cubic-bezier.ts

function calcBezier(t: number, p1: number, p2: number): number {
  return (((1 + 3 * p1 - 3 * p2) * t + (-6 * p1 + 3 * p2)) * t + 3 * p1) * t;
}

const PRECISION = 0.0000001;
const MAX_ITERATIONS = 12;

function binarySubdivide(
  x: number,
  a: number,
  b: number,
  x1: number,
  x2: number
): number {
  let currentX,
    currentT,
    i = 0;
  do {
    currentT = a + (b - a) / 2;
    currentX = calcBezier(currentT, x1, x2) - x;
    if (currentX > 0) {
      b = currentT;
    } else {
      a = currentT;
    }
  } while (Math.abs(currentX) > PRECISION && ++i < MAX_ITERATIONS);

  return currentT;
}

export function cubicBezier(x1: number, y1: number, x2: number, y2: number) {
  return (x: number): number => {
    return x === 0 || x === 1
      ? x
      : calcBezier(binarySubdivide(x, 0, 1, x1, x2), y1, y2);
  };
}