code-understand

# Code Understand — On-Demand Architecture Focus Map

You are about to work in a codebase you don't fully know yet. Instead of grepping around or
reading files at random, ask the local extractor for a **focus map**: the ranked symbols the
architecture hangs on, each with a `file:line` citation and evidence type. Read only what it
points at — this is a map, not a substitute for reading the actual code it cites.

## When to reach for this

- Before editing an unfamiliar module — see what calls it and what it calls before changing
  its shape.
- The user asks "how does X work", "what's the impact of changing Y", "who else uses this
  function" for a codebase, not the wiki.
- You're about to do a large refactor and want a ranked list of load-bearing files instead of
  reading the whole tree.
- You already have a diff or a set of changed files and want to know their blast radius before
  finishing the change.

Not for: summarizing what a project *does* for the wiki (that's `wiki-update` Step 3b, which
calls this same command as part of a bigger sync flow) — use this skill directly only when you
need the focus map for your own immediate work, not to persist knowledge.

## Running it

```bash
obsidian-wiki code-understand --project <dir> [--backend auto|builtin|codegraph] \
    [--changed <file>...] [--since <sha>] [--max-symbols N] [--pretty]
```

- `--project` — defaults to the current directory.
- No `--changed`/`--since` — seeds from every tracked file (full-project scan).
- `--changed <file>` (repeatable) — seed from specific files you already know are relevant
  (e.g. files in the diff you're about to make).
- `--since <sha>` — seed from everything changed since a git ref (e.g. `--since HEAD~5` or a
  base branch).
- `--backend` — `auto` (default) uses CodeGraph when installed, falls back to the builtin
  extractor otherwise; force `builtin` for the dependency-free path, or `codegraph` to require
  the enhanced backend (fails loudly if it's not installed rather than silently degrading).
- `--max-symbols` — cap the focus map size (default 50); keep this small for a quick
  orientation pass, raise it for a thorough one.

**GUARD:** if the command is unavailable or errors, skip it and fall back to your normal
exploration (grep/read) — this is an accelerant, not a dependency. Don't block work on it.

## Reading the output

1. **Evidence type matters.** When `backend: codegraph`, focus-map entries are structural
   facts (real call-graph edges) — cite them directly. When `backend: builtin`, `defines`/
   `imports`/`changed-file` entries are facts, but `rg-reference` entries are text-match
   evidence only — open the file and confirm before treating it as a real relationship.
2. **Open what it cites, nothing else first.** The focus map tells you *where* to read; it
   never contains source bodies. Go read those `file:line` locations before forming an opinion
   on the architecture.
3. **Cite it in your own output.** If you explain the architecture back to the user or write
   it into a PR description/commit message, keep the `(file:lines)` citations from the focus
   map or from the source you opened — don't assert structure without a pointer to it.
4. **It's a cache, not a deliverable.** Never write the raw JSON or `.codegraph/` into the
   wiki vault, a PR description, or committed docs — it's a disposable sidecar in the project
   repo (git-ignored). Re-run it fresh next time rather than treating old output as durable.

## If CodeGraph isn't installed

`auto`/`builtin` still work with zero setup (uses the regex AST extractor + `rg`). If you want
the enhanced cross-file call-graph evidence and the user is fine with installing something:

```bash
npm install -g @colbymchenry/codegraph
```

or point `CODE_UNDERSTANDING_CODEGRAPH_BIN` at an existing binary. Never install without the
user's go-ahead — ask first, then re-run the command.

Check `obsidian-wiki doctor --project <dir>` for the `code-understanding.*` capability lines
(`builtin`, `rg`, `codegraph`, `codegraph-index`, `codegraph-fresh`, `codegraph-gitignore`) to
see what's available before deciding which backend to request explicitly.