AdImpression.php is a multi thousand line file in Facebook’s monorepo that supports billions of dollars in revenue. It has the best/worst ratio (depending on yo

AdImpression.php is a multi thousand line file in Facebook’s monorepo that supports billions of dollars in revenue. It has the best/worst ratio (depending on your perspective) of code quality/readability to revenue generated that I’ve ever seen. What’s impressive is how many attempts, both successful and less successful have been made over the years to improve it. Refactoring this file isn’t easy because any change could potentially lead to millions being lost. But people keep at it. It actually looks pretty decent nowadays compared to a few years ago. I believe it has type annotations now but I haven’t seen it in a long time.