Explain this code

Understand unfamiliar code fast: purpose, flow, gotchas.

Explain the code below to a developer who knows {{LANGUAGE}} but has never seen this codebase.

1. What it does, in 2 sentences.
2. Step-by-step flow, referencing line numbers or function names.
3. Inputs, outputs and side effects (I/O, network, global state).
4. Non-obvious parts: tricks, clever code, implicit assumptions.
5. Possible bugs or edge cases that are not handled.

Do not rewrite the code unless I ask.

```
{{CODE}}
```