1 files changed, 39 insertions, 8 deletions

diff --git a/aoc-2020-gleam/src/days/day24.gleam b/aoc-2020-gleam/src/days/day24.gleam
index 66bedb8..f380530 100644
--- a/aoc-2020-gleam/src/days/day24.gleam
+++ b/aoc-2020-gleam/src/days/day24.gleam
@@ -1,13 +1,14 @@
 import gleam/io
-import gleam/set
+import gleam/bool
 import gleam/list
+import gleam/set.{type Set}
 import ext/setx
 import ext/resultx as resx
 import util/input_util
 import util/parser as p
 import util/hex.{type Hex}
 
-fn parse_tiles(lines: List(String)) -> List(Hex) {
+fn parse_flips(lines: List(String)) -> List(Hex) {
   let tile_parser =
     [
       p.replace(p.literal("e"), with: hex.e),
@@ -28,20 +29,50 @@ fn parse_tiles(lines: List(String)) -> List(Hex) {
   })
 }
 
+fn get_black_tiles(flips: List(Hex)) -> Set(Hex) {
+  list.fold(over: flips, from: set.new(), with: setx.toggle)
+}
+
+fn cycle(prev: Set(Hex), times: Int) -> Set(Hex) {
+  use <- bool.guard(when: times == 0, return: prev)
+
+  prev
+  |> setx.flat_map(with: hex.neighbours7)
+  |> set.fold(from: set.new(), with: fn(acc, tile) {
+    let was_black = set.contains(in: prev, this: tile)
+    let adjacent =
+      tile
+      |> hex.neighbours6
+      |> set.intersection(prev)
+      |> set.size
+
+    case #(was_black, adjacent) {
+      #(True, 1) | #(_, 2) -> set.insert(into: acc, this: tile)
+      _ -> acc
+    }
+  })
+  |> cycle(times - 1)
+}
+
 fn part1(lines: List(String)) -> Int {
-  let tiles = parse_tiles(lines)
-  let blacks = list.fold(over: tiles, from: set.new(), with: setx.toggle)
-  set.size(blacks)
+  lines
+  |> parse_flips
+  |> get_black_tiles
+  |> set.size
 }
 
-fn part2(lines: List(String)) -> Nil {
-  Nil
+fn part2(lines: List(String)) -> Int {
+  lines
+  |> parse_flips
+  |> get_black_tiles
+  |> cycle(100)
+  |> set.size
 }
 
 pub fn main() -> Nil {
   let testing = input_util.read_lines("test24")
   let assert 10 = part1(testing)
-  io.debug(part2(testing))
+  let assert 2208 = part2(testing)
 
   let input = input_util.read_lines("day24")
   io.debug(part1(input))