Grabbed dndbeyond's source code

```
~/go/bin/sourcemapper -output ddb -jsurl https://media.dndbeyond.com/character-app/static/js/main.90aa78c5.js
```

ddb_main/node_modules/@serprex/react-motion/lib/stepper.js

@@ -0,0 +1,34 @@
+"use strict";
+
+exports.__esModule = true;
+exports["default"] = stepper;
+// stepper is used a lot. Saves allocation to return the same array wrapper.
+// This is fine and danger-free against mutations because the callsite
+// immediately destructures it and gets the numbers inside without passing the
+// array reference around.
+var reusedTuple = [0, 0];
+
+function stepper(secondPerFrame, x, v, destX, k, b, precision) {
+  // Spring stiffness, in kg / s^2
+  // for animations, destX is really spring length (spring at rest). initial
+  // position is considered as the stretched/compressed position of a spring
+  var Fspring = -k * (x - destX); // Damping, in kg / s
+
+  var Fdamper = -b * v; // usually we put mass here, but for animation purposes, specifying mass is a
+  // bit redundant. you could simply adjust k and b accordingly
+  // let a = (Fspring + Fdamper) / mass;
+
+  var a = Fspring + Fdamper;
+  var newV = v + a * secondPerFrame;
+  var newX = x + newV * secondPerFrame;
+
+  if (Math.abs(newV) < precision && Math.abs(newX - destX) < precision) {
+    reusedTuple[0] = destX;
+    reusedTuple[1] = 0;
+    return reusedTuple;
+  }
+
+  reusedTuple[0] = newX;
+  reusedTuple[1] = newV;
+  return reusedTuple;
+}