commit 26bec5c3c0fe91896209dbe262ba539ee2bac3c0 · hauleth.dev/advent-of-code

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2025/day04.livemd

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       # Day 04

     

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       ```elixir

     

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       Mix.install([:kino_aoc])

     

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       ```

     

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       ## Setup

     

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       <!-- livebook:{"attrs":"eyJhc3NpZ25fdG8iOiJwdXp6bGVfaW5wdXQiLCJkYXkiOiI0Iiwic2Vzc2lvbl9zZWNyZXQiOiJBRFZFTlRfT0ZfQ09ERV9TRVNTSU9OIiwieWVhciI6IjIwMjUifQ","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", "4", System.fetch_env!("LB_ADVENT_OF_CODE_SESSION"))

     

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       ```

     

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       ```elixir

     

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       rolls =

     

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         puzzle_input

     

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         |> String.split("\n", trim: true)

     

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         |> Enum.with_index()

     

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         |> Enum.flat_map(fn {line, row} ->

     

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           line

     

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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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           |> Enum.map(&{elem(&1, 1), row})

     

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         end)

     

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         |> MapSet.new()

     

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       ```

     

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       ## Implementation

     

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       ```elixir

     

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       defmodule PaperRolls do

     

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         def adjacent({x, y}) do

     

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           for dx <- -1..1,

     

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               dy <- -1..1,

     

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               {dx, dy} != {0, 0},

     

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               do: {x + dx, y + dy}

     

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         end

     

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         def free?(pos, map) do

     

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           pos

     

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           |> adjacent()

     

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           |> Enum.count(&(&1 in map))

     

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           |> then(&(&1 < 4))

     

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         end

     

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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.count(rolls, &PaperRolls.free?(&1, rolls))

     

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       ```

     

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       ## Part 2

     

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       ```elixir

     

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       cleaned =

     

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         Stream.repeatedly(fn -> [] end)

     

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         |> Enum.reduce_while(rolls, fn _, acc ->

     

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           removable =

     

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             Enum.filter(acc, &PaperRolls.free?(&1, acc))

     

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             |> MapSet.new()

     

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           case MapSet.difference(acc, removable) do

     

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             ^acc -> {:halt, acc}

     

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             remaining -> {:cont, remaining}

     

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           end

     

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         end)

     

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       ```

     

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       ```elixir

     

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       MapSet.size(rolls) - MapSet.size(cleaned)

     

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       ```

     

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