Day 6: Wait For It – Advent of Code 2023

input =
  File.stream!("/Users/pw/src/weiland/adventofcode/2023/input/06.txt")
  |> Stream.map(&String.trim/1)
  |> Enum.to_list()

test_input =
  "Time:      7  15   30
Distance:  9  40  200"
  |> String.split("\n")

Parsing

parse_pairs = fn input ->
  input
  |> Enum.map(fn line ->
    String.split(line, ":")
    |> Enum.at(1)
    |> String.split(" ", trim: true)
    |> Enum.map(&String.to_integer/1)
  end)
  |> Enum.zip()
end

parse_pair = fn input ->
  input
  |> Enum.map(fn line ->
    String.split(line, ":")
    |> Enum.at(1)
    |> String.replace(" ", "")
    |> String.to_integer()
  end)
  |> Enum.chunk_every(1)
  |> Enum.zip()
  |> Enum.at(0)
end

Part One

find_options = fn {max_time, max_distance} ->
  Enum.to_list(2..(max_time - 1))
  |> Enum.reject(fn time -> time * (max_time - time) <= max_distance end)
  |> Enum.count()
end

part_one = fn input ->
  input
  |> parse_pairs.()
  |> Enum.map(find_options)
  |> Enum.product()
end

IO.inspect(part_one.(test_input) == 288, label: "smoke test part two")

part_one.(input)

Part Two

find_options_fast = fn {max_time, max_distance} ->
  # solve for time: time * (max_time - time) > max_distance
  (max_time ** 2)
  |> Kernel.-(max_distance * 4)
  # alternative |> Kernel.**(1/2)
  |> :math.sqrt()
  |> trunc
end

part_two = fn input ->
  input
  |> parse_pair.()
  |> find_options_fast.()
end

IO.inspect(part_two.(test_input) == 71503, label: "smoke test part two")

part_two.(input)

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