🎄 Year 2023 🔔 Day 06

Setup

filename = "#{__DIR__}/../../../inputs/2023/day06.txt"

races =
  File.read!(filename)
  |> String.split("\n", trim: true)
  |> Enum.map(fn line ->
    String.split(line, " ", trim: true)
    |> Enum.drop(1)
    |> Enum.map(&String.to_integer/1)
  end)
  |> Enum.zip()

races_part_two =
  File.read!(filename)
  |> String.split("\n", trim: true)
  |> Enum.map(fn line ->
    String.split(line, " ", trim: true)
    |> Enum.drop(1)
    |> Enum.join("")
    |> String.to_integer()
  end)
  |> List.to_tuple()

Part 1

races
|> Enum.map(fn {time, distance} ->
  1..(time - 1)
  |> Enum.drop_while(fn n -> n * (time - n) <= distance end)
  |> Enum.take_while(fn n -> n * (time - n) > distance end)
  |> Enum.count()
end)
|> Enum.product()

Part 2

defmodule Helper do
  def binary_search(min, max, {time, distance}) do
    current = div(max - min, 2) + min
    prev = current - 1

    current_result = current * (time - current) > distance
    prev_result = prev * (time - prev) > distance

    case {prev_result, current_result} do
      {false, true} -> current
      {true, true} -> binary_search(min, current, {time, distance})
      {false, false} -> binary_search(current, max, {time, distance})
    end
  end
end

{time, distance} = races_part_two

Helper.binary_search(1, time - 1, {time, distance})
|> then(fn result -> Enum.count(result..(time - result)) end)