Going-Commando/AGENTS.md

AGENTS.md — working agreements for this repository

Guidance for coding agents and for anyone picking this up cold. Read this before changing anything.

What this project is

A from-scratch native PC reimplementation of Ratchet & Clank: Going Commando (PlayStation 2, 2003), written in Rust. The game is reimplemented, not emulated and not translated: there is no MIPS interpreter and no CPU emulator anywhere in this repository.

The goal is a build that plays the game the way the original does. "Close enough" is not the bar — the point is that it feels like the game you remember.

Hard rules

1. No game data in this repository. Ever.

Not the ISO, not extracted assets, not textures, not models, not audio, not BIOS dumps, not SDK binaries. Not in a branch, not in a stash, not "just for a test".

• Everything is covered by .gitignore.
• .githooks/pre-commit blocks disc images, asset directories, container signatures and files over 50 MiB. It is activated via git config core.hooksPath .githooks — check that this is still set in a fresh clone, or run git config core.hooksPath .githooks.
• To undo the hook for a legitimate source change: git commit --no-verify. Legitimate source changes should not need it.
• Game data is an input, supplied by the user from their own legally obtained disc. Repositories that need the disc state its serial and hashes.

2. Reproduce the PS2 float model.

The Emotion Engine's FPU is not IEEE-754. It has no denormals, and rounding and exception behaviour differ from x86 in ways that are visible in gameplay.

Gameplay and animation maths must model this deliberately rather than using Rust's native f32 and hoping. When porting a routine, find out how the original computes it and reproduce that, including the odd cases — ReRAC's reference implementation deliberately reproduces a "post-scale list bug" in the moby animation evaluator because doing so is what makes the result match.

Do not "fix" something that looks like a bug until you have confirmed the original does the same thing. Fidelity beats correctness.

3. Never report a number without its bounds and its method.

Any size, count or distribution quoted in a commit message, a doc or a test comes with two things attached: which byte range it describes, and how it was measured.

This is not ceremony. Four separate numbers in one session looked load-bearing and turned out to be artifacts:

• a format identified as VIF-packed, from a handful of recurring byte patterns, where a control put it on the random baseline;
• a body size computed over a span whose bounds were wrong, so the last class absorbed most of the lump's tail;
• a "6.2 MB of unidentified data" region that was texture data;
• "3 unexplained blocks" that were an artifact of sampling too coarsely.

All four shared one cause: measuring before bounding, and naming bytes before looking at them. None of them errored, which is why they survived long enough to be believed.

3a. Partition the measurement, or you have not measured it.

A number computed over a heterogeneous population describes nobody. Report a measurement per partition -- per level, per chunk type, per record size -- before reporting it in aggregate.

Three errors in one stretch were all of this kind, and all three survived the control check that rule 3 asks for, because a matched control bounds significance while saying nothing about scope:

• a 368-byte "period" attributed to a whole region, when it was the stride of one chunk among ~15; averaging them put the peak at the noise floor, which read as "no structure" rather than "wrong scope";
• a ten-entry table disconfirmed, when it existed at a fixed stride among several;
• a moby class array reported at u16 slots 129-140, which was simply the largest-stride chunk type dominating a pooled scan. Per type, every one of the 26 places its array somewhere else.

In every case the aggregate number was arithmetically correct and the conclusion drawn from it was wrong. The re-run that caught all three was the same each time: group by the thing that varies, then look.

• State the partition before the number, not after.
• If a claim is made over a pooled set, also report it for each partition, or say explicitly that it is pooled and unverified.
• A structure found in one partition is not a property of the whole until every partition agrees.
• When a result changes under partitioning, the pooled result was not a weak version of the truth. It was a different and wrong claim. Retract it rather than soften it.

4. Controls, and heuristic counts that are not identifications.

Concretely:

• Derive ranges from gc_formats::regions::core_layout, not by running from one known offset to core_data.len(). Where a bound is unknown it is None, not a guess.
• Before calling a byte pattern a format, test it against a control. If a statistic cannot tell your format apart from uniform random, it cannot tell you anything.
• A heuristic count is not an identification. Count distinct byte values, then look at the blocks that stand out before naming them.

5. Verify against the real disc, not against a guess.

A parser is correct when it round-trips your own disc's bytes, checked by hash. Commit size-and-hash tables (never content) so a test can assert "this matches retail" rather than "this matches whatever was on my disk".

6. Bound every read.

Asset data is untrusted input from a 2003 disc format. Parsers return typed errors; they never panic and never read past the end of a buffer.

Orientation

Read these before working in an unfamiliar area:

Path	What it is
docs/disc-identification.md	Which disc we build against, verified
docs/roadmap.md	Phases, gates, and what is done
docs/plan/	One file per subsystem, written as work proceeds
docs/formats/	Format specs, written as formats are reversed
PROVENANCE.md	Upstream projects we build on, and their licences
docs/plan/pcsx2-verification.md	PCSX2 is set up on this machine, and how to run the comparison loop
vendor/reRAC/	Pinned reference checkout (not tracked)

Crates

Crate	Responsibility
gc-formats	Parse asset formats into plain Rust values. No I/O beyond the caller's buffer.
gc-extract	Disc pipeline: identify, unpack, verify.
gc-game	Game logic: hero, weapons, classes, progression, saves.
gc-video	IPU/MPEG-2 decode.
gc-engine	Bevy app: GS, VU0/VU1, renderer, frame loop. Also owns the hardware models — ps2.rs is the Emotion Engine's FPU, and the tfrag lighting pass runs on it.

The dependency direction is gc-formats ← gc-extract; gc-formats ← gc-game ← gc-engine; gc-video ← gc-game ← gc-engine. gc-formats depends on nothing of ours. Do not invert these — a format parser that can reach into the engine cannot be tested in isolation.

A hardware model belongs in gc-engine, not gc-formats. gc-formats decodes inputs — including the data a hardware model consumes, such as the overlay's sine table — but arithmetic that reproduces how the PS2 computes something is an engine concern. tfrag_light.rs (formats) holds the normal table and the light bank framing; tfrag_light.rs (engine) holds the pass that uses them.

Conventions

• Rust 2021 edition, workspace inheritance for all shared config. Do not hardcode a version in a member crate.
• rustfmt defaults. Clippy warnings are errors in spirit; do not silence them without a comment explaining why.
• Error types: thiserror for library errors in gc-formats and gc-game, anyhow at the binary/xtask boundary.
• Comments explain why, especially for anything that looks like a bug. The .githooks and crate docs deliberately have few inline comments; match the surrounding density rather than adding noise.
• Tests live beside the code (#[cfg(test)] mod tests) for unit-level checks, and in tests/ for golden-file checks against real disc data.

Working rhythm

This is a long project. Keep it navigable:

1. Work in small, coherent commits with a substantive body explaining what was non-obvious. Match the existing style in git log.
2. Update docs/roadmap.md when a phase advances or a gate is met.
3. Write the docs/plan/*.md file for a subsystem when you start it, and update it as you learn. The reasoning is more valuable than the code; the code is in git.
4. Update this file if you change a convention.

When you get stuck

Reverse engineering means long stretches of not knowing. That is normal, not a failure. Write down what you tried and what it ruled out in the relevant docs/plan/ file before moving on — the next attempt (which may be you, in three months) needs the negative results as much as the positive ones.

If something is genuinely blocked, say so plainly and say what would unblock it. Do not paper over a gap with an approximation that looks like it works.

Legal

Ratchet & Clank is a trademark of Sony Interactive Entertainment, developed by Insomniac Games. This is an unofficial fan project, not affiliated with or endorsed by either. We redistribute no game content. Upstream projects we learn from keep their own licences — see PROVENANCE.md, and keep those notices intact when porting their code.