Advent of Code 2023

Mix.install([
  {:req, "~> 0.3.2"}
])

Day 6

input =
  "https://adventofcode.com/2023/day/6/input"
  |> Req.get!(headers: [cookie: "session=#{System.get_env("AOC_COOKIE")}"])
  |> Map.get(:body)

sample = """
Time:      7  15   30
Distance:  9  40  200
"""

defmodule A do
  def parse(input) do
    input
    |> String.split("\n", trim: true)
    |> Enum.map(fn line ->
      String.split(line, ~r/\s+/) |> tl() |> Enum.map(&String.to_integer/1)
    end)
    |> Enum.zip()
  end

  def parse2(input) do
    input
    |> String.split("\n", trim: true)
    |> Enum.map(fn line ->
      line
      |> String.replace(" ", "")
      |> String.split(":", trim: true)
      |> Enum.at(1)
      |> String.to_integer()
    end)
  end

  def num_ways_to_win(t, d) do
    # assume we can always win
    mid = div(t, 2)

    mid =
      if rem(t, 2) == 0 do
        mid
      else
        mid + 1
      end

    low = calc_lower_bound(0, mid, t, d, 0)

    if rem(t, 2) == 0 do
      (mid - low) * 2 + 1
    else
      (mid - low) * 2
    end
  end

  # def calc_lower_bound(_, _, _, _, i) when i > 10, do: raise("abort")
  def calc_lower_bound(lo, hi, _, _, _i) when lo == hi, do: lo

  def calc_lower_bound(lo, hi, t, d, i) do
    mid =
      case hi - lo do
        1 -> lo
        n -> lo + div(n, 2)
      end

    distance = mid * (t - mid)
    # IO.inspect([lo, hi, mid, distance], label: "calc")

    {lo, hi} =
      if distance > d do
        {lo, mid}
      else
        case hi - mid do
          1 ->
            {hi, hi}

          _ ->
            {mid, hi}
        end
      end

    calc_lower_bound(lo, hi, t, d, i + 1)
  end

  def part1(input) do
    input
    |> parse()
    |> Enum.map(fn {t, d} -> num_ways_to_win(t, d) end)
    |> Enum.product()
  end

  def part2(input) do
    input
    |> parse2()
    |> then(fn [t, d] ->
      num_ways_to_win(t, d)
    end)
  end
end

Part 1

input
# sample
|> A.part1()

Part 2

input
|> A.part2()