gzh_chuanyuan/uni_modules/lime-circle/components/l-circle/animation/bezier.ts

export function cubicBezier(p1x: number, p1y: number, p2x: number, p2y: number) {
  const ZERO_LIMIT = 1e-6;
  // Calculate the polynomial coefficients,
  // implicit first and last control points are (0,0) and (1,1).
  const ax = 3 * p1x - 3 * p2x + 1;
  const bx = 3 * p2x - 6 * p1x;
  const cx = 3 * p1x;

  const ay = 3 * p1y - 3 * p2y + 1;
  const by = 3 * p2y - 6 * p1y;
  const cy = 3 * p1y;

  function sampleCurveDerivativeX(t: number) {
    // `ax t^3 + bx t^2 + cx t` expanded using Horner's rule
    return (3 * ax * t + 2 * bx) * t + cx;
  }

  function sampleCurveX(t: number) {
    return ((ax * t + bx) * t + cx) * t;
  }

  function sampleCurveY(t: number) {
    return ((ay * t + by) * t + cy) * t;
  }

  // Given an x value, find a parametric value it came from.
  function solveCurveX(x: number) {
    let t2 = x;
    let derivative: number;
    let x2: number;

    // https://trac.webkit.org/browser/trunk/Source/WebCore/platform/animation
    // first try a few iterations of Newton's method -- normally very fast.
    // http://en.wikipedia.org/wikiNewton's_method
    for (let i = 0; i < 8; i++) {
      // f(t) - x = 0
      x2 = sampleCurveX(t2) - x;
      if (Math.abs(x2) < ZERO_LIMIT) {
        return t2;
      }
      derivative = sampleCurveDerivativeX(t2);
      // == 0, failure
      /* istanbul ignore if */
      if (Math.abs(derivative) < ZERO_LIMIT) {
        break;
      }
      t2 -= x2 / derivative;
    }

    // Fall back to the bisection method for reliability.
    // bisection
    // http://en.wikipedia.org/wiki/Bisection_method
    let t1 = 1;
    /* istanbul ignore next */
    let t0 = 0;

    /* istanbul ignore next */
    t2 = x;
    /* istanbul ignore next */
    while (t1 > t0) {
      x2 = sampleCurveX(t2) - x;
      if (Math.abs(x2) < ZERO_LIMIT) {
        return t2;
      }
      if (x2 > 0) {
        t1 = t2;
      } else {
        t0 = t2;
      }
      t2 = (t1 + t0) / 2;
    }

    // Failure
    return t2;
  }

  function solve(x: number) {
    return sampleCurveY(solveCurveX(x));
  }

  return solve;
}