Nim

For part 3 I parse gears as tuples, with regular gears having same value on both ends e.g.

3|5 -> (3, 5)
3   -> (3, 3)

proc parseGears(input: string): seq[int] =
  for line in input.splitLines():
    result.add parseInt(line)

proc parseNestedGears(input: string): seq[(int, int)] =
  for line in input.splitLines():
    let nested = line.split('|').mapIt(it.parseInt)
    result.add:
      if nested.len == 1: (nested[0], nested[0])
      else: (nested[0], nested[1])

proc solve_part1*(input: string): Solution =
  let gears = parseGears(input)
  result := 2025 * gears[0] div gears[^1]

proc solve_part2*(input: string): Solution =
  let gears = parseGears(input)
  result := ceil(10000000000000'f64 / (gears[0] / gears[^1])).int

proc solve_part3*(input: string): Solution =
  let gears = parseNestedGears(input)
  let ratios = (0..gears.high-1).mapIt(gears[it][1] / gears[it+1][0])
  result := int(100 * ratios.prod)

Full solution at Codeberg: solution.nim