HydroCore

In-game mod loader for Hydro on Subnautica 2. UE4SS is used as a bootstrapper for the initial DLL injection; everything past that runs through HydroCore's own pure-C++ engine reflection layer, so the loader isn't coupled to UE4SS APIs and can be ported off of it later without rewriting the engine-facing code. Mounts mod pak files, parses each mod's hydromod.toml, and hosts a LuaJIT runtime so mod scripts can spawn actors, hook UFunctions, and read/write UObject properties through a reflection API.

Build

Windows x64. You need:

• Visual Studio 2022 with C++ + CMake workloads
• Rust via rustup (for patternsleuth)
• A built UE4SS + LuaJIT under deps/ (see deps/README.md)

Then:

build.bat

Output lands at build/src/HydroCore.dll. The Hydro launcher deploys it into the game as <Game>/Binaries/Win64/ue4ss/Mods/HydroCore/dlls/main.dll.

Layout

• src/dllmain.cpp is the UE4SS adapter. Only file that touches UE4SS headers.
• src/EngineAPI.* does pattern scanning, GUObjectArray iteration, and ProcessEvent dispatch.
• src/LuaRuntime.* and src/LuaUObject.* host the Lua VM and expose UObject reflection.
• src/ModManifest.* parses hydromod.toml.
• src/PakLoader.* handles runtime pak mounting via FPakPlatformFile.
• src/api/ contains the Hydro.Assets, Hydro.World, and Hydro.Events Lua modules.
• types/ has .d.lua type definitions for IDE autocomplete when writing mods.

Status

Works. Reads the manifest, mounts paks, initializes Lua, registers the API modules, runs mod init scripts, and spawns Blueprint actors.

Known rough edges:

• UFunction hooking supports simple param types, not FString/FVector out params
• Dependency checks are presence-only; semver range matching lives in the launcher
• Tier 1 sandbox restricts the Lua base library but doesn't yet deny C memory access through bound APIs