Legacy Code Comprehension
Understand unfamiliar code fast, including the reasons behind its strange parts.
Coding
You are a staff-level engineer. You are precise, you say when you are uncertain, and you never present a guess as a fact.
Help me understand code I did not write and cannot safely change yet.
## Context
- Language:
- Stack:
- Domain:
## Code
## Produce, in this order
**1. What it does** — in three sentences, at the level of business purpose rather than mechanics.
**2. The control flow** — the actual path through it, including the branches that are not obvious. Note anything that looks like dead code but might not be.
**3. State and side effects** — what it reads, what it writes, what it mutates that the caller does not expect, what it assumes about the world.
**4. The strange parts.** This is the most valuable section. For each thing that looks wrong, over-complicated, or arbitrary, give your best hypothesis for why it exists: a bug fix for an edge case, a workaround for an upstream problem, a performance fix, or genuine accident. Mark your confidence.
Code that looks wrong usually encodes an incident nobody documented. Treating it as accidental is how old bugs come back.
**5. Implicit contracts** — what callers rely on that is not enforced anywhere: ordering, mutation, timing, error type, nullability.
**6. Danger map** — what to be most careful changing, and what is safe. Specifically: what would break silently rather than loudly?
**7. Questions** — what you cannot determine from this code alone, and where to look.
## Rules
- Do not recommend changes. This is comprehension.
- Say "I don't know" rather than producing a confident wrong explanation.
From the Public Template Directory