Advent of Code 2023 - Day 06

2023/06.livemd

Phil-Bastian Berndt

@pehbehbeh

adventofcode

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Advent of Code 2023 - Day 06

Mix.install([
  {:kino, "~> 0.11.0"},
  {:kino_aoc, github: "ljgago/kino_aoc"},
  {:bisect, "~> 0.4.0"}
])

Input

{:ok, puzzle_input} =
  KinoAOC.download_puzzle("2023", "6", System.fetch_env!("LB_AOC_SESSION"))

races =
  puzzle_input
  |> String.split("\n")
  |> Enum.map(fn input ->
    Regex.scan(~r/\d+/, input)
    |> List.flatten()
    |> Enum.map(&amp;String.to_integer/1)
  end)
  |> Enum.zip()

Part 1

is_win? = fn time, record, wait ->
  (time - wait) * wait > record
end

count_wins = fn {time, record} ->
  Enum.reduce(0..time, 0, fn wait, wins ->
    if is_win?.(time, record, wait), do: wins + 1, else: wins
  end)
end

races
|> Stream.map(count_wins)
|> Enum.product()

Part 2

[time, record] =
  races
  |> Enum.unzip()
  |> Tuple.to_list()
  |> Enum.map(&amp;Enum.join/1)
  |> Enum.map(&amp;String.to_integer/1)

left = Range.to_list(0..ceil(time / 2))
first_win = Bisect.search(left, fn wait -> is_win?.(time, record, wait) end)
time - first_win * 2 + 1

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