🚀 demo + spring

packages/demo/springUtils.ts

@@ -0,0 +1,63 @@
+const epsilon = 0.01;
+
+export const clamp = (a: number, b: number) => (x: number) =>
+  Math.max(a, Math.min(b, x));
+
+/**
+ * step the spring, mutate the state to reflect the state at t+dt
+ *
+ */
+const stepSpringOne = (
+  s: { x: number; v: number },
+  {
+    tension,
+    friction,
+    maxVelocity = Infinity,
+  }: { tension: number; friction: number; maxVelocity?: number },
+  target: number,
+  dt = 1 / 60
+) => {
+  const a = -tension * (s.x - target) - friction * s.v;
+
+  s.v += a * dt;
+  s.v = clamp(-maxVelocity / dt, maxVelocity / dt)(s.v);
+  s.x += s.v * dt;
+};
+
+/**
+ * return true if the spring is to be considered in a stable state
+ * ( close enough to the target and with a small enough velocity )
+ */
+export const isStable = (
+  s: { x: number; v: number },
+  target: number,
+  dt = 1 / 60
+) => Math.abs(s.x - target) < epsilon && Math.abs(s.v * dt) < epsilon;
+
+export const isStableAndBound = (
+  s: { x: number; v: number },
+  target: number,
+  dt?: number
+) => {
+  const stable = isStable(s, target, dt);
+  if (stable) {
+    s.x = target;
+    s.v = 0;
+  }
+  return stable;
+};
+
+export const stepSpring = (
+  s: { x: number; v: number },
+  params: { tension: number; friction: number; maxVelocity?: number },
+  target: number,
+  dt = 1 / 60
+) => {
+  const interval = 1 / 60;
+
+  while (dt > 0) {
+    stepSpringOne(s, params, target, Math.min(interval, dt));
+    // eslint-disable-next-line no-param-reassign
+    dt -= interval;
+  }
+};