1 files changed, 107 insertions, 0 deletions
diff --git a/aoc-2020-gleam/src/days/day20.gleam b/aoc-2020-gleam/src/days/day20.gleam
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+++ b/aoc-2020-gleam/src/days/day20.gleam
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+import gleam/io
+import gleam/int
+import gleam/list
+import gleam/dict
+import gleam/set.{type Set}
+import ext/intx
+import ext/listx
+import ext/resultx as resx
+import ext/genericx as genx
+import util/grid
+import util/input_util
+import util/pos2.{type Pos2}
+import util/parser as p
+
+const tile_size = 10
+
+type Tile {
+  Tile(id: Int, filled: Set(Pos2))
+}
+
+fn edge_ids(tile: Tile) -> List(Int) {
+  let extract = fn(filter, map) {
+    tile.filled
+    |> set.to_list
+    |> list.filter(keeping: filter)
+    |> list.map(with: fn(pos) { int.bitwise_shift_left(1, map(pos)) })
+    |> int.sum
+  }
+
+  let dedup = fn(edge_id) {
+    int.min(
+      edge_id,
+      edge_id
+      |> intx.reverse_bits(tile_size),
+    )
+  }
+
+  [
+    extract(fn(b) { pos2.y(b) == 0 }, pos2.x),
+    extract(fn(b) { pos2.x(b) == 9 }, pos2.y),
+    extract(fn(b) { pos2.y(b) == 9 }, pos2.x),
+    extract(fn(b) { pos2.x(b) == 0 }, pos2.y),
+  ]
+  |> list.map(with: dedup)
+}
+
+fn parse_tiles(input: String) -> List(Tile) {
+  let tile_parser =
+    p.literal("Tile ")
+    |> p.proceed(with: p.int())
+    |> p.skip(p.literal(":\n"))
+    |> p.then(p.any_str_of_len(tile_size * tile_size + { tile_size - 1 }))
+    |> p.map2(fn(id, content) {
+      Tile(
+        id,
+        content
+        |> grid.parse_grid(with: fn(x, y) { #(x, y) }),
+      )
+    })
+
+  let assert Ok(tiles) =
+    input
+    |> p.parse_entire(
+      with: tile_parser
+      |> p.sep1(by: p.nlnl())
+      |> p.skip_ws,
+    )
+  tiles
+}
+
+fn part1(input: String) -> Int {
+  let tiles = parse_tiles(input)
+
+  let counts =
+    tiles
+    |> list.flat_map(with: edge_ids)
+    |> listx.counts
+
+  tiles
+  |> list.filter(keeping: fn(tile) {
+    tile
+    |> edge_ids
+    |> list.map(with: fn(edge_id) {
+      counts
+      |> dict.get(edge_id)
+      |> resx.assert_unwrap
+    })
+    |> listx.count(satisfying: genx.equals(_, 1))
+    |> genx.equals(2)
+  })
+  |> list.map(with: fn(tile) { tile.id })
+  |> int.product
+}
+
+fn part2() -> Nil {
+  todo
+}
+
+pub fn main() -> Nil {
+  let testing = input_util.read_text("test20")
+  let assert 20_899_048_083_289 = part1(testing)
+
+  let input = input_util.read_text("day20")
+  io.debug(part1(input))
+
+  Nil
+}