The context

At Aptimiz, the engine turns the GPS positions of farm machines into activities, journeys and breaks. Written in 2020 and extended for years, it had become hard to change: the business logic was driven by tables of numbered rules, effective but impossible to test.

What I did

I took the engine over and rewrote it in TypeScript as explicit steps. To guarantee that nothing changed for users, I put regression tests in place at three levels: against the old rules, against real cases captured in staging, and on complete runs compared item by item. Duplicate rules were merged, dead code removed.

In short

Millions of positions processed every day, and identical behaviour before and after the rewrite.