Day 06

Common

# Time:      7  15   30
# Distance:  9  40  200
example_input = [{7, 9}, {15, 40}, {30, 200}]

# Time:        41     66     72     66
# Distance:   244   1047   1228   1040
real_input = [{41, 244}, {66, 1047}, {72, 1228}, {66, 1040}]

defmodule Race do
  def count_possibilities({time, distance}) do
    0..time
    |> Stream.map(&((time - &1) * &1))
    |> Stream.drop_while(&amp;(&amp;1 <= distance))
    |> Stream.take_while(&amp;(&amp;1 > distance))
    |> Enum.count()
  end
end

ExUnit.start(autorun: false)

defmodule RaceTest do
  use ExUnit.Case, async: true

  describe "count_possibilities/1" do
    test "returns correct count for {7, 9}" do
      assert 4 = Race.count_possibilities({7, 9})
    end

    test "returns correct count for {30, 200}" do
      assert 9 = Race.count_possibilities({30, 200})
    end
  end
end

ExUnit.run()

Part 1

# Time:      7  15   30
# Distance:  9  40  200
example_input = [{7, 9}, {15, 40}, {30, 200}]

# Time:        41     66     72     66
# Distance:   244   1047   1228   1040
real_input = [{41, 244}, {66, 1047}, {72, 1228}, {66, 1040}]

real_input
|> Enum.map(&Race.count_possibilities/1)
|> Enum.product()

Part 2

# Time:      7  15   30
# Distance:  9  40  200
example_input = {71530, 940_200}

# Time:        41     66     72     66
# Distance:   244   1047   1228   1040
real_input = {41_667_266, 244_104_712_281_040}

Race.count_possibilities(real_input)