this repo has no description
hauleth.dev
1# Day 07
2
3```elixir
4Mix.install([:kino_aoc, :image])
5```
6
7## Parse
8
9<!-- livebook:{"attrs":"eyJhc3NpZ25fdG8iOiJwdXp6bGVfaW5wdXQiLCJkYXkiOiI3Iiwic2Vzc2lvbl9zZWNyZXQiOiJBRFZFTlRfT0ZfQ09ERV9TRVNTSU9OIiwieWVhciI6IjIwMjUifQ","chunks":null,"kind":"Elixir.KinoAOC.HelperCell","livebook_object":"smart_cell"} -->
10
11```elixir
12{:ok, puzzle_input} =
13 KinoAOC.download_puzzle("2025", "7", System.fetch_env!("LB_ADVENT_OF_CODE_SESSION"))
14```
15
16```elixir
17[start | rest] = String.split(puzzle_input)
18
19start_col = byte_size(start) - byte_size(String.trim_leading(start, "."))
20```
21
22```elixir
23splitters =
24 Enum.map(rest, fn row ->
25 row
26 |> String.to_charlist()
27 |> Enum.with_index()
28 |> Enum.filter(&(elem(&1, 0) == ?^))
29 |> MapSet.new(&elem(&1, 1))
30 end)
31```
32
33## Part 1
34
35```elixir
36Enum.reduce(splitters, {MapSet.new([start_col]), 0}, fn splits, {beams, count} ->
37 import MapSet, only: [intersection: 2, difference: 2, union: 2]
38
39 hits = intersection(beams, splits)
40 new_beams = for hit <- hits, dx <- [-1, 1], into: MapSet.new(), do: hit + dx
41 beams = beams |> difference(hits) |> union(new_beams)
42
43 {beams, MapSet.size(hits) + count}
44end)
45```
46
47## Part 2
48
49```elixir
50Enum.reduce(splitters, %{start_col => 1}, fn splits, beams ->
51 Enum.reduce(splits, beams, fn s, acc ->
52 case Map.pop(acc, s) do
53 {nil, map} ->
54 map
55
56 {count, map} ->
57 Map.merge(
58 map,
59 %{
60 (s + 1) => count,
61 (s - 1) => count
62 },
63 fn _k, a, b -> a + b end
64 )
65 end
66 end)
67end)
68|> Enum.sum_by(&elem(&1, 1))
69```
70
71<!-- livebook:{"offset":1576,"stamp":{"token":"XCP.4O_0Nj7V9wS7w6P8UvTBgpp4vhW57_8fy1nGUUO-kK-Q74Dt1KmWL8qx_FolCKDbpEooq8lZ8yn9tXw9AlKXzLPCKp2hEtIvQvKWtBiSmje0vN9ms0Uqud5m2GxlgCHmtg","version":2}} -->