boing.js (second pass)

boing.js

// Create a spring object.
export const Spring = (curr, prev, dest, stiffness, damping) => ({
  curr,                // The current position of spring 
  prev,                // The position of spring at previous tick
  dest,                // The destination of spring
  stiffness,           // How hard it springs back
  damping,             // Friction. 1.0 means no bounce.
});

// Given numbers, calculate the next position for a spring.
Spring.next = (curr, prev, dest, stiffness, damping) => {
  // Calculate spring force
  const fspring = -stiffness * (curr - dest);

  // Calculate velocity
  const vel = curr - prev;

  // Calculate damping force.
  const fdamping = -damping * vel;

  // Calc acceleration by adjusting spring force to damping force
  const acc = fspring + fdamping;

  // Calculate new velocity after adding acceleration. Scale to framerate.
  const nextv = (vel + acc);

  // Calculate next position by integrating velocity. Scale to framerate.
  const next = (curr + nextv);

  return next;
}

// Some arbitrarily small positive number used as threshold value.
const epsilon = 0.001;

// Given numbers, calcluate if a spring is at rest.
Spring.isResting = (curr, prev, dest) =>
  Math.abs(curr - prev) < epsilon && Math.abs(curr - dest) < epsilon;

// Create a new spring at location.
Spring.at = (pos, stiffness, damping) =>
  Spring(pos, pos, pos, stiffness, damping);

// Copy a spring
Spring.copy = (spring) => Spring(
  spring.curr, spring.prev, spring.dest,
  spring.stiffness, spring.damping
);

// Set coords on a spring, mutating spring
Spring.coords = (spring, curr, prev, dest) => {
  spring.curr = curr;
  spring.prev = prev;
  spring.dest = dest;
  return spring;
}

// Set the destination of spring A to the current location of spring B.
Spring.connect = (a, b) =>
  Spring.coords(a, a.curr, a.prev, b.curr);

Spring.animate = (spring) =>
  !Spring.isResting(spring.curr, spring.prev, spring.dest) ?
    Spring.coords(
      spring,
      Spring.next(
        spring.curr, spring.prev, spring.dest,
        spring.stiffness, spring.damping),
      spring.curr, spring.dest) :
  Spring.coords(spring, spring.dest, spring.dest, spring.dest);

Spring.animate2d = (spring) =>
  (!Spring.isResting(spring.curr[0], spring.prev[0], spring.dest[0]) ||
   !Spring.isResting(spring.curr[1], spring.prev[1], spring.dest[1])) ?
    Spring.coords(
      spring,
      [Spring.next(
        spring.curr[0], spring.prev[0], spring.dest[0],
        spring.stiffness, spring.damping),
      Spring.next(
        spring.curr[1], spring.prev[1], spring.dest[1],
        spring.stiffness, spring.damping)],
      spring.curr, spring.dest) :
  Spring.coords(spring, spring.dest, spring.dest, spring.dest);