nescript/CLAUDE.md

# CLAUDE.md

Guidance for Claude Code (and any other AI agents) working in this repo.
Keep it short and practical — it's here so the next agent doesn't have to
re-derive the project conventions from scratch.

---

## Project shape

- **NEScript** is a Rust-based compiler that turns `.ne` source files into
  iNES ROMs. Single binary, no external assemblers, no external linkers.
- `src/` is a flat module layout: each compiler phase is its own directory
  with `mod.rs` + `tests.rs`. See `docs/architecture.md` for the phase
  pipeline.
- Examples live in `examples/*.ne`. Every example is expected to compile
  cleanly and has a pinned emulator golden — see below.
- **`examples/*.nes` is committed.** The compiler is deterministic
  (same source → byte-identical ROM), so the ROMs travel with the
  repo. If you edit any `.ne` file you **must** rebuild its `.nes`
  in the same commit — CI's `examples` job rebuilds each ROM into a
  tmp path and fails if the committed version differs, pointing at
  the exact `cargo run -- build examples/<name>.ne` to run. The
  pre-commit hook under `scripts/pre-commit` catches this locally.
- `docs/future-work.md` lists the remaining gaps. If you implement
  something from that file, update the doc in the same PR.

## Running the basics

```bash
cargo build --release          # build the compiler
cargo test                     # all Rust tests (lib + integration)
cargo fmt                      # mandatory before committing
cargo clippy --all-targets     # mandatory before committing; fix or #[allow]
./target/release/nescript build examples/hello_sprite.ne   # build one ROM
```

Compile every example at once:

```bash
for f in examples/*.ne; do cargo run --release -- build "$f"; done
```

## The jsnes emulator harness

This is the most important piece of project-specific tooling. Every `.ne`
example has a **pixel-exact PNG golden** and a **sample-exact audio hash**
committed under `tests/emulator/goldens/`. Any compiler change that alters
observable behaviour — codegen, optimizer, runtime, linker, asset pipeline
— will flip at least one golden, and CI will fail loudly with a visible
diff. Do not skip or weaken this check.

### Layout

```
tests/emulator/
  harness.html          # thin wrapper around jsnes; exposes window.nesHarness
                        # with loadRomBase64, runFrames, rawPixelsBase64,
                        # audioHash, audioWavBase64
  run_examples.mjs      # puppeteer-driven runner (headless Chrome)
  package.json          # depends on jsnes, pngjs, puppeteer
  goldens/
    <name>.png          # 256×240 RGBA framebuffer at frame 180 (~3s at 60fps)
    <name>.audio.hash   # one line: "<fnv1a-hex> <sample-count>"
  actual/               # gitignored; written on every run for diff artifacts
```

### Running it locally

The harness is **separate** from `cargo test`. You have to run it by hand:

```bash
# 1. Rebuild every example with the current compiler. The harness
#    reads whatever sits under examples/*.nes — if you want to test
#    your working copy you have to rebuild them first.
cargo build --release
for f in examples/*.ne; do ./target/release/nescript build "$f"; done

# 2. Install node deps (once per worktree; node_modules/ is gitignored).
cd tests/emulator
npm install          # or `npm ci` in CI

# 3. Verify every ROM still matches its golden.
node run_examples.mjs
# → "22/22 ROMs match their goldens" on success
# → FAIL / MISS lines + `actual/<name>.png`, `actual/<name>.diff.png`,
#   `actual/<name>.wav` written for any ROM that mismatched
```

The harness always runs against whatever sits in `examples/*.nes`,
so iterating on the compiler means rebuilding the example first.
CI's `emulator` job does this too — it builds the compiler, compiles
every `.ne` into the workspace (overwriting the committed ROMs,
which are ephemeral in the CI checkout), and then runs the harness.
The committed ROMs are a PR-review convenience and a "did this
change affect codegen" tripwire via the `examples` job's
reproducibility diff; they are **not** what the emulator job tests.

### Updating goldens

If a change is supposed to flip goldens (you added a new example, changed
a rendering path, fixed a bug that was baked into the old output), update
them with:

```bash
cd tests/emulator
UPDATE_GOLDENS=1 node run_examples.mjs     # rewrites every mismatched golden
# or
node run_examples.mjs --update-goldens
```

Then `git diff tests/emulator/goldens/` the result, eyeball each change,
and include the updated PNG+hash files in the same commit as the code
change. Goldens are the contract; the commit message should explain why
each diff is legitimate. **Never** `UPDATE_GOLDENS=1` just to silence a
failing CI — that defeats the entire purpose of the harness.

### Adding a new example

1. Write `examples/<name>.ne`.
2. Build it with the release compiler so a `.nes` file lands next to it.
3. Run `UPDATE_GOLDENS=1 node run_examples.mjs` to generate
   `goldens/<name>.png` and `goldens/<name>.audio.hash`. Both files must
   be committed — the runner treats missing goldens as a hard failure.
4. Verify visually that the generated PNG is what you actually intended
   (open it; you can use Read on the PNG file to have Claude display it).
5. Add the example to the tables in `README.md` and `examples/README.md`.

### What the harness tests (and doesn't)

- **Tests**: final rendered framebuffer at frame 180, full audio sample
  stream over the same window. Catches codegen miscompiles, runtime
  bugs, linker layout changes, PPU timing regressions, APU regressions,
  asset pipeline bugs — essentially anything that affects the observable
  behaviour of a whole program.
- **Does not test**: input handling (no buttons pressed during the run),
  anything past frame 180 (~3 seconds), state transitions that require
  user input. Examples that need input to look non-trivial should
  structure themselves so a good demo happens on autopilot — e.g. a
  frame counter that drives the interesting state (`examples/palette_and_background.ne`
  is a working pattern).

### CI integration

The `emulator` job in `.github/workflows/ci.yml` installs Chrome deps,
builds all examples, then runs the harness. On failure it uploads the
`actual/` directory and `report.json` as an artifact named `emulator-diff`
so reviewers can download and inspect the pixel diffs without cloning the
repo. The CI job does **not** pass `UPDATE_GOLDENS`; if it flips, the
change needs a manual update + review.

## Conventions worth knowing

- Every `src/**/mod.rs` has a co-located `tests.rs`. Add unit tests
  there, not in a separate file.
- Big cross-phase tests go in `tests/integration_test.rs`. Use the
  `compile` / `compile_banked` helpers at the top of that file instead
  of re-building the pipeline by hand.
- Error codes live in `src/errors/diagnostic.rs`. Don't add a new code
  without emitting it from somewhere — clippy will catch unused variants,
  but past agents have also let them sit as dead code.
- Zero page is tight. `$00-$0F` is reserved for the runtime (frame
  flag, input, OAM cursor, sfx/music pointers). `$11-$17` is reserved
  for PPU palette/background updates **when the program declares them**
  (the analyzer bumps the user ZP start from `$10` to `$18` in that
  case — programs without palette/bg keep the old `$10` layout to
  preserve their goldens). User vars go at `$10+` or `$18+`; IR temps
  land at `$80+`.
- `docs/future-work.md` is the authoritative roadmap. If you finish an
  item, delete its section; if you add a new gap, write one.

## Things to avoid

- **Don't add backwards-compat shims.** The repo is pre-1.0; breaking
  changes are fine if they improve the code. Delete dead code outright
  rather than `#[allow(dead_code)]`-ing it.
- **Don't skip `cargo fmt` / `cargo clippy`.** CI runs both and they
  are cheap.
- **Don't `UPDATE_GOLDENS=1` without reading the diff.** If you can't
  explain why a golden flipped, the change is probably wrong.
- **Don't commit `tests/emulator/actual/` or `tests/emulator/node_modules/`.**
  Both are gitignored, but it's worth double-checking before a commit
  that touches the emulator directory.