuint;
        }

        if (easingFunction == Ease.EaseInOutQuint)
        {
            return EaseInOutQuint;
        }

        if (easingFunction == Ease.EaseInSine)
        {
            return EaseInSine;
        }

        if (easingFunction == Ease.EaseOutSine)
        {
            return EaseOutSine;
        }

        if (easingFunction == Ease.EaseInOutSine)
        {
            return EaseInOutSine;
        }

        if (easingFunction == Ease.EaseInExpo)
        {
            return EaseInExpo;
        }

        if (easingFunction == Ease.EaseOutExpo)
        {
            return EaseOutExpo;
        }

        if (easingFunction == Ease.EaseInOutExpo)
        {
            return EaseInOutExpo;
        }

        if (easingFunction == Ease.EaseInCirc)
        {
            return EaseInCirc;
        }

        if (easingFunction == Ease.EaseOutCirc)
        {
            return EaseOutCirc;
        }

        if (easingFunction == Ease.EaseInOutCirc)
        {
            return EaseInOutCirc;
        }

        if (easingFunction == Ease.Linear)
        {
            return Linear;
        }

        if (easingFunction == Ease.Spring)
        {
            return Spring;
        }

        if (easingFunction == Ease.EaseInBounce)
        {
            return EaseInBounce;
        }

        if (easingFunction == Ease.EaseOutBounce)
        {
            return EaseOutBounce;
        }

        if (easingFunction == Ease.EaseInOutBounce)
        {
            return EaseInOutBounce;
        }

        if (easingFunction == Ease.EaseInBack)
        {
            return EaseInBack;
        }

        if (easingFunction == Ease.EaseOutBack)
        {
            return EaseOutBack;
        }

        if (easingFunction == Ease.EaseInOutBack)
        {
            return EaseInOutBack;
        }

        if (easingFunction == Ease.EaseInElastic)
        {
            return EaseInElastic;
        }

        if (easingFunction == Ease.EaseOutElastic)
        {
            return EaseOutElastic;
        }

        if (easingFunction == Ease.EaseInOutElastic)
        {
            return EaseInOutElastic;
        }

        return null;
    }

    /// <summary>
    /// Gets the derivative function of the appropriate easing function. If you use an easing function for position then this
    /// function can get you the speed at a given time (normalized).
    /// </summary>
    /// <param name="easingFunction"></param>
    /// <returns>The derivative function</returns>
    public static Function GetEasingFunctionDerivative(Ease easingFunction)
    {
        if (easingFunction == Ease.EaseInQuad)
        {
            return EaseInQuadD;
        }

        if (easingFunction == Ease.EaseOutQuad)
        {
            return EaseOutQuadD;
        }

        if (easingFunction == Ease.EaseInOutQuad)
        {
            return EaseInOutQuadD;
        }

        if (easingFunction == Ease.EaseInCubic)
        {
            return EaseInCubicD;
        }

        if (easingFunction == Ease.EaseOutCubic)
        {
            return EaseOutCubicD;
        }

        if (easingFunction == Ease.EaseInOutCubic)
        {
            return EaseInOutCubicD;
        }

        if (easingFunction == Ease.EaseInQuart)
        {
            return EaseInQuartD;
        }

        if (easingFunction == Ease.EaseOutQuart)
        {
            return EaseOutQuartD;
        }

        if (easingFunction == Ease.EaseInOutQuart)
        {
            return EaseInOutQuartD;
        }

        if (easingFunction == Ease.EaseInQuint)
        {
            return EaseInQuintD;
        }

        if (easingFunction == Ease.EaseOutQuint)
        {
            return EaseOutQuintD;
        }

        if (easingFunction == Ease.EaseInOutQuint)
        {
            return EaseInOutQuintD;
        }

        if (easingFunction == Ease.EaseInSine)
        {
            return EaseInSineD;
        }

        if (easingFunction == Ease.EaseOutSine)
        {
            return EaseOutSineD;
        }

        if (easingFunction == Ease.EaseInOutSine)
        {
            return EaseInOutSineD;
        }

        if (easingFunction == Ease.EaseInExpo)
        {
            return EaseInExpoD;
        }

        if (easingFunction == Ease.EaseOutExpo)
        {
            return EaseOutExpoD;
        }

        if (easingFunction == Ease.EaseInOutExpo)
        {
            return EaseInOutExpoD;
        }

        if (easingFunction == Ease.EaseInCirc)
        {
            return EaseInCircD;
        }

        if (easingFunction == Ease.EaseOutCirc)
        {
            return EaseOutCircD;
        }

        if (easingFunction == Ease.EaseInOutCirc)
        {
            return EaseInOutCircD;
        }

        if (easingFunction == Ease.Linear)
        {
            return LinearD;
        }

        if (easingFunction == Ease.Spring)
        {
            return SpringD;
        }

        if (easingFunction == Ease.EaseInBounce)
        {
            return EaseInBounceD;
        }

        if (easingFunction == Ease.EaseOutBounce)
        {
            return EaseOutBounceD;
        }

        if (easingFunction == Ease.EaseInOutBounce)
        {
            return EaseInOutBounceD;
        }

        if (easingFunction == Ease.EaseInBack)
        {
            return EaseInBackD;
        }

        if (easingFunction == Ease.EaseOutBack)
        {
            return EaseOutBackD;
        }

        if (easingFunction == Ease.EaseInOutBack)
        {
            return EaseInOutBackD;
        }

        if (easingFunction == Ease.EaseInElastic)
        {
            return EaseInElasticD;
        }

        if (easingFunction == Ease.EaseOutElastic)
        {
            return EaseOutElasticD;
        }

        if (easingFunction == Ease.EaseInOutElastic)
        {
            return EaseInOutElasticD;
        }

        return null;
    }
}

A couple optimizations

1. Cache function references to avoid repeated allocation
2. Could be the compiler would do this anyway, but using a switch statement is faster than doing a ton of compares on an enum.

/*
 * Created by C.J. Kimberlin
 *
 * The MIT License (MIT)
 *
 * Copyright (c) 2019
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in all
 * copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 * SOFTWARE.
 *
 *
 * TERMS OF USE - EASING EQUATIONS
 * Open source under the BSD License.
 * Copyright (c)2001 Robert Penner
 * All rights reserved.
 * Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:
 * Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer.
 * Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution.
 * Neither the name of the author nor the names of contributors may be used to endorse or promote products derived from this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
 * THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
 * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 *
 *
 * ============= Description =============
 *
 * Below is an example of how to use the easing functions in the file. There is a getting function that will return the function
 * from an enum. This is useful since the enum can be exposed in the editor and then the function queried during Start().
 *
 * EasingFunction.Ease ease = EasingFunction.Ease.EaseInOutQuad;
 * EasingFunction.EasingFunc func = GetEasingFunction(ease;
 *
 * float value = func(0, 10, 0.67f);
 *
 * EasingFunction.EaseingFunc derivativeFunc = GetEasingFunctionDerivative(ease);
 *
 * float derivativeValue = derivativeFunc(0, 10, 0.67f);
 */

using UnityEngine;

public static class EasingFunction
{
    #region Cached Function References
    static Function _linear;
    static Function _spring;
    static Function _easeInQuad;
    static Function _easeOutQuad;
    static Function _easeInOutQuad;
    static Function _easeInCubic;
    static Function _easeOutCubic;
    static Function _easeInOutCubic;
    static Function _easeInQuart;
    static Function _easeOutQuart;
    static Function _easeInOutQuart;
    static Function _easeInQuint;
    static Function _easeOutQuint;
    static Function _easeInOutQuint;
    static Function _easeInSine;
    static Function _easeOutSine;
    static Function _easeInOutSine;
    static Function _easeInExpo;
    static Function _easeOutExpo;
    static Function _easeInOutExpo;
    static Function _easeInCirc;
    static Function _easeOutCirc;
    static Function _easeInOutCirc;
    static Function _easeInBounce;
    static Function _easeOutBounce;
    static Function _easeInOutBounce;
    static Function _easeInBack;
    static Function _easeOutBack;
    static Function _easeInOutBack;
    static Function _easeInElastic;
    static Function _easeOutElastic;
    static Function _easeInOutElastic;
    static Function _linearD;
    static Function _easeInQuadD;
    static Function _easeOutQuadD;
    static Function _easeInOutQuadD;
    static Function _easeInCubicD;
    static Function _easeOutCubicD;
    static Function _easeInOutCubicD;
    static Function _easeInQuartD;
    static Function _easeOutQuartD;
    static Function _easeInOutQuartD;
    static Function _easeInQuintD;
    static Function _easeOutQuintD;
    static Function _easeInOutQuintD;
    static Function _easeInSineD;
    static Function _easeOutSineD;
    static Function _easeInOutSineD;
    static Function _easeInExpoD;
    static Function _easeOutExpoD;
    static Function _easeInOutExpoD;
    static Function _easeInCircD;
    static Function _easeOutCircD;
    static Function _easeInOutCircD;
    static Function _easeInBounceD;
    static Function _easeOutBounceD;
    static Function _easeInOutBounceD;
    static Function _easeInBackD;
    static Function _easeOutBackD;
    static Function _easeInOutBackD;
    static Function _easeInElasticD;
    static Function _easeOutElasticD;
    static Function _easeInOutElasticD;
    static Function _springD;
    #endregion

    static EasingFunction()
    {
        _linear = Linear;
        _spring = Spring;
        _easeInQuad = EaseInQuad;
        _easeOutQuad = EaseOutQuad;
        _easeInOutQuad = EaseInOutQuad;
        _easeInCubic = EaseInCubic;
        _easeOutCubic = EaseOutCubic;
        _easeInOutCubic = EaseInOutCubic;
        _easeInQuart = EaseInQuart;
        _easeOutQuart = EaseOutQuart;
        _easeInOutQuart = EaseInOutQuart;
        _easeInQuint = EaseInQuint;
        _easeOutQuint = EaseOutQuint;
        _easeInOutQuint = EaseInOutQuint;
        _easeInSine = EaseInSine;
        _easeOutSine = EaseOutSine;
        _easeInOutSine = EaseInOutSine;
        _easeInExpo = EaseInExpo;
        _easeOutExpo = EaseOutExpo;
        _easeInOutExpo = EaseInOutExpo;
        _easeInCirc = EaseInCirc;
        _easeOutCirc = EaseOutCirc;
        _easeInOutCirc = EaseInOutCirc;
        _easeInBounce = EaseInBounce;
        _easeOutBounce = EaseOutBounce;
        _easeInOutBounce = EaseInOutBounce;
        _easeInBack = EaseInBack;
        _easeOutBack = EaseOutBack;
        _easeInOutBack = EaseInOutBack;
        _easeInElastic = EaseInElastic;
        _easeOutElastic = EaseOutElastic;
        _easeInOutElastic = EaseInOutElastic;
        _linearD = LinearD;
        _easeInQuadD = EaseInQuadD;
        _easeOutQuadD = EaseOutQuadD;
        _easeInOutQuadD = EaseInOutQuadD;
        _easeInCubicD = EaseInCubicD;
        _easeOutCubicD = EaseOutCubicD;
        _easeInOutCubicD = EaseInOutCubicD;
        _easeInQuartD = EaseInQuartD;
        _easeOutQuartD = EaseOutQuartD;
        _easeInOutQuartD = EaseInOutQuartD;
        _easeInQuintD = EaseInQuintD;
        _easeOutQuintD = EaseOutQuintD;
        _easeInOutQuintD = EaseInOutQuintD;
        _easeInSineD = EaseInSineD;
        _easeOutSineD = EaseOutSineD;
        _easeInOutSineD = EaseInOutSineD;
        _easeInExpoD = EaseInExpoD;
        _easeOutExpoD = EaseOutExpoD;
        _easeInOutExpoD = EaseInOutExpoD;
        _easeInCircD = EaseInCircD;
        _easeOutCircD = EaseOutCircD;
        _easeInOutCircD = EaseInOutCircD;
        _easeInBounceD = EaseInBounceD;
        _easeOutBounceD = EaseOutBounceD;
        _easeInOutBounceD = EaseInOutBounceD;
        _easeInBackD = EaseInBackD;
        _easeOutBackD = EaseOutBackD;
        _easeInOutBackD = EaseInOutBackD;
        _easeInElasticD = EaseInElasticD;
        _easeOutElasticD = EaseOutElasticD;
        _easeInOutElasticD = EaseInOutElasticD;
        _springD = SpringD;
    }

    public enum Ease
    {
        EaseInQuad = 0,
        EaseOutQuad,
        EaseInOutQuad,
        EaseInCubic,
        EaseOutCubic,
        EaseInOutCubic,
        EaseInQuart,
        EaseOutQuart,
        EaseInOutQuart,
        EaseInQuint,
        EaseOutQuint,
        EaseInOutQuint,
        EaseInSine,
        EaseOutSine,
        EaseInOutSine,
        EaseInExpo,
        EaseOutExpo,
        EaseInOutExpo,
        EaseInCirc,
        EaseOutCirc,
        EaseInOutCirc,
        Linear,
        Spring,
        EaseInBounce,
        EaseOutBounce,
        EaseInOutBounce,
        EaseInBack,
        EaseOutBack,
        EaseInOutBack,
        EaseInElastic,
        EaseOutElastic,
        EaseInOutElastic,
    }

    private const float NATURAL_LOG_OF_2 = 0.693147181f;

    //
    // Easing functions
    //

    public static float Linear(float start, float end, float value)
    {
        return Mathf.Lerp(start, end, value);
    }

    public static float Spring(float start, float end, float value)
    {
        value = Mathf.Clamp01(value);
        value =
            (Mathf.Sin(value * Mathf.PI * (0.2f + 2.5f * value * value * value)) * Mathf.Pow(1f - value, 2.2f) + value)
            * (1f + (1.2f * (1f - value)));
        return start + (end - start) * value;
    }

    public static float EaseInQuad(float start, float end, float value)
    {
        end -= start;
        return end * value * value + start;
    }

    public static float EaseOutQuad(float start, float end, float value)
    {
        end -= start;
        return -end * value * (value - 2) + start;
    }

    public static float EaseInOutQuad(float start, float end, float value)
    {
        value /= .5f;
        end -= start;
        if (value < 1)
            return end * 0.5f * value * value + start;
        value--;
        return -end * 0.5f * (value * (value - 2) - 1) + start;
    }

    public static float EaseInCubic(float start, float end, float value)
    {
        end -= start;
        return end * value * value * value + start;
    }

    public static float EaseOutCubic(float start, float end, float value)
    {
        value--;
        end -= start;
        return end * (value * value * value + 1) + start;
    }

    public static float EaseInOutCubic(float start, float end, float value)
    {
        value /= .5f;
        end -= start;
        if (value < 1)
            return end * 0.5f * value * value * value + start;
        value -= 2;
        return end * 0.5f * (value * value * value + 2) + start;
    }

    public static float EaseInQuart(float start, float end, float value)
    {
        end -= start;
        return end * value * value * value * value + start;
    }

    public static float EaseOutQuart(float start, float end, float value)
    {
        value--;
        end -= start;
        return -end * (value * value * value * value - 1) + start;
    }

    public static float EaseInOutQuart(float start, float end, float value)
    {
        value /= .5f;
        end -= start;
        if (value < 1)
            return end * 0.5f * value * value * value * value + start;
        value -= 2;
        return -end * 0.5f * (value * value * value * value - 2) + start;
    }

    public static float EaseInQuint(float start, float end, float value)
    {
        end -= start;
        return end * value * value * value * value * value + start;
    }

    public static float EaseOutQuint(float start, float end, float value)
    {
        value--;
        end -= start;
        return end * (value * value * value * value * value + 1) + start;
    }

    public static float EaseInOutQuint(float start, float end, float value)
    {
        value /= .5f;
        end -= start;
        if (value < 1)
            return end * 0.5f * value * value * value * value * value + start;
        value -= 2;
        return end * 0.5f * (value * value * value * value * value + 2) + start;
    }

    public static float EaseInSine(float start, float end, float value)
    {
        end -= start;
        return -end * Mathf.Cos(value * (Mathf.PI * 0.5f)) + end + start;
    }

    public static float EaseOutSine(float start, float end, float value)
    {
        end -= start;
        return end * Mathf.Sin(value * (Mathf.PI * 0.5f)) + start;
    }

    public static float EaseInOutSine(float start, float end, float value)
    {
        end -= start;
        return -end * 0.5f * (Mathf.Cos(Mathf.PI * value) - 1) + start;
    }

    public static float EaseInExpo(float start, float end, float value)
    {
        end -= start;
        return end * Mathf.Pow(2, 10 * (value - 1)) + start;
    }

    public static float EaseOutExpo(float start, float end, float value)
    {
        end -= start;
        return end * (-Mathf.Pow(2, -10 * value) + 1) + start;
    }

    public static float EaseInOutExpo(float start, float end, float value)
    {
        value /= .5f;
        end -= start;
        if (value < 1)
            return end * 0.5f * Mathf.Pow(2, 10 * (value - 1)) + start;
        value--;
        return end * 0.5f * (-Mathf.Pow(2, -10 * value) + 2) + start;
    }

    public static float EaseInCirc(float start, float end, float value)
    {
        end -= start;
        return -end * (Mathf.Sqrt(1 - value * value) - 1) + start;
    }

    public static float EaseOutCirc(float start, float end, float value)
    {
        value--;
        end -= start;
        return end * Mathf.Sqrt(1 - value * value) + start;
    }

    public static float EaseInOutCirc(float start, float end, float value)
    {
        value /= .5f;
        end -= start;
        if (value < 1)
            return -end * 0.5f * (Mathf.Sqrt(1 - value * value) - 1) + start;
        value -= 2;
        return end * 0.5f * (Mathf.Sqrt(1 - value * value) + 1) + start;
    }

    public static float EaseInBounce(float start, float end, float value)
    {
        end -= start;
        float d = 1f;
        return end - EaseOutBounce(0, end, d - value) + start;
    }

    public static float EaseOutBounce(float start, float end, float value)
    {
        value /= 1f;
        end -= start;
        if (value < (1 / 2.75f))
        {
            return end * (7.5625f * value * value) + start;
        }
        else if (value < (2 / 2.75f))
        {
            value -= (1.5f / 2.75f);
            return end * (7.5625f * (value) * value + .75f) + start;
        }
        else if (value < (2.5 / 2.75))
        {
            value -= (2.25f / 2.75f);
            return end * (7.5625f * (value) * value + .9375f) + start;
        }
        else
        {
            value -= (2.625f / 2.75f);
            return end * (7.5625f * (value) * value + .984375f) + start;
        }
    }

    public static float EaseInOutBounce(float start, float end, float value)
    {
        end -= start;
        float d = 1f;
        if (value < d * 0.5f)
            return EaseInBounce(0, end, value * 2) * 0.5f + start;
        else
            return EaseOutBounce(0, end, value * 2 - d) * 0.5f + end * 0.5f + start;
    }

    public static float EaseInBack(float start, float end, float value)
    {
        end -= start;
        value /= 1;
        float s = 1.70158f;
        return end * (value) * value * ((s + 1) * value - s) + start;
    }

    public static float EaseOutBack(float start, float end, float value)
    {
        float s = 1.70158f;
        end -= start;
        value = (value) - 1;
        return end * ((value) * value * ((s + 1) * value + s) + 1) + start;
    }

    public static float EaseInOutBack(float start, float end, float value)
    {
        float s = 1.70158f;
        end -= start;
        value /= .5f;
        if ((value) < 1)
        {
            s *= (1.525f);
            return end * 0.5f * (value * value * (((s) + 1) * value - s)) + start;
        }
        value -= 2;
        s *= (1.525f);
        return end * 0.5f * ((value) * value * (((s) + 1) * value + s) + 2) + start;
    }

    public static float EaseInElastic(float start, float end, float value)
    {
        end -= start;

        float d = 1f;
        float p = d * .3f;
        float s;
        float a = 0;

        if (value == 0)
            return start;

        if ((value /= d) == 1)
            return start + end;

        if (a == 0f || a < Mathf.Abs(end))
        {
            a = end;
            s = p / 4;
        }
        else
        {
            s = p / (2 * Mathf.PI) * Mathf.Asin(end / a);
        }

        return -(a * Mathf.Pow(2, 10 * (value -= 1)) * Mathf.Sin((value * d - s) * (2 * Mathf.PI) / p)) + start;
    }

    public static float EaseOutElastic(float start, float end, float value)
    {
        end -= start;

        float d = 1f;
        float p = d * .3f;
        float s;
        float a = 0;

        if (value == 0)
            return start;

        if ((value /= d) == 1)
            return start + end;

        if (a == 0f || a < Mathf.Abs(end))
        {
            a = end;
            s = p * 0.25f;
        }
        else
        {
            s = p / (2 * Mathf.PI) * Mathf.Asin(end / a);
        }

        return (a * Mathf.Pow(2, -10 * value) * Mathf.Sin((value * d - s) * (2 * Mathf.PI) / p) + end + start);
    }

    public static float EaseInOutElastic(float start, float end, float value)
    {
        end -= start;

        float d = 1f;
        float p = d * .3f;
        float s;
        float a = 0;

        if (value == 0)
            return start;

        if ((value /= d * 0.5f) == 2)
            return start + end;

        if (a == 0f || a < Mathf.Abs(end))
        {
            a = end;
            s = p / 4;
        }
        else
        {
            s = p / (2 * Mathf.PI) * Mathf.Asin(end / a);
        }

        if (value < 1)
            return -0.5f * (a * Mathf.Pow(2, 10 * (value -= 1)) * Mathf.Sin((value * d - s) * (2 * Mathf.PI) / p)) + start;
        return a * Mathf.Pow(2, -10 * (value -= 1)) * Mathf.Sin((value * d - s) * (2 * Mathf.PI) / p) * 0.5f + end + start;
    }

    //
    // These are derived functions that the motor can use to get the speed at a specific time.
    //
    // The easing functions all work with a normalized time (0 to 1) and the returned value here
    // reflects that. Values returned here should be divided by the actual time.
    //
    // TODO: These functions have not had the testing they deserve. If there is odd behavior around
    //       dash speeds then this would be the first place I'd look.

    public static float LinearD(float start, float end, float value)
    {
        return end - start;
    }

    public static float EaseInQuadD(float start, float end, float value)
    {
        return 2f * (end - start) * value;
    }

    public static float EaseOutQuadD(float start, float end, float value)
    {
        end -= start;
        return -end * value - end * (value - 2);
    }

    public static float EaseInOutQuadD(float start, float end, float value)
    {
        value /= .5f;
        end -= start;

        if (value < 1)
        {
            return end * value;
        }

        value--;

        return end * (1 - value);
    }

    public static float EaseInCubicD(float start, float end, float value)
    {
        return 3f * (end - start) * value * value;
    }

    public static float EaseOutCubicD(float start, float end, float value)
    {
        value--;
        end -= start;
        return 3f * end * value * value;
    }

    public static float EaseInOutCubicD(float start, float end, float value)
    {
        value /= .5f;
        end -= start;

        if (value < 1)
        {
            return (3f / 2f) * end * value * value;
        }

        value -= 2;

        return (3f / 2f) * end * value * value;
    }

    public static float EaseInQuartD(float start, float end, float value)
    {
        return 4f * (end - start) * value * value * value;
    }

    public static float EaseOutQuartD(float start, float end, float value)
    {
        value--;
        end -= start;
        return -4f * end * value * value * value;
    }

    public static float EaseInOutQuartD(float start, float end, float value)
    {
        value /= .5f;
        end -= start;

        if (value < 1)
        {
            return 2f * end * value * value * value;
        }

        value -= 2;

        return -2f * end * value * value * value;
    }

    public static float EaseInQuintD(float start, float end, float value)
    {
        return 5f * (end - start) * value * value * value * value;
    }

    public static float EaseOutQuintD(float start, float end, float value)
    {
        value--;
        end -= start;
        return 5f * end * value * value * value * value;
    }

    public static float EaseInOutQuintD(float start, float end, float value)
    {
        value /= .5f;
        end -= start;

        if (value < 1)
        {
            return (5f / 2f) * end * value * value * value * value;
        }

        value -= 2;

        return (5f / 2f) * end * value * value * value * value;
    }

    public static float EaseInSineD(float start, float end, float value)
    {
        return (end - start) * 0.5f * Mathf.PI * Mathf.Sin(0.5f * Mathf.PI * value);
    }

    public static float EaseOutSineD(float start, float end, float value)
    {
        end -= start;
        return (Mathf.PI * 0.5f) * end * Mathf.Cos(value * (Mathf.PI * 0.5f));
    }

    public static float EaseInOutSineD(float start, float end, float value)
    {
        end -= start;
        return end * 0.5f * Mathf.PI * Mathf.Sin(Mathf.PI * value);
    }

    public static float EaseInExpoD(float start, float end, float value)
    {
        return (10f * NATURAL_LOG_OF_2 * (end - start) * Mathf.Pow(2f, 10f * (value - 1)));
    }

    public static float EaseOutExpoD(float start, float end, float value)
    {
        end -= start;
        return 5f * NATURAL_LOG_OF_2 * end * Mathf.Pow(2f, 1f - 10f * value);
    }

    public static float EaseInOutExpoD(float start, float end, float value)
    {
        value /= .5f;
        end -= start;

        if (value < 1)
        {
            return 5f * NATURAL_LOG_OF_2 * end * Mathf.Pow(2f, 10f * (value - 1));
        }

        value--;

        return (5f * NATURAL_LOG_OF_2 * end) / (Mathf.Pow(2f, 10f * value));
    }

    public static float EaseInCircD(float start, float end, float value)
    {
        return ((end - start) * value) / Mathf.Sqrt(1f - value * value);
    }

    public static float EaseOutCircD(float start, float end, float value)
    {
        value--;
        end -= start;
        return (-end * value) / Mathf.Sqrt(1f - value * value);
    }

    public static float EaseInOutCircD(float start, float end, float value)
    {
        value /= .5f;
        end -= start;

        if (value < 1)
        {
            return (end * value) / (2f * Mathf.Sqrt(1f - value * value));
        }

        value -= 2;

        return (-end * value) / (2f * Mathf.Sqrt(1f - value * value));
    }

    public static float EaseInBounceD(float start, float end, float value)
    {
        end -= start;
        float d = 1f;

        return EaseOutBounceD(0, end, d - value);
    }

    public static float EaseOutBounceD(float start, float end, float value)
    {
        value /= 1f;
        end -= start;

        if (value < (1 / 2.75f))
        {
            return 2f * end * 7.5625f * value;
        }
        else if (value < (2 / 2.75f))
        {
            value -= (1.5f / 2.75f);
            return 2f * end * 7.5625f * value;
        }
        else if (value < (2.5 / 2.75))
        {
            value -= (2.25f / 2.75f);
            return 2f * end * 7.5625f * value;
        }
        else
        {
            value -= (2.625f / 2.75f);
            return 2f * end * 7.5625f * value;
        }
    }

    public static float EaseInOutBounceD(float start, float end, float value)
    {
        end -= start;
        float d = 1f;

        if (value < d * 0.5f)
        {
            return EaseInBounceD(0, end, value * 2) * 0.5f;
        }
        else
        {
            return EaseOutBounceD(0, end, value * 2 - d) * 0.5f;
        }
    }

    public static float EaseInBackD(float start, float end, float value)
    {
        float s = 1.70158f;

        return 3f * (s + 1f) * (end - start) * value * value - 2f * s * (end - start) * value;
    }

    public static float EaseOutBackD(float start, float end, float value)
    {
        float s = 1.70158f;
        end -= start;
        value = (value) - 1;

        return end * ((s + 1f) * value * value + 2f * value * ((s + 1f) * value + s));
    }

    public static float EaseInOutBackD(float start, float end, float value)
    {
        float s = 1.70158f;
        end -= start;
        value /= .5f;

        if ((value) < 1)
        {
            s *= (1.525f);
            return 0.5f * end * (s + 1) * value * value + end * value * ((s + 1f) * value - s);
        }

        value -= 2;
        s *= (1.525f);
        return 0.5f * end * ((s + 1) * value * value + 2f * value * ((s + 1f) * value + s));
    }

    public static float EaseInElasticD(float start, float end, float value)
    {
        return EaseOutElasticD(start, end, 1f - value);
    }

    public static float EaseOutElasticD(float start, float end, float value)
    {
        end -= start;

        float d = 1f;
        float p = d * .3f;
        float s;
        float a = 0;

        if (a == 0f || a < Mathf.Abs(end))
        {
            a = end;
            s = p * 0.25f;
        }
        else
        {
            s = p / (2 * Mathf.PI) * Mathf.Asin(end / a);
        }

        return (a * Mathf.PI * d * Mathf.Pow(2f, 1f - 10f * value) * Mathf.Cos((2f * Mathf.PI * (d * value - s)) / p)) / p
            - 5f * NATURAL_LOG_OF_2 * a * Mathf.Pow(2f, 1f - 10f * value) * Mathf.Sin((2f * Mathf.PI * (d * value - s)) / p);
    }

    public static float EaseInOutElasticD(float start, float end, float value)
    {
        end -= start;

        float d = 1f;
        float p = d * .3f;
        float s;
        float a = 0;

        if (a == 0f || a < Mathf.Abs(end))
        {
            a = end;
            s = p / 4;
        }
        else
        {
            s = p / (2 * Mathf.PI) * Mathf.Asin(end / a);
        }

        if (value < 1)
        {
            value -= 1;

            return -5f * NATURAL_LOG_OF_2 * a * Mathf.Pow(2f, 10f * value) * Mathf.Sin(2 * Mathf.PI * (d * value - 2f) / p)
                - a * Mathf.PI * d * Mathf.Pow(2f, 10f * value) * Mathf.Cos(2 * Mathf.PI * (d * value - s) / p) / p;
        }

        value -= 1;

        return a * Mathf.PI * d * Mathf.Cos(2f * Mathf.PI * (d * value - s) / p) / (p * Mathf.Pow(2f, 10f * value))
            - 5f * NATURAL_LOG_OF_2 * a * Mathf.Sin(2f * Mathf.PI * (d * value - s) / p) / (Mathf.Pow(2f, 10f * value));
    }

    public static float SpringD(float start, float end, float value)
    {
        value = Mathf.Clamp01(value);
        end -= start;

        // Damn... Thanks http://www.derivative-calculator.net/
        // TODO: And it's a little bit wrong
        return end
                * (6f * (1f - value) / 5f + 1f)
                * (
                    -2.2f * Mathf.Pow(1f - value, 1.2f) * Mathf.Sin(Mathf.PI * value * (2.5f * value * value * value + 0.2f))
                    + Mathf.Pow(1f - value, 2.2f)
                        * (Mathf.PI * (2.5f * value * value * value + 0.2f) + 7.5f * Mathf.PI * value * value * value)
                        * Mathf.Cos(Mathf.PI * value * (2.5f * value * value * value + 0.2f))
                    + 1f
                )
            - 6f * end * (Mathf.Pow(1 - value, 2.2f) * Mathf.Sin(Mathf.PI * value * (2.5f * value * value * value + 0.2f)) + value / 5f);
    }

    public delegate float Function(float s, float e, float v);

    /// <summary>
    /// Returns the function associated to the easingFunction enum. This value returned should be cached as it allocates memory
    /// to return.
    /// </summary>
    /// <param name="easingFunction">The enum associated with the easing function.</param>
    /// <returns>The easing function</returns>
    public static Function GetEasingFunction(Ease easingFunction)
    {
        switch (easingFunction)
        {
            case Ease.EaseInQuad:
                return _easeInQuad;
            case Ease.EaseOutQuad:
                return _easeOutQuad;
            case Ease.EaseInOutQuad:
                return _easeInOutQuad;
            case Ease.EaseInCubic:
                return _easeInCubic;
            case Ease.EaseOutCubic:
                return _easeOutCubic;
            case Ease.EaseInOutCubic:
                return _easeInOutCubic;
            case Ease.EaseInQuart:
                return _easeInQuart;
            case Ease.EaseOutQuart:
                return _easeOutQuart;
            case Ease.EaseInOutQuart:
                return _easeInOutQuart;
            case Ease.EaseInQuint:
                return _easeInQuint;
            case Ease.EaseOutQuint:
                return _easeOutQuint;
            case Ease.EaseInOutQuint:
                return _easeInOutQuint;
            case Ease.EaseInSine:
                return _easeInSine;
            case Ease.EaseOutSine:
                return _easeOutSine;
            case Ease.EaseInOutSine:
                return _easeInOutSine;
            case Ease.EaseInExpo:
                return _easeInExpo;
            case Ease.EaseOutExpo:
                return _easeOutExpo;
            case Ease.EaseInOutExpo:
                return _easeInOutExpo;
            case Ease.EaseInCirc:
                return _easeInCirc;
            case Ease.EaseOutCirc:
                return _easeOutCirc;
            case Ease.EaseInOutCirc:
                return _easeInOutCirc;
            case Ease.Linear:
                return _linear;
            case Ease.Spring:
                return _spring;
            case Ease.EaseInBounce:
                return _easeInBounce;
            case Ease.EaseOutBounce:
                return _easeOutBounce;
            case Ease.EaseInOutBounce:
                return _easeInOutBounce;
            case Ease.EaseInBack:
                return _easeInBack;
            case Ease.EaseOutBack:
                return _easeOutBack;
            case Ease.EaseInOutBack:
                return _easeInOutBack;
            case Ease.EaseInElastic:
                return _easeInElastic;
            case Ease.EaseOutElastic:
                return _easeOutElastic;
            case Ease.EaseInOutElastic:
                return _easeInOutElastic;
            default:
                return null;
        }
    }

    /// <summary>
    /// Gets the derivative function of the appropriate easing function. If you use an easing function for position then this
    /// function can get you the speed at a given time (normalized).
    /// </summary>
    /// <param name="easingFunction"></param>
    /// <returns>The derivative function</returns>
    public static Function GetEasingFunctionDerivative(Ease easingFunction)
    {
        switch (easingFunction)
        {
            case Ease.EaseInQuad:
                return _easeInQuadD;
            case Ease.EaseOutQuad:
                return _easeOutQuadD;
            case Ease.EaseInOutQuad:
                return _easeInOutQuadD;
            case Ease.EaseInCubic:
                return _easeInCubicD;
            case Ease.EaseOutCubic:
                return _easeOutCubicD;
            case Ease.EaseInOutCubic:
                return _easeInOutCubicD;
            case Ease.EaseInQuart:
                return _easeInQuartD;
            case Ease.EaseOutQuart:
                return _easeOutQuartD;
            case Ease.EaseInOutQuart:
                return _easeInOutQuartD;
            case Ease.EaseInQuint:
                return _easeInQuintD;
            case Ease.EaseOutQuint:
                return _easeOutQuintD;
            case Ease.EaseInOutQuint:
                return _easeInOutQuintD;
            case Ease.EaseInSine:
                return _easeInSineD;
            case Ease.EaseOutSine:
                return _easeOutSineD;
            case Ease.EaseInOutSine:
                return _easeInOutSineD;
            case Ease.EaseInExpo:
                return _easeInExpoD;
            case Ease.EaseOutExpo:
                return _easeOutExpoD;
            case Ease.EaseInOutExpo:
                return _easeInOutExpoD;
            case Ease.EaseInCirc:
                return _easeInCircD;
            case Ease.EaseOutCirc:
                return _easeOutCircD;
            case Ease.EaseInOutCirc:
                return _easeInOutCircD;
            case Ease.Linear:
                return _linearD;
            case Ease.Spring:
                return _springD;
            case Ease.EaseInBounce:
                return _easeInBounceD;
            case Ease.EaseOutBounce:
                return _easeOutBounceD;
            case Ease.EaseInOutBounce:
                return _easeInOutBounceD;
            case Ease.EaseInBack:
                return _easeInBackD;
            case Ease.EaseOutBack:
                return _easeOutBackD;
            case Ease.EaseInOutBack:
                return _easeInOutBackD;
            case Ease.EaseInElastic:
                return _easeInElasticD;
            case Ease.EaseOutElastic:
                return _easeOutElasticD;
            case Ease.EaseInOutElastic:
                return _easeInOutElasticD;
            default:
                return null;
        }
    }
}

@cjddmut Based on the activity in this gist over the years and the number of individual forks, let me suggest you to create a fully featured repository out of it. This way, we can split this code scroll into separate files and make it easier for others to contribute. I can even help you publish it to UPM, if you'd like, so that script users can receive updates whenever changes are made.

UPD: Also, I've checked and noticed, that because you do not apply suggestions to the initial version, all the forks do almost the same changes after all.

Just out of interest, how come we are dividing 'value' by 0.5f instead of simply multiplying it by 2f? I was under the impression that division is always slower than multiplication when it comes to programming.

Wow! I've made some tests and you are right... that was one of the fundamental knowledges I somehow got wrong a long time ago, and not questioning it once after that. Thank you! That feels like you believe in some conspiracy theory for the whole of your life XD

@cjddmut Based on the activity in this gist over the years and the number of individual forks, let me suggest you to create a fully featured repository out of it. This way, we can split this code scroll into separate files and make it easier for others to contribute. I can even help you publish it to UPM, if you'd like, so that script users can receive updates whenever changes are made.

UPD: Also, I've checked and noticed, that because you do not apply suggestions to the initial version, all the forks do almost the same changes after all.

I'll take a look this weekend to shift it over if it'll be helpful :)