Cave Story Engine 2

CSE2:

Creator(s):	Clownacy Et al.
Year:	2019
Platform:	Mac OS, Linux OS, Windows OS
Status:	DMCA'd
Info:	Original Forum Thread

Cave Story Engine 2, commonly referred to as CSE2, is a fan-made decompilation of the original freeware Cave Story, started in 2019. The port aimed to recreate a byte-for-byte identical copy of the original CS freeware executable from source code.

CSE2 is popular among modders for its relatively clean code and accurate-to-original behavior. The engine is often either modded directly or used as a reference guide for freeware hacking, as many of the functions are identical to the originals.

Background

From the original source code’s readme, the port’s history is as follows:

When Pixel made Cave Story, he compiled the original Windows EXE with no optimizations. This left the generated assembly code extremely verbose and easy to read. It also made the code very decompiler-friendly, since the assembly could be mapped directly back to the original C++ code.

Technically, this alone made a decompilation feasible, as was the case for the Super Mario 64 decompilation project - however, there was more to be found…

In 2007, a Linux port of Cave Story was made by Simon Parzer and Peter Mackay. Details about it can be found on Peter’s old blog. This port received an update in 2011, including two shiny new executables. What they didn’t realize was that they left huge amounts of debugging information in these executables, including the names of every C++ source file, as well as the variables, functions, and structs they contained.

This was a goldmine of information about not just the game’s inner-workings, but its source code. This is the same lucky-break the Diablo decompilation project had. With it, much of the game’s code was pre-documented and explained, saving us the effort of doing it ourselves. In fact, the combination of easy-to-decompile code, and a near-full set of function/variable names, reduced much of the decompilation process to mere copy-paste.

To top it all off, some of Cave Story’s original source code would eventually see the light of day:

In early 2018, the Organya music engine was released on GitHub by an old friend of Pixel’s. On top of providing an insight into Pixel’s coding style, this helped with figuring out one of the most complex parts of Cave Story’s codebase.

And… that’s it! It’s not often that a game this decompilable comes along, so I’m glad that Cave Story was one of them. Patching a dusty old executable from 2005 sucks.

Official Branches

When CSE2 was publicly available, these branches existed:

• Accurate: The main decompilation branch. The code is intended to be as close to the original as possible, down to all the bugs and platform-dependencies.

• Portable: This branch ports the engine away from WinAPI and DirectX, and addresses numerous portability issues, allowing it to run on other platforms.

• Enhanced-Lite: Based on the portable branch, this branch is intended for modders, and adds several enhancements and features to the engine:
  • Widescreen
  • Options menu:
    • Control remapping (keyboard and gamepad)
    • Soundtrack selection
    • V-sync toggle
    • 50FPS/60FPS toggle
    • Option to disable the design choice that locks sprites to a 320x240 grid when drawn, making them move smoother
  • Bugfixes
• Enhanced: Based on the enhanced-lite branch, this branch is intended for players, and adds further enhancements and features to the engine:
  • Support for alternate soundtracks

• Emscripten: This branch allows the game to be compiled with Emscripten, enabling it to be played in a browser. This requires fairly extensive (and messy) modifications to the engine, to remove its dependency on “mainloops” - the infinite loops games put themselves in so they can run indefinitely. It’s because of this that Emscripten support is limited to this branch: it’s just so invasive. A proof-of-concept can be found here.

• Wii: Ports the engine to the Nintendo Wii.

DMCA Takedown

Unfortunately, Nicalis issued a DMCA takedown of CSE2 when development was nearing completion. The first of these takedowns was submitted on November 19, 2020, with a second one following up on December 8, 2020. Despite this, late copies of the engine are complete, playable from start to finish.

In contrast to the original engine, Nicalis has not gone after mods based on CSE2, and it still remains popular for modding usage.

Compiled mods using CSE2 are freely distributed and can commonly be found in modding databases. However, publicly sharing the source code is not advised and strongly discouraged by the modding community. Because of this, a download link will not be provided, but the code is common enough that a simple internet search will often yield the desired results.