commit 8003c4178f16d2811aeba3a3270c3da4c576a78f · hauleth.dev/advent-of-code

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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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       <!-- livebook:{"attrs":"eyJhc3NpZ25fdG8iOiJwdXp6bGVfaW5wdXQiLCJkYXkiOiI3Iiwic2Vzc2lvbl9zZWNyZXQiOiJBRFZFTlRfT0ZfQ09ERV9TRVNTSU9OIiwieWVhciI6IjIwMjUifQ","chunks":null,"kind":"Elixir.KinoAOC.HelperCell","livebook_object":"smart_cell"} -->

     

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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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