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Day 07 of 2025
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2025/day07.livemd
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2025/day07.livemd
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# Day 07
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```elixir
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Mix.install([:kino_aoc, :image])
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```
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## Parse
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```elixir
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{:ok, puzzle_input} =
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KinoAOC.download_puzzle("2025", "7", System.fetch_env!("LB_ADVENT_OF_CODE_SESSION"))
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```
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```elixir
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[start | rest] = String.split(puzzle_input)
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start_col = byte_size(start) - byte_size(String.trim_leading(start, "."))
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```
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```elixir
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splitters =
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Enum.map(rest, fn row ->
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row
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|> String.to_charlist()
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|> Enum.with_index()
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|> Enum.filter(&(elem(&1, 0) == ?^))
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|> MapSet.new(&elem(&1, 1))
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end)
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```
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## Part 1
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```elixir
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Enum.reduce(splitters, {MapSet.new([start_col]), 0}, fn splits, {beams, count} ->
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import MapSet, only: [intersection: 2, difference: 2, union: 2]
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hits = intersection(beams, splits)
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new_beams = for hit <- hits, dx <- [-1, 1], into: MapSet.new(), do: hit + dx
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beams = beams |> difference(hits) |> union(new_beams)
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{beams, MapSet.size(hits) + count}
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end)
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```
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## Part 2
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```elixir
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Enum.reduce(splitters, %{start_col => 1}, fn splits, beams ->
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Enum.reduce(splits, beams, fn s, acc ->
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case Map.pop(acc, s) do
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{nil, map} ->
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map
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{count, map} ->
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Map.merge(
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map,
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%{
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(s + 1) => count,
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(s - 1) => count
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},
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fn _k, a, b -> a + b end
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)
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end
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end)
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end)
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|> Enum.sum_by(&elem(&1, 1))
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```
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