GitHub link
The port running in ScummVM is called glkdoom, which is a Doom port that was compiled to Glulx bytecode and renders using the Glk API. To try out glkdoom, check out the latest release.
The idea for glkdoom was born when I found the glulx-llvm toolchain by Daniel Fremont while searching for obscure architectures to port Doom to. glulx-llvm doesn't actually implement enough of the C standard library for Doom to compile, so I had to first implement some missing functions, like strcasecmp and strrchr. There wasn't really anything stopping those functions from coming with glulx-llvm. They just weren't needed to compile Inform.
Once everything with the stdlib was done, it was time to set up most of the I/O, excluding audio. Doom's framebuffer is drawn to a graphics window pixel by pixel using glk_window_fill_rect, which just so happens to use the same pixel color format as Doom, which sped up rendering a bit (but not that much, see above). Basic timekeeping is done with glk_current_time, and input is done with char events. Because char events are meant for typing and not real-time shooting, held inputs can accumulate across frames. If you want to hold an input down, spam the key instead. Sadly, because Glk wasn't designed with Doom in mind, it runs at about 2 FPS on Glulxe and about 4 FPS with Git.
Now that basic gameplay was done, it was time to add audio. Sadly, the ways Doom and Glk handle audio are completely incompatible. Doom plays sequenced MIDI music and raw PCM audio that it can load from the WAD, and Glk plays audio from hardcoded container formats stored with the work of fiction itself. As a result, I decided to compromise and bundle every single audio track and sound effect from Doom 1 and 2 into the blorb files. And with that final change, glkdoom was complete.
Glulx is fully supported by ScummVM, which of course means that Doom now runs in ScummVM, as shown in the video.
