DOS + BIOS boot (hybrid binary)

Repo | day01.asm | day01.c (prototype) | .COM download

Written in x86-16 assembly. Works as a DOS program but also as disk image for older PCs with BIOS support (or older VMs). Getting this setup to work was tricky, especially since when I started this, I had only basic x86-16 experience! Needless to say I’ve spent much time staring at hex numbers.

The very start of the file determines if it’s running in DOS or as a bootloader, in which case it’ll have to load the remainder of the file from disk and rearrange the memory layout to emulate the DOS situation.

Right now this is using just one segment of memory (64K). Input data is compressed using a custom run-length encoding scheme. The compressed background image is directly decoded into the VGA framebuffer, after which it is overwritten with the decompressed input file. Space is tight!

My main goal is to finish a few days at least with one or more game consoles supported (GameBoy Advance would be cool) and to do some cool palette tricks, like a day/night transition or animated water and stars.

Nim

That was the rough first day for me. Part 1 was ok. For part 2 I didn’t want to go the easy route, so I was trying to find simple formulaic solution, but my answer was always off by some amount. And debugging was hard, because I I was getting the right answer for example input.
After 40 minutes I wiped everything clean and wrote a bruteforce.

Later that day I returned and solved this one properly. I had to draw many schemes and consider all the edge cases carefully to come up with code below.

type
  AOCSolution[T,U] = tuple[part1: T, part2: U]

proc solve(input: string): AOCSolution[int, int] =
  var dial = 50
  for line in input.splitLines():
    let value = parseInt(line[1..^1])
    let sign = if line[0] == 'L': -1 else: 1
    let offset = value mod 100
    result.part2 += value div 100

    if dial != 0:
      if sign < 0 and offset >= dial or
         sign > 0 and offset >= (100-dial): inc result.part2

    dial = (dial + offset * sign).euclmod(100)
    if dial == 0: inc result.part1

Full solution at Codeberg: solution.nim