Day 14 part 2

const MAPLEN = 1000

type
  Point = tuple
    x,y: int
  GridType = enum
    Air, Rock, Sand
  Map = array[MAPLEN, array[MAPLEN, GridType]]

var
  answer, leftHeap, rightHeap, floorLevel: Natural

proc newPoint(s: string): Point = assert scanf(s, "$i,$i", result.x, result.y)

proc isStanding(heap: var Natural, y: Natural): bool =
  let h = MAPLEN - y
  if heap > (h-1) * h div 2: return true
  heap.inc

proc dropSand(m: var Map): bool =
  result = true
  var 
    s: Point = (500, 0)
    i = 0
  if m[s.x][s.y] == Sand: return false
  while true:
    if s.y+i == floorLevel: 
      m[s.x][s.y+i-1] = Rock
      return true
    if m[s.x][s.y+i] == Air: 
      i.inc
      continue
    if m[s.x][s.y+i] in [Rock, Sand]:
      if s.x == 0:
        if leftHeap.isStanding(s.y+i): m[s.x][s.y+i-1] = Rock
        return true
      elif s.x == MAPLEN-1:
        if rightHeap.isStanding(s.y+i): m[s.x][s.y+i-1] = Rock
        return true
      if m[s.x-1][s.y+i] == Air: 
        s.x.dec
      elif m[s.x+1][s.y+i] == Air: 
        s.x.inc
      else: 
        m[s.x][s.y+i-1] = Sand
        return true

proc createRockLine(map: var Map, a, b: Point) =
  floorLevel = max(floorLevel, max(a.y+2, b.y+2))
  if a.x < b.x:
    for x in a.x..b.x: map[x][a.y] = Rock
  elif a.x > b.x:
    for x in b.x..a.x: map[x][a.y] = Rock
  elif a.y < b.y:
    for y in a.y..b.y: map[a.x][y] = Rock
  elif a.y > b.y:
    for y in b.y..a.y: map[a.x][y] = Rock

proc createRocks(map: var Map, points: var seq[string]) =
  var
    rockStart = newPoint(points.pop)
    rockEnd = newPoint(points.pop)
  map.createRockLine(rockStart, rockEnd)
  while points.len > 0:
    rockStart = rockEnd
    rockEnd = newPoint(points.pop)
    map.createRockLine(rockStart, rockEnd)

proc loadMap(): Map =
  for line in "input.txt".lines:
    var points = line.split(" -> ")
    result.createRocks(points)
      
var map = loadMap()
while map.dropSand: answer.inc

Answer is: 25771