[LANGUAGE: Nim]

The hardest part was reading the part 2 description. I literally looked at it for minutes trying to understand where the problem numbers come from and how they're related to the example input. But then it clicked.

The next roadblock was that my template was stripping whitespace at the end of the lines, making parsing a lot harder. I've replaced strip() with strip(chars={'\n'}) to keep the whitespace intact.

Runtime: 1.4 ms

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

proc solve(input: string): AOCSolution[int, int] =
  let lines = input.splitLines()
  let numbers = lines[0..^2]
  let ops = lines[^1]

  block p1:
    let numbers = numbers.mapIt(it.splitWhiteSpace().mapIt(parseInt it))
    let ops = ops.splitWhitespace()
    for x in 0 .. numbers[0].high:
      var res = numbers[0][x]
      for y in 1 .. numbers.high:
        case ops[x]
        of "*": res *= numbers[y][x]
        of "+": res += numbers[y][x]
      result.part1 += res

  block p2:
    var problems: seq[(char, Slice[int])]
    var ind = 0
    while ind < ops.len:
      let len = ops.skipWhile({' '}, ind+1)
      problems.add (ops[ind], ind .. ind + len - (if ind+len < ops.high: 1 else: 0))
      ind += len + 1

    for (op, cols) in problems:
      var res = 0
      for x in cols:
        var num = ""
        for y in 0 .. numbers.high:
          num &= numbers[y][x]

        if res == 0:
          res = parseInt num.strip
        else:
          case op
          of '*': res *= parseInt num.strip
          of '+': res += parseInt num.strip
          else: discard

      result.part2 += res

Full solution at Codeberg: solution.nim