2025 Day 1

Day 1: Secret Entrance

Day 1: Secret Entrance

Part 1

defmodule AdventOfCode2025Day1Part1 do
  def run(input) do
    input
    |> parse_input()
    |> solve()
  end
  
  defp parse_input(input) do
    input
    |> String.split("\n", trim: true)
    |> Enum.map(&parse/1)
  end

  defp solve(list) do
    list
    |> Enum.reduce([50], &rotate/2)
    |> Enum.count(& &1 == 0)
  end

  defp rotate({:left, number}, [current | _] = acc) do
    new_current = normalize(current - number)
    [new_current | acc]
  end

  defp rotate({:right, number}, [current | _] = acc) do
    new_current = normalize(current + number)
    [new_current | acc]
  end

  defp normalize(number) when number > 99, do: normalize(number - 100)
  defp normalize(number) when number < 0, do: normalize(100 + number)
  defp normalize(number), do: number

  defp parse(<<"L", number::binary>>), do: {:left, String.to_integer(number)}
  defp parse(<<"R", number::binary>>), do: {:right, String.to_integer(number)}
end

input = """
L68
L30
R48
L5
R60
L55
L1
L99
R14
L82
"""

AdventOfCode2025Day1Part1.run(input)

Part 2

defmodule AdventOfCode2025Day1Part2 do
  def run(input) do
    input
    |> String.split("\n", trim: true)
    |> Enum.map(&parse/1)
    |> Enum.reduce({[50], 0}, &rotate/2)
    |> elem(1)
  end

  defp rotate({:left, number}, {[current | _] = list, count}) do
    {new_current, new_count} = move(current, -number)
    {[new_current | list], count + new_count}
  end

  defp rotate({:right, number}, {[current | _] = list, count}) do
    {new_current, new_count} = move(current, number)
    {[new_current | list], count + new_count}
  end

  defp move(0, distance) when distance < 0 do
    {normalize(distance), div(distance, 100) * -1}
  end

  defp move(current, distance) when (current + distance) == 0 do
    {0, 1}
  end

  defp move(current, distance) when distance < 0 and (current + distance) < 0 do
    {normalize(current + distance), div(current + distance, 100) * -1 + 1}
  end

  defp move(current, distance) when distance < 0 do
    {normalize(current + distance), div(current + distance, 100) * -1}
  end

  defp move(current, distance) do
    {normalize(current + distance), div(current + distance, 100)}
  end

  defp normalize(number) when number > 99, do: normalize(number - 100)
  defp normalize(number) when number < 0, do: normalize(100 + number)
  defp normalize(number), do: number

  defp parse(<<"L", num::binary>>), do: {:left, String.to_integer(num)}
  defp parse(<<"R", num::binary>>), do: {:right, String.to_integer(num)}
end

input = """
L68
L30
R48
L5
R60
L55
L1
L99
R14
L82
"""

AdventOfCode2025Day1Part2.run(input)