6 files changed, 233 insertions(+), 3 deletions(-)

M .gitignore
A input/day12.example.txt
A input/day12.txt
M src/aoc2022.gleam
A src/aoc2022/day12.gleam
M src/aoc2022/utils.gleam

@@ 2,3 2,4 @@
 *.ez
 build
 erl_crash.dump
+/tmp/<
\ No newline at end of file

@@ 0,0 1,5 @@
+Sabqponm
+abcryxxl
+accszExk
+acctuvwj
+abdefghi<
\ No newline at end of file

@@ 0,0 1,41 @@
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\ No newline at end of file

@@ 1,9 1,9 @@
 import gleam/io
-import aoc2022/day10
+import aoc2022/day12
 
 pub fn main() {
   io.print("Part 1: ")
-  io.debug(day10.part1())
+  io.debug(day12.part1())
   io.print("Part 2: ")
-  io.debug(day10.part2())
+  io.debug(day12.part2())
 }

@@ 0,0 1,176 @@
+//// --- Day 12: Hill Climbing Algorithm ---
+//// https://adventofcode.com/2022/day/12
+
+import aoc2022/utils
+import gleam/int
+import gleam/list
+import gleam/map
+import gleam/option.{None, Option, Some}
+import gleam/queue
+import gleam/result
+import gleam/string
+
+type Pos {
+  Pos(x: Int, y: Int)
+}
+
+type HeightMap =
+  map.Map(Pos, Int)
+
+type Visited =
+  map.Map(Pos, Option(Pos))
+
+type Queue =
+  queue.Queue(Pos)
+
+type State {
+  State(end: Pos, height_map: HeightMap, queue: Queue, visited: Visited)
+}
+
+pub fn part1() {
+  let #(start, end, height_map) = parse_input()
+  play(start, end, height_map)
+}
+
+pub fn part2() {
+  let #(_start, end, height_map) = parse_input()
+
+  height_map
+  |> map.filter(fn(_pos, height) { height == 0 })
+  |> map.keys
+  |> list.map(play(_, end, height_map))
+  |> list.filter(result.is_ok)
+  |> list.map(utils.assert_ok)
+  |> list.reduce(int.min)
+}
+
+fn play(start, end, height_map) {
+  let visited = map.from_list([#(start, None)])
+  let queue = queue.from_list([start])
+  let state = State(end, height_map, queue, visited)
+  do_play(state)
+}
+
+// BFS to find path from start to end
+// Could be improved by A* but not really required for this one
+fn do_play(state: State) -> Result(Int, Nil) {
+  case queue.pop_front(state.queue) {
+    Ok(#(current_pos, queue)) ->
+      case current_pos == state.end {
+        True -> {
+          let path = unravel_path(state.visited, current_pos)
+          Ok(list.length(path) - 1)
+        }
+        False -> {
+          let next =
+            next_positions(current_pos, state.height_map, state.visited)
+          let visited = add_visited(state.visited, current_pos, next)
+          let queue = add_queue(queue, next)
+          let state = State(state.end, state.height_map, queue, visited)
+          do_play(state)
+        }
+      }
+    // Queue depleted, no path found
+    Error(Nil) -> Error(Nil)
+  }
+}
+
+fn unravel_path(visited, end_pos) -> List(Pos) {
+  do_unravel_path(visited, end_pos, [])
+}
+
+fn do_unravel_path(visited, pos, path) -> List(Pos) {
+  case utils.assert_ok(map.get(visited, pos)) {
+    Some(prev_pos) -> do_unravel_path(visited, prev_pos, [pos, ..path])
+    None -> [pos, ..path]
+  }
+}
+
+fn add_visited(visited: Visited, current_pos: Pos, next: List(Pos)) {
+  list.fold(
+    next,
+    visited,
+    fn(visited, next_pos) { map.insert(visited, next_pos, Some(current_pos)) },
+  )
+}
+
+fn add_queue(queue: Queue, next: List(Pos)) {
+  list.fold(
+    next,
+    queue,
+    fn(queue, next_pos) { queue.push_back(queue, next_pos) },
+  )
+}
+
+fn next_positions(pos, height_map, visited) -> List(Pos) {
+  pos
+  |> neighbours()
+  // Remove out of bounds
+  |> list.filter(map.has_key(height_map, _))
+  // Remove visited
+  |> list.filter(fn(neighbour) { !map.has_key(visited, neighbour) })
+  // Remove neighbours too high to climb
+  |> list.filter(fn(neighbour) {
+    assert Ok(pos_height) = map.get(height_map, pos)
+    assert Ok(neighbour_height) = map.get(height_map, neighbour)
+    neighbour_height <= pos_height + 1
+  })
+}
+
+fn neighbours(pos: Pos) -> List(Pos) {
+  [
+    Pos(pos.x + 1, pos.y),
+    Pos(pos.x - 1, pos.y),
+    Pos(pos.x, pos.y + 1),
+    Pos(pos.x, pos.y - 1),
+  ]
+}
+
+// -----------------------------------------------------------------------------
+// Input Parsing
+// -----------------------------------------------------------------------------
+
+fn parse_input() -> #(Pos, Pos, HeightMap) {
+  let char_map =
+    "day12.txt"
+    |> utils.read_input
+    |> parse_to_char_map
+
+  let start = find_char(char_map, "S")
+  let end = find_char(char_map, "E")
+  let height_map = map.map_values(char_map, fn(_, v) { char_to_height(v) })
+  #(start, end, height_map)
+}
+
+fn char_to_height(char) {
+  let char = case char {
+    "S" -> "a"
+    "E" -> "z"
+    other -> other
+  }
+
+  utils.ord(char) - 97
+}
+
+fn find_char(map, char) -> Pos {
+  assert [#(pos, _)] =
+    map
+    |> map.filter(fn(_, v) { v == char })
+    |> map.to_list
+
+  pos
+}
+
+fn parse_to_char_map(input) {
+  input
+  |> parse_to_nested_list
+  |> list.flatten
+  |> map.from_list
+}
+
+fn parse_to_nested_list(input) {
+  let rows = string.split(input, "\n")
+  use y, row <- list.index_map(rows)
+  use x, char <- list.index_map(string.to_graphemes(row))
+  #(Pos(x, y), char)
+}

@@ 1,4 1,5 @@
 import gleam/int
+import gleam/io
 import gleam/list
 
 // Externals


@@ 52,3 53,9 @@ pub fn list_at(list: List(a), at: Int) -> Result(a, Nil) {
     False -> Error(Nil)
   }
 }
+
+pub fn inspect(term: anything, label: String) -> anything {
+  io.print(label)
+  io.print(": ")
+  io.debug(term)
+}