Day 12 part 2

const 
  XLEN = 101
  YLEN = 41

type
  Position = tuple
    x, y: int
  Edge = tuple
    head, tail: Position
  HeightMap = array[XLEN, array[YLEN, char]]

var
  hMap: HeightMap
  answer: Natural = Natural.high
  startPos, endPos: Position
  edges: Deque[Edge]

proc `$`(p: Position): string = fmt"[{hMap[p.x][p.y]}]({p.x+1},{p.y+1})"
proc `-`(a, b: Position): int = ord(hMap[a.x][a.y]) - ord(hMap[b.x][b.y])

proc parent(p: Position): Position =
  for e in edges:
    if e.tail == p: return e.head
  raise newException(OSError, fmt"No parent for {p}. Node unreachable.")

iterator neighbors(p: Position): Position =
  for i in [-1, 1]:
    if p.x+i >= 0 and p.x+i < XLEN:
      var neigh: Position = (p.x+i, p.y)
      if p - neigh <= 1: yield neigh
    if p.y+i >= 0 and p.y+i < YLEN:
      var neigh: Position = (p.x, p.y+i)
      if p - neigh <= 1: yield neigh

proc readFile() =
  var y: Natural
  for line in "input.txt".lines:
    for x in 0..<line.len:
      hMap[x][y] = line[x]
      if line[x] == 'S': 
        startPos = (x, y)
        hMap[x][y] = 'a'
      if line[x] == 'E': 
        endPos = (x, y)
        hMap[x][y] = 'z'
    y.inc

proc buildTree(root: Position) =
  var 
    queue: Deque[Position]
    visited: HashSet[Position]
  queue.addLast(root)
  while queue.len > 0:
    var head = queue.popFirst
    for tail in head.neighbors:
      if tail in visited: continue
      queue.addLast(tail)
      var e: Edge = (head, tail)
      edges.addLast(e)
      visited.incl tail
    visited.incl head

proc countSteps(target: Position): Natural =
  var node = target
  while node != endPos:
    result.inc
    node = node.parent

proc getTargets(): seq[Position] = 
  result = newSeq[Position]()
  for x in 0..<hMap.len:
    for y in 0..<hMap[x].len:
      if hMap[x][y] == 'a': 
        var p: Position = (x, y)
        result.add p

readFile()

buildTree(endPos)

for t in getTargets():
  try:
    let steps = countSteps(t)
    answer = min(answer, steps)
  except OSError: continue

Answer is: 354