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.

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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

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:

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:

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

# 2. Verify every committed 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 reads the committed examples/*.nes files directly — since those are rebuilt and diffed by the examples CI job, the emulator job doesn't need a compiler toolchain at all. If you're iterating on the compiler locally, rebuild the example ROM(s) you care about before re-running the harness:

cargo build --release
./target/release/nescript build examples/hello_sprite.ne
# then:
(cd tests/emulator && node run_examples.mjs)

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:

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.