CSS `cubic-bezier()` timing function

CubicBezier.es6.js

'use strict';

export default function cubicBezier(x1, y1, x2, y2) {
  let ax, bx, cx, ay, by, cy;

  ax = 3 * (x1 - x2) + 1;
  bx = 3 * x2 - 6 * x1;
  cx = 3 * x1;

  ay = 3 * (y1 - y2) + 1;
  by = 3 * y2 - 6 * y1;
  cy = 3 * y1;

  return function(x, accuracy) {
    let t, i, sample, derivative, low, high;

    if (x <= 0) {
      return 0;
    } else if (x >= 1) {
      return 1;
    }

    t = x, i = 8;

    while (i--) {
      sample = ((ax * t + bx) * t + cx) * t - x;

      if (-accuracy < sample && sample < accuracy) {
        return ((ay * t + by) * t + cy) * t;
      }

      derivative = (3 * ax * t + 2 * bx) * t + cx;

      if (-0.000001 < derivative && derivative < 0.000001) {
        break;
      }

      t = t - sample / derivative;
    }

    t = x, low = 0, high = 1;

    while (low < high) {
      sample = ((ax * t + bx) * t + cx) * t - x;

      if (-accuracy < sample && sample < accuracy) {
        break;
      }

      sample < 0 ? (low = t) : (high = t);
      t = 0.5 * (high + low);
    }

    return ((ay * t + by) * t + cy) * t;
  }
}