LibGM

A tool for unpacking, decompiling and modding GameMaker games such as Undertale or Deltarune.

This is effectively a Rust port of UndertaleModTool (specifically UndertaleModLib).

Benefits of this Rust port

• Runtime for parsing and building is ~8x faster than UndertaleModLib.

  • This can be further accelerated with the upcoming threaded chunk reading feature (opt-in).

• Thorough documentation on docs.rs.

• Clean and maintainable library code.

• Helpful error messages:

  • No NullReferenceException, ever

  • No meaningless stack traces over 50 lines long

  • Still more information than just "Reading out of bounds"

  • Strict data integrity checks catch errors earlier, making debugging easier

  • Example trace printed out using .chain_pretty():

    sprite::swf::item::shape::style_group::fill::gradient::Record count 1065353216 implies data size 8.5 GB which exceeds failsafe size 10.0 MB
    ↳ while reading simple list
    ↳ while deserializing element 1/2 of sprite::swf::item::shape::style_group::StyleGroup<sprite::swf::item::subshape::Data> simple list
    ↳ while deserializing element 0/1 of sprite::swf::item::Item simple list
    ↳ while deserializing element 3/60 of GMSprite pointer list
    ↳ while deserializing chunk 'SPRT'
    ↳ while parsing GameMaker data file ./gm48_datafiles/a-loop_detective.win
    

• Configurable lenient options for trying to parse half-broken data files.

Disadvantages / TODOs

• Null pointers are not yet supported.
• GML Decompiler and Compiler not yet implemented (help would be greatly appreciated!)
• No GUI yet, only a Rust library.

How to use as a dependency

Add this line in the [dependencies] section of your Cargo.toml file:

libgm = "0.3.2"

Or if you want bleeding edge:

libgm = { git = "https://github.com/BioTomateDE/LibGM" }

Now you can use these functions exposed by LibGM:

• parse_file(data_file_path: impl AsRef<Path>) -> Result<GMData>
• parse_bytes(raw_data: impl AsRef<[u8]>) -> Result<GMData>
• build_data(gm_data: &GMData, path: impl AsRef<Path>) -> Result<()>
• build_bytes(gm_data: &GMData) -> Result<Vec<u8>>

If you need more control over how the data file should be read, you can also use the DataParserOptions struct to modify parsing options:

// Create a parser with custom options
let parser = DataParserOptions::new()
    .verify_alignment(false)
    .allow_unread_chunks(true);

// Parse multiple files
for path in data_files {
    let data: GMData = parser.parse_file(path)?;
    // Process the parsed data...
}

// Parse from a byte vector
let raw_data: Vec<u8> = read_from_zip(zip_file, "Undertale/assets/game.unx")?;
let data: GMData = parser.parse_bytes(raw_data)?;

// Parse from a byte slice reference
let byte_slice: &[u8] = &[0x46, 0x4F, 0x52, 0x4D, /* ... */];
let data: GMData = parser.parse_bytes(byte_slice)?;

// You can also parse directly from borrowed data
let buffer: Vec<u8> = std::fs::read("./data.win")?;
let data: GMData = parser.parse_bytes(&buffer)?;
// buffer is still accessible here since we passed a reference

Crate features

• catch-panic catches panics in GameMaker (de)serialization functions and returns them as a LibGM error.
• check-integrity enables alignment and constant validation while parsing. These checks may still be demoted to a warning using ParsingOptions. Some checks regarding panic safety or memory allocation are always enabled.
• game-creation-timestamp exposes the creation-timestamp field in GMGeneralInfo. Otherwise, it will be stored as an i64 iternally.
• bzip2-image enables deserialization of BZip2+QOI encoded texture pages. If *_dynamic_image is called on a GMImage that contains Bzip2-Qoi data and this feature is disabled, an error will be returned.
• png-image enables PNG deserialization and all serialization of texture pages. If you deserialize texture pages into DynamicImages with this feature disabled, you will not be able to serialize the data.

Credits

Huge thanks to the Underminers Team! Without UndertaleModTool, this project would've been impossible. I also want to thank the people in the Underminers Discord Server who helped me along the way, especially @colinator27.

Licencing

This project is licenced under the GNU Public Licence v3.0 (GPL-3).

Contributing

All contributions are welcome! Whether that's a pull request, a feature you would like to see added, a bug you found; just create an Issue/PR in this repo.

• Everything related to GameMaker is located in libgm/src/gamemaker/.
• There is a basic CLI to interact with LibGM. Its code is located in libgm-cli/src/.

Roadmap

• Add QOI and Bz2Qoi image serialization
• Implement threading for parser
• Add crate features (maybe use prefix for dependency disablers?):
  • bzip2 (opt-out): Enables Bz2Qoi image support
  • png (opt-out: Enables PNG image support
  • chrono (opt-out): Exposes the general info creation timestamp field. stored as raw data otherwise
  • check-integrity (opt-out): Enables all data integrity checks (pointer validation, constant validation etc). These checks may still be demoted to a warning using ParsingOptions. Some checks regarding panic safety or memory allocation should always be enabled.
  • panic-catching (opt-out): Sets a panic handler before data [de]serialization, returning a LibGM error if a panic occurred
• Overhaul the CLI: Allow for viewing of relevant data, exporting assembly and more
• Maybe move the CLI to a different repo / publish it?
• Add helpers to Instruction (like fn get_variable(&self) -> Option<&CodeVariable> that returns some for push, pushloc, pushglb, pushbltn or pop)
• "Fix" disassembler for child code entries (right now they will generate empty string)
• Maybe add some sort of header for assembly for entire code entries (name, local count, arg count) so u can assemble_code directly
• Add comments to assembly? Which style tho? How much does it fuck Up efficiency and maintainability?