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20240428.md

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+# 2024-04-28
+
+## Deadbeat escapement simulator
+
+I've been reading more Goodrich, and his explanation of laying out a deadbeat escapement is quite clear.
+I am once again motivated to try to make an escapement simulator. But I'd rather not fight Matter.js
+again, so I'm going to use one of the JavaScript Box2D ports, which Luke Wallin suggested to me.
+
+[Which Box2D JavaScript library should I use?](https://stackoverflow.com/a/7968954)
+
+https://github.com/piqnt/planck.js has the most github stars so let's start there.
+
+Plan is maybe:
+
+1. make a pendulum
+2. make the rest of the escapement?
+
+If I can get a pendulum working tonight I shall be pleased.
+
+I'm following the "script tag" instructions on https://piqnt.com/planck.js/docs/install to try
+to get `planck-with-testbed.min.js`. It works if I literally copy and paste the example, but
+if I download `planck-with-testbed.min.js` from jsDelivr and try to load it locally it doesn't work,
+either from a `file:///` URL ot with `SimpleHTTPServer`. Does it need to be loaded over https? Nope,
+if I host it under `https://incoherency.co.uk/` it still doesn't work.
+
+The error I'm getting is `Uncaught TypeError: t is null` at `planck-with-testbed.min.js:38`. So
+what is it about jsDelivr that is making it work?
+
+Oh! It's actually not jsDelivr. It's that if I write
+
+    <script src="..."></script>
+
+then it doesn't work, but if I write:
+
+    <script src="..." />
+
+then it does. Wtf?
+
+Is it actually working? Maybe it doesn't work anyway but the ` />` form hides the error message?
+
+I think the latter. If I put a `console.log` in my code it doesn't even execute that.
+
+I've reported an issue on github: https://github.com/piqnt/planck.js/issues/276
+
+OK, figured it, it's because it assumes the existence of some HTML elements that it hasn't created.
+
+Woo, got a working pendulum: https://img.incoherency.co.uk/5418