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Day 04 od 2025
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2025/day04.livemd
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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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```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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