day 10 gleam complete

1 files changed, 154 insertions, 2 deletions

diff --git a/aoc2023/src/day10/solve.gleam b/aoc2023/src/day10/solve.gleam
index 43dba6d..c33634d 100644
--- a/aoc2023/src/day10/solve.gleam
+++ b/aoc2023/src/day10/solve.gleam
@@ -1,12 +1,164 @@
 import adglent.{First, Second}
+import gleam/bool
+import gleam/dict.{type Dict}
+import gleam/list
+import gleam/int
 import gleam/io
+import gleam/set.{type Set}
+import gleam/string
+
+type Posn {
+  Posn(row: Int, col: Int)
+}
+
+fn add_posns(p1: Posn, p2: Posn) -> Posn {
+  Posn(p1.row + p2.row, p1.col + p2.col)
+}
+
+type PipeGrid =
+  Dict(Posn, String)
+
+const north = Posn(-1, 0)
+
+const south = Posn(1, 0)
+
+const east = Posn(0, 1)
+
+const west = Posn(0, -1)
+
+const initial_directions = [
+  #(north, ["|", "7", "F"]),
+  #(south, ["|", "J", "L"]),
+  #(east, ["-", "J", "7"]),
+  #(west, ["-", "F", "L"]),
+]
+
+fn pipe_neighbors(pipe: String) -> List(Posn) {
+  case pipe {
+    "|" -> [north, south]
+    "-" -> [east, west]
+    "L" -> [north, east]
+    "F" -> [south, east]
+    "7" -> [south, west]
+    "J" -> [north, west]
+    _ -> panic as "bad pipe"
+  }
+}
+
+fn make_grid(input: String) -> PipeGrid {
+  {
+    use r, row <- list.index_map(string.split(input, "\n"))
+    use c, col <- list.index_map(string.to_graphemes(row))
+    #(Posn(r, c), col)
+  }
+  |> list.flatten
+  |> dict.from_list
+}
+
+fn valid_start_direction(grid: PipeGrid, s: Posn) {
+  let assert [dir, ..] = {
+    use d <- list.filter_map(initial_directions)
+    let #(delta, valids) = d
+    let neighbor = add_posns(s, delta)
+    case dict.get(grid, neighbor) {
+      Ok(pipe) ->
+        case list.contains(valids, pipe) {
+          True -> Ok(neighbor)
+          False -> Error(Nil)
+        }
+      Error(_) -> Error(Nil)
+    }
+  }
+  dir
+}
+
+fn to_next_pipe(current: Posn, grid: PipeGrid, acc: List(Posn)) {
+  let assert [prev, ..] = acc
+  let assert Ok(pipe) = dict.get(grid, current)
+  use <- bool.guard(pipe == "S", [current, ..acc])
+  let assert [next] = {
+    pipe
+    |> pipe_neighbors
+    |> list.filter_map(fn(p) {
+      case add_posns(p, current) {
+        neighbor if neighbor == prev -> Error(Nil)
+        neighbor -> Ok(neighbor)
+      }
+    })
+  }
+  to_next_pipe(next, grid, [current, ..acc])
+}
 
 pub fn part1(input: String) {
-  todo as "Implement solution to part 1"
+  let grid =
+    input
+    |> make_grid
+
+  let assert Ok(s) =
+    grid
+    |> dict.filter(fn(_, v) { v == "S" })
+    |> dict.keys
+    |> list.first
+
+  grid
+  |> valid_start_direction(s)
+  |> to_next_pipe(grid, [s])
+  |> list.length
+  |> fn(i) { { { i - 1 } / 2 } }
+  |> string.inspect
+}
+
+fn trace_ray(p: Posn, loop: Set(Posn), grid: PipeGrid) -> Bool {
+  use <- bool.guard(set.contains(loop, p), False)
+  int.is_odd(count_crossings(p, loop, grid, 0, ""))
+}
+
+fn count_crossings(
+  p: Posn,
+  loop: Set(Posn),
+  grid: PipeGrid,
+  acc: Int,
+  corner: String,
+) {
+  let maybe_cell = dict.get(grid, p)
+  use <- bool.guard(maybe_cell == Error(Nil), acc)
+  let assert Ok(cell) = maybe_cell
+  let next = add_posns(p, east)
+  case set.contains(loop, p) {
+    False -> count_crossings(next, loop, grid, acc, corner)
+    True ->
+      case corner, cell {
+        _, "|" -> count_crossings(next, loop, grid, acc + 1, corner)
+        _, "F" | _, "L" -> count_crossings(next, loop, grid, acc, cell)
+        "F", "J" | "L", "7" -> count_crossings(next, loop, grid, acc + 1, "")
+        "F", "7" | "L", "J" -> count_crossings(next, loop, grid, acc, "")
+        _, _ -> count_crossings(next, loop, grid, acc, corner)
+      }
+  }
 }
 
 pub fn part2(input: String) {
-  todo as "Implement solution to part 2"
+  let grid =
+    input
+    |> make_grid
+
+  let assert Ok(s) =
+    grid
+    |> dict.filter(fn(_, v) { v == "S" })
+    |> dict.keys
+    |> list.first
+
+  let loop_pipes =
+    grid
+    |> valid_start_direction(s)
+    |> to_next_pipe(grid, [s])
+    |> set.from_list
+
+  grid
+  |> dict.keys
+  |> list.filter(trace_ray(_, loop_pipes, grid))
+  |> list.length()
+  |> string.inspect
 }
 
 pub fn main() {