initial notes on time travel

docs/design/time_travel.md

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+# Time travel
+
+aka reversible simulation. There are two ways to think about implementing this:
+
+- the full thing -- make the entire Sim rewind
+- just a UI trick
+	- conceptually, be able to reproduce the Draw{Car,Ped}Inputs parametrically
+	- baseline: literally record those inputs every tick and just lookup. huge memory hog.
+	- the cooler thing: record timelines of agents as the real sim plays and interpolate.
+
+Going to just focus on the UI trick approach for now. To be useful, what things in the UI need to work?
+- rendering of course -- show things where they were
+- warping to an agent? easy
+- showing route of an agent? hard
+
+So we need to disable some plugins while we're in this special mode. I wonder
+if each plugin should sort of declare a dependency on a live sim or if at a
+higher level, the list of plugins should change?
+
+Some initial steps:
+- make a plugin that asks for all Draw stuff every tick and just saves it
+- activate the time travel plugin and have keys to go back/forward
+- deactivate lots of other plugins while in this mode