hauleth.dev
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Day 08 of 2025
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2025/day08.livemd
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2025/day08.livemd
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# Day 08
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```elixir
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Mix.install([:kino_aoc])
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```
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## Parse
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<!-- livebook:{"attrs":"eyJhc3NpZ25fdG8iOiJwdXp6bGVfaW5wdXQiLCJkYXkiOiI4Iiwic2Vzc2lvbl9zZWNyZXQiOiJBRFZFTlRfT0ZfQ09ERV9TRVNTSU9OIiwieWVhciI6IjIwMjUifQ","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", "8", System.fetch_env!("LB_ADVENT_OF_CODE_SESSION"))
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```
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```elixir
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boxes =
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puzzle_input
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|> String.split()
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|> Enum.map(fn raw ->
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raw
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|> String.split(",")
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|> Enum.map(&String.to_integer/1)
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|> List.to_tuple()
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end)
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|> Enum.sort()
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```
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## Setup
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```elixir
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defmodule JunctionBoxes do
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def all_pairs([]), do: []
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def all_pairs([x | rest]) do
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for(y <- rest, do: {x, y}) ++ all_pairs(rest)
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end
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def dist2({ax, ay, az}, {bx, by, bz}) do
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dx = ax - bx
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dy = ay - by
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dz = az - bz
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dx ** 2 + dy ** 2 + dz ** 2
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end
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def group([], {a, b}), do: [MapSet.new([a, b])]
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def group([set | rest], {a, b}) do
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cond do
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a in set and b in set -> [set | rest]
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a in set or b in set -> set |> MapSet.put(a) |> MapSet.put(b) |> do_squash(rest, a, b, [])
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true -> [set | group(rest, {a, b})]
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end
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end
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defp do_squash(curr, [], _, _, acc), do: [curr | acc]
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defp do_squash(curr, [x | rest], a, b, acc) do
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if a in x or b in x do
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[MapSet.union(curr, x) | acc ++ rest]
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else
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do_squash(curr, rest, a, b, [x | acc])
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end
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end
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end
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```
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```elixir
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sorted_pairs =
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JunctionBoxes.all_pairs(boxes)
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|> Enum.sort_by(fn {a, b} -> JunctionBoxes.dist2(a, b) end)
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```
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<!-- livebook:{"branch_parent_index":1} -->
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## Part 1
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```elixir
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sorted_pairs
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|> Enum.take(1000)
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|> Enum.reduce([], &JunctionBoxes.group(&2, &1))
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|> Enum.map(&MapSet.size/1)
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|> Enum.sort(:desc)
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|> Enum.take(3)
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|> Enum.product()
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```
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<!-- livebook:{"branch_parent_index":1} -->
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## Part 2
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```elixir
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box_count = length(boxes)
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```
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```elixir
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{a, b} =
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sorted_pairs
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|> Enum.reduce_while([], fn pair, acc ->
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new_acc = JunctionBoxes.group(acc, pair)
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if MapSet.size(hd(new_acc)) == box_count do
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{:halt, pair}
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else
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{:cont, new_acc}
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end
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end)
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```
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```elixir
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{ax, _, _} = a
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{bx, _, _} = b
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ax * bx
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```
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