A work-in-progress decompilation of Castlevania Symphony of the Night for PlayStation 1. It aims to recreate the source code from the existing binaries using static and/or dynamic analysis. The code compiles byte-for-byte to the same binaries of the game, effectively being a matching decompilation. Currently it only supports the US version of the game `SLUS-00067`.
This repo does not include any assets or assembly code necessary for compiling the binaries. A prior copy of the game is required to extract the required assets.
*`SLUS_000.67` the main executable. It contains all the hardware API (eg. gamepad, CD, memory card, GPU renderer) of the PlayStation 1 console. It does not contain any game logic.
*`DRA` the game engine. It contains the business logic (eg. gameloop, API to draw maps, entities, load levels, handle entities, animations and collisions) and some data such as Alucard's sprites or the loading/save rooms.
*`ST/` the overlays for each area. An area (eg. Castle's entrance, Alchemy Laboratory, etc.) contains all the unique logic to handle map specific events, cutscenes, enemy AI, collisions and more. It also contains the rooms and entities layout. Each overlay can be considered as its own mini-game. The title screen `SEL.BIN` is an example of how a stage overlay can act very differently.
Even if different overlays are loaded at the same time in memory, like `DRA` and stages, they never communicate each other directly. Instead they share the same memory area where `SLUS_000.67` is located. Each overlay exposes their API as function pointers in the shared memory area, effectively allowing overlays to communicate without directly coupling them. One prime example is `struct GameApi`, which exposes `DRA` APIs to the stages and stage APIs to `DRA`. All the shared area is defined in `game.h`.
Some non-matching functions are present in the source preprocessed by the macro `NON_MATCHING`. You can still compile the game binaries by running `CPP_FLAGS=-DNON_MATCHING make`. In theory they might be logically equivalent in-game, but I cannot promise that. Few of them could match by tuning or changing the compiler.
1. Run `./tools/decompile.py func_801873A0` to decompile the function in the C source code where the function is supposed to be located
1. If the function does not compile, try addressing the compilation errors until `make` compiles
1. If the function does not match, invoke `python3 ./tools/asm-differ/diff.py -mwo --overlay st/wrp func_801873A0` and refactor the code until it matches
1. If the function matches, try refactoring to keep the code clean while checking if the function still matches once in a while
1. Use [decomp.me](https://decomp.me/) with PSY-Q 4.0. Be aware that the repo is using GCC 2.6.x, so decomp.me will sometimes give a slightly different output.
1. Use [decomp-permuter](https://github.com/simonlindholm/decomp-permuter) to solve some mismatches.
1. Use [this](https://github.com/mkst/sssv/wiki/Jump-Tables) and [this](https://github.com/pmret/papermario/wiki/GCC-2.8.1-Tips-and-Tricks) guide to understand how some compiler patterns work.
1. Use the `#ifndef NON_MATCHING` if your code is logically equivalent but you cannot yet fully match it.
Due to how the game is structured, a lot of duplicate code can be found across the different overlays. We [track a live list of possible duplicate functions](https://raw.githubusercontent.com/Xeeynamo/sotn-decomp/gh-duplicates/duplicates.txt) that is useful to avoid trying to decompile functions that have been already decompiled elsewhere.
* PS1’s CPU R3000 instruction [manual](https://cgi.cse.unsw.edu.au/~cs3231/doc/R3000.pdf) and [cheat sheet](https://vhouten.home.xs4all.nl/mipsel/r3000-isa.html)
* The debug room overlay `ST/MAD.BIN` was compiled earlier than the first retail release of the game. All the offsets that refers to DRA.BIN points to invalid portions of data or to the wrong API calls, effectively breaking the majority of its original functionalities. That is why the debug room does not contain any object. By compiling the debug room with make mad_fix you can restore it by redirecting the old pointers to the retail version of the game.
Be aware that not all the offsets have been yet redirected, so it will still be not entirely functional until further update.
* I suspect that GCC 2.6.x / PSY-Q 3.4 have been used to originally compile DRA.BIN
*`main.exe` uses PS-X libraries that might have been created with a different compiler and with `-O1` rather than `-O2`
