C

There are interesting analytical observations to be made about the problem to sidestep most of the actual iteration, but I wasn’t up to working it all out and the runtime was pretty much instant anyway.

Here’s my original solution with ints, using divions with powers of 10 to do the splitting: day02-u64.c

But I’m only doing the C implementations to prototype my assembly solutions, and dealing with 64-bit numbers, especially divisions, on x86-16 is painful, so I rewrote the solution using a fixed-length string “numbers” instead: day02.c

Still working on the assembly version

Assembly update: have part 1 working now! It’s dog slow on DOSBox, but on QEMU it’s good: day02.asm

Awesome! I was looking for this type of solution and made some progress but couldn’t get to it

Nim

Easy one today. Part 2 is pretty forgiving on performance, so regex bruteforce was only a couple seconds . But eventually I’ve cleaned it up and did a solution that runs in ~340 ms.

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

proc isRepeating(str:string, sectorLength=1): bool =
  if str.len mod sectorLength != 0: return false
  for i in countUp(0, str.len - sectorLength, sectorLength):
    if str.toOpenArray(i, i+sectorLength-1) != str.toOpenArray(0, sectorLength-1):
      return false
  true

proc solve(input: string): AOCSolution[int, int] =
  let ranges = input.split(',').mapIt:
    let parts = it.split('-')
    (parseInt parts[0], parseInt parts[1])

  for (a, b) in ranges:
    for num in a .. b:
      if num < 10: continue
      let strnum = $num
      let half = strnum.len div 2

      for i in countDown(half, 1):
        if strnum.isRepeating(i):
          if i == half and strnum.len mod 2 == 0:
            result.part1 += num
          result.part2 += num
          break

Full solution at Codeberg: solution.nim

DOS + BIOS boot (hybrid binary)

Repo | day02.asm | .COM download

Got the x86-16 assembly implementation working at last! Here, 64-bit integer math, especially lots of divisions by power of 10, wasn’t going to do so the code instead operates on fixed-width, zero-padded numeric strings. Lots of time lost to debugging control flow/logic mistakes this time. I need to make printf() a priority!

Short input so no space issues, sitting comfortably at 45K, well below the 64K COM limit! Sadly no time yet to add animations or anything cool.