Door 3: three layers of settings
Why it matters By now you have a toolbox: reframes, indexes, forgetting rules, event logs. The last door checks that you still know when to leave the toolbox shut.
A web app has three layers of settings: the product defaults, the team's choices and the user's own. Every request works out the effective settings. The app serves about 50 requests a second, and each layer is a flat dict of about 30 keys (text, numbers, booleans), loaded from the database with the user.
It sounds like it might want a cache, a class, or a layered config library. Read the numbers before deciding.
Write effective_settings(defaults, team, user). Return a new dict with exactly the keys of defaults; each value comes from user if the key is there, else from team, else the default. Keys not in defaults are ignored. False, 0 and "" are real values and still win. Do not change the dicts you are given, and callers may change the dict you return. Rule: your whole code may have at most 60 syntax nodes (code_size).
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