2 files changed, 120 insertions, 3 deletions

diff --git a/aoc2023/src/day17/solve.gleam b/aoc2023/src/day17/solve.gleam
index 431dcc7..200565a 100644
--- a/aoc2023/src/day17/solve.gleam
+++ b/aoc2023/src/day17/solve.gleam
@@ -1,11 +1,111 @@
 import adglent.{First, Second}
 import gleam/io
 import gleam/string
+import gleam/list
+import gleam/result
 import utilities/array2d.{type Posn, Posn}
+import gleam/bool
+import gleam/dict
+import gleam/queue.{type Queue}
+import gleam/set
+
+type State {
+  State(posn: Posn, heatloss: Int, previous: Posn, history: List(Posn))
+}
+
+const deltas = [Posn(-1, 0), Posn(1, 0), Posn(0, -1), Posn(0, 1)]
+
+fn make_key(s: State) {
+  #(s.posn, same_dir(s))
+}
+
+fn same_dir(s: State) {
+  case s.history {
+    [] -> []
+    [first, ..] as deltas -> list.take_while(deltas, fn(d) { d == first })
+  }
+}
+
+fn is_goal(s: State, min_run: Int, goal: Posn) {
+  goal == s.posn && list.length(same_dir(s)) >= min_run
+}
+
+fn find_good_neighbors(max: Int, min: Int, s: State, grid) {
+  deltas
+  |> list.filter(eliminate_bad_neighbors(_, s, max, min, grid))
+  |> list.map(make_state(_, s, grid))
+}
+
+fn eliminate_bad_neighbors(d: Posn, s: State, max, min, grid) {
+  let neighbor = array2d.add_posns(d, s.posn)
+
+  use <- bool.guard(
+    neighbor == s.previous || !dict.has_key(grid, neighbor),
+    False,
+  )
+  case same_dir(s), list.length(same_dir(s)) {
+    [prev, ..], l if l == max -> d != prev
+    _, 0 -> True
+    [prev, ..], l if l < min -> d == prev
+    _, _ -> True
+  }
+}
+
+fn make_state(d: Posn, s: State, grid) {
+  let neighbor = array2d.add_posns(d, s.posn)
+  let assert Ok(heat_lost) = dict.get(grid, neighbor)
+  State(
+    posn: neighbor,
+    heatloss: s.heatloss + heat_lost,
+    previous: s.posn,
+    history: [d, ..s.history],
+  )
+}
+
+fn find_path(grid, goal, seen, queue: Queue(State), neighbor_fn, goal_fn) {
+  let assert Ok(#(s, rest)) = queue.pop_front(queue)
+  let key = make_key(s)
+
+  case set.contains(seen, key) {
+    True -> find_path(grid, goal, seen, rest, neighbor_fn, goal_fn)
+    False -> {
+      let seen = set.insert(seen, key)
+      let neighbors: List(State) = neighbor_fn(s)
+
+      case list.find(neighbors, goal_fn) {
+        Ok(final) -> final.heatloss
+        Error(Nil) -> {
+          let queue = list.fold(neighbors, queue, queue.push_back)
+          find_path(grid, goal, seen, queue, neighbor_fn, goal_fn)
+        }
+      }
+    }
+  }
+}
 
 pub fn part1(input: String) {
-  input
-  |> array2d.parse_grid
+  let raw_grid =
+    input
+    |> array2d.to_list_of_lists
+
+  let grid = array2d.to_2d_intarray(raw_grid)
+
+  let rmax = list.length(raw_grid)
+  let assert Ok(cmax) =
+    raw_grid
+    |> list.first
+    |> result.map(list.length)
+
+  let goal = Posn(rmax, cmax)
+
+  find_path(
+    grid,
+    goal,
+    set.new(),
+    queue.from_list([State(Posn(0, 0), 0, Posn(0, 0), [])]),
+    find_good_neighbors(0, 3, _, grid),
+    is_goal(_, 1, goal),
+  )
   |> string.inspect
 }
 
diff --git a/aoc2023/src/utilities/array2d.gleam b/aoc2023/src/utilities/array2d.gleam
index 83ec8ae..9d3b966 100644
--- a/aoc2023/src/utilities/array2d.gleam
+++ b/aoc2023/src/utilities/array2d.gleam
@@ -1,6 +1,7 @@
 import gleam/list
 import gleam/dict.{type Dict}
 import gleam/string
+import gleam/int
 
 pub type Posn {
   Posn(r: Int, c: Int)
@@ -25,9 +26,25 @@ pub fn to_2d_array(xss: List(List(a))) -> Array2D(a) {
   |> dict.from_list
 }
 
-pub fn parse_grid(str: String) -> Array2D(String) {
+pub fn to_2d_intarray(xss: List(List(String))) -> Array2D(Int) {
+  {
+    use r, row <- list.index_map(xss)
+    use c, cell <- list.index_map(row)
+    let assert Ok(n) = int.parse(cell)
+    #(Posn(r, c), n)
+  }
+  |> list.flatten
+  |> dict.from_list
+}
+
+pub fn to_list_of_lists(str: String) -> List(List(String)) {
   str
   |> string.split("\n")
   |> list.map(string.to_graphemes)
+}
+
+pub fn parse_grid(str: String) -> Array2D(String) {
+  str
+  |> to_list_of_lists
   |> to_2d_array
 }