From f5ecea2b0c263f97e6d3afbb9a20969131676e44 Mon Sep 17 00:00:00 2001
From: Ryan Kiley <ryanekiley@gmail.com>
Date: Sat, 10 Oct 2026 16:11:49 -0700
Subject: [PATCH] Round a spring's derived physics to ten significant figures

The duration round trip handed back 300.00000000000006 for a stiffness of 300, and the noise reached changes() and the copied values. Stiffness and damping derived from a duration now snap to ten significant figures, so given values come back exact and derived ones read cleanly.
---
 src/tweaks/controls/spring.ts |  5 ++++-
 test/docs-snapshot.json       |  8 ++++----
 test/motion.test.mjs          | 14 ++++++++++++++
 3 files changed, 22 insertions(+), 5 deletions(-)

diff --git a/src/tweaks/controls/spring.ts b/src/tweaks/controls/spring.ts
index d85abd1..a09fd2f 100644
--- a/src/tweaks/controls/spring.ts
+++ b/src/tweaks/controls/spring.ts
@@ -34,7 +34,10 @@ function createSpring(meta: Meta, onChange: OnChange): Control {
   const DUR_MIN = 0.1, DUR_MAX = 1;
   const clampDur = (v: unknown) => clamp(has(v) ? +v : 0.5, DUR_MIN, DUR_MAX);
   const clampBounce = (v: unknown) => clamp(has(v) ? +v : 0.2, 0, 1);
-  const timeToPhysics = (t: SpringTime): Phys => { const k = (2 * Math.PI / t.visualDuration) ** 2, z = 1 - t.bounce, m = 1; return { stiffness: k, damping: 2 * z * Math.sqrt(k * m), mass: m }; };
+  // Derived values snap to ten significant figures: the trig round trip otherwise hands back
+  // 300.00000000000006 for a stiffness of 300, and that noise lands in changes() and the copy.
+  const sig = (x: number) => +x.toPrecision(10);
+  const timeToPhysics = (t: SpringTime): Phys => { const k = (2 * Math.PI / t.visualDuration) ** 2, z = 1 - t.bounce, m = 1; return { stiffness: sig(k), damping: sig(2 * z * Math.sqrt(k * m)), mass: m }; };
 
   // Initial config — accept top-level props (the inline shorthand
   // `{ type:"spring", visualDuration:0.3, bounce:0.2 }` or `{ …, stiffness, damping, mass }`)
diff --git a/test/docs-snapshot.json b/test/docs-snapshot.json
index 6bd3171..ba5dc6a 100644
--- a/test/docs-snapshot.json
+++ b/test/docs-snapshot.json
@@ -129,8 +129,8 @@
           },
           "bounce": {
             "mode": "spring",
-            "stiffness": 109.6622711232151,
-            "damping": 10.471975511965978,
+            "stiffness": 109.6622711,
+            "damping": 10.47197551,
             "mass": 1,
             "visualDuration": 0.6,
             "bounce": 0.5,
@@ -216,8 +216,8 @@
       "values": {
         "move": {
           "mode": "spring",
-          "stiffness": 194.9551486634935,
-          "damping": 19.54768762233649,
+          "stiffness": 194.9551487,
+          "damping": 19.54768762,
           "mass": 1,
           "visualDuration": 0.45,
           "bounce": 0.3,
diff --git a/test/motion.test.mjs b/test/motion.test.mjs
index f74f0a5..909754f 100644
--- a/test/motion.test.mjs
+++ b/test/motion.test.mjs
@@ -152,3 +152,17 @@ test("review follow-up: `value` as a bare keyword or 4-tuple is the curve on the
   assert.deepEqual(a.params.t.curve, [0.42, 0, 1, 1]);
   a.destroy();
 });
+
+test("a spring's physics round-trips clean: stiffness 300 comes back 300, not 300.00000000000006", async () => {
+  const p = tweaks("Noise", { m: { type: "motion", stiffness: 300, damping: 20 }, s: { type: "spring", stiffness: 300, damping: 20 } });
+  document.body.append(p.el); await p.ready;
+  for (const k of ["m", "s"]) {
+    assert.equal(p.params[k].stiffness, 300, `${k}: stiffness`);
+    assert.equal(p.params[k].damping, 20, `${k}: damping`);
+  }
+  p.set("m", { stiffness: 500, damping: 25 });
+  const c = p.changes().m;
+  assert.equal(c.stiffness, 500, "changes() carries the stiffness as set");
+  assert.equal(c.damping, 25, "and the damping");
+  p.destroy();
+});