Nim

As many others today, I’ve solved part 2 first and then fixed a ‘bug’ to solve part 1. =)

type Vec2 = tuple[x,y:int]
const Adjacent = [(x:1,y:0),(-1,0),(0,1),(0,-1)]

proc path(start: Vec2, grid: seq[string]): tuple[ends, trails: int] =
  var queue = @[@[start]]
  var endNodes: HashSet[Vec2]
  while queue.len > 0:
    let path = queue.pop()
    let head = path[^1]
    let c = grid[head.y][head.x]

    if c == '9':
      inc result.trails
      endNodes.incl head
      continue

    for d in Adjacent:
      let nd = (x:head.x + d.x, y:head.y + d.y)
      if nd.x < 0 or nd.y < 0 or nd.x > grid[0].high or nd.y > grid.high:
        continue
      if grid[nd.y][nd.x].ord - c.ord != 1: continue
      queue.add path & nd
  result.ends = endNodes.len

proc solve(input: string): AOCSolution[int, int] =
  let grid = input.splitLines()
  var trailstarts: seq[Vec2]

  for y, line in grid:
    for x, c in line:
      if c == '0':
        trailstarts.add (x,y)

  for start in trailstarts:
    let (ends, trails) = start.path(grid)
    result.part1 += ends
    result.part2 += trails

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