Easing.js

/**
 * Copyright (c) 2015-present, Facebook, Inc.
 * All rights reserved.
 *
 * This source code is licensed under the BSD-style license found in the
 * LICENSE file in the root directory of this source tree. An additional grant
 * of patent rights can be found in the PATENTS file in the same directory.
 *
 * @providesModule Easing
 */
'use strict';

/**
 * This class implements common easing functions. The math is pretty obscure,
 * but this cool website has nice visual illustrations of what they represent:
 * http://xaedes.de/dev/transitions/
 */
class Easing {
  static step0(n) {
    return n > 0 ? 1 : 0;
  }

  static step1(n) {
    return n >= 1 ? 1 : 0;
  }

  static linear(t) {
    return t;
  }

  static ease(t: number): number {
    return ease(t);
  }

  static quad(t) {
    return t * t;
  }

  static cubic(t) {
    return t * t * t;
  }

  static poly(n) {
    return (t) => Math.pow(t, n);
  }

  static sin(t) {
    return 1 - Math.cos(t * Math.PI / 2);
  }

  static circle(t) {
    return 1 - Math.sqrt(1 - t * t);
  }

  static exp(t) {
    return Math.pow(2, 10 * (t - 1));
  }

  static elastic(a: number, p: number): (t: number) => number {
    var tau = Math.PI * 2;
    // flow isn't smart enough to figure out that s is always assigned to a
    // number before being used in the returned function
    var s: any;
    if (arguments.length < 2) {
      p = 0.45;
    }
    if (arguments.length) {
      s = p / tau * Math.asin(1 / a);
    } else {
      a = 1;
      s = p / 4;
    }
    return (t) => 1 + a * Math.pow(2, -10 * t) * Math.sin((t - s) * tau / p);
  };

  static back(s: number): (t: number) => number {
    if (s === undefined) {
      s = 1.70158;
    }
    return (t) => t * t * ((s + 1) * t - s);
  };

  static bounce(t: number): number {
    if (t < 1 / 2.75) {
      return 7.5625 * t * t;
    }

    if (t < 2 / 2.75) {
      t -= 1.5 / 2.75;
      return 7.5625 * t * t + 0.75;
    }

    if (t < 2.5 / 2.75) {
      t -= 2.25 / 2.75;
      return 7.5625 * t * t + 0.9375;
    }

    t -= 2.625 / 2.75;
    return 7.5625 * t * t + 0.984375;
  };

  static bezier(
    x1: number,
    y1: number,
    x2: number,
    y2: number,
    epsilon?: ?number
  ): (t: number) => number {
    if (epsilon === undefined) {
      // epsilon determines the precision of the solved values
      // a good approximation is:
      var duration = 500; // duration of animation in milliseconds.
      epsilon = (1000 / 60 / duration) / 4;
    }

    return bezier(x1, y1, x2, y2, epsilon);
  }

  static in(
    easing: (t: number) => number
  ): (t: number) => number {
    return easing;
  }

  static out(
    easing: (t: number) => number
  ): (t: number) => number {
    return (t) => 1 - easing(1 - t);
  }

  static inOut(
    easing: (t: number) => number
  ): (t: number) => number {
    return (t) => {
      if (t < 0.5) {
        return easing(t * 2) / 2;
      }
      return 1 - easing((1 - t) * 2) / 2;
    };
  }
}

var ease = Easing.bezier(0.42, 0, 1, 1);

module.exports = Easing;