Advent of Code 2021 - Day 5

Setup

Mix.install([
  {:kino, "~> 0.4.1"}
])

:ok

example_input = Kino.Input.textarea("Please enter the example input:")

my_input = Kino.Input.textarea("Please enter your input:")

defmodule Parser do
  def parse_input(input) do
    input
    |> String.split("\n", trim: true)
    |> Enum.map(&String.trim/1)
    |> Enum.flat_map(&String.split(&1, " -> ", trim: true))
    |> Enum.flat_map(&String.split(&1, ","))
    |> Enum.map(&String.to_integer/1)
    |> Enum.chunk_every(2)
    |> Enum.map(&List.to_tuple/1)
    |> Enum.chunk_every(2)
    |> Enum.map(&List.to_tuple/1)
  end
end

example_input =
  example_input
  |> Kino.Input.read()
  |> Parser.parse_input()

my_input =
  my_input
  |> Kino.Input.read()
  |> Parser.parse_input()

:ok

:ok

Part 1

my_input
|> Enum.filter(fn {{from_row, from_col}, {to_row, to_col}} ->
  from_row == to_row or from_col == to_col
end)
# |> IO.inspect()
|> Enum.flat_map(fn {{from_row, from_col}, {to_row, to_col}} ->
  for row <- from_row..to_row,
      col <- from_col..to_col do
    {row, col}
  end
end)
|> Enum.frequencies()
# |> IO.inspect()
|> Enum.filter(fn {_point, count} -> count > 1 end)
|> Enum.count()

5092

Part 2

I’m having trouble getting this to finish. If I use this point generation function in part 1, part 1 runs faster. So the problem isn’t that the point generation algorithm is slower than the one in part 1, the problem is its not fast enough to handle the lines we aren’t ignoring anymore.

Apparently, modules are faster because they actually get compiled. This runs much faster.

defmodule Day5.Part1 do
  def solve(input) do
    input
    |> Enum.filter(fn {{from_row, from_col}, {to_row, to_col}} ->
      from_row == to_row or from_col == to_col
    end)
    # |> IO.inspect()
    |> Enum.flat_map(fn {{from_row, from_col}, {to_row, to_col}} ->
      for row <- from_row..to_row,
          col <- from_col..to_col do
        {row, col}
      end
    end)
    |> Enum.frequencies()
    |> Enum.filter(fn {_point, count} -> count > 1 end)
    |> Enum.count()
  end
end

Day5.Part1.solve(my_input)

5092

defmodule Day5.Part2 do
  def solve(input) do
    input
    |> Enum.flat_map(fn {{from_row, from_col}, {to_row, to_col}} ->
      distance = Enum.max([abs(from_row - to_row), abs(from_col - to_col)])

      for step <- 0..distance do
        row =
          cond do
            from_row == to_row -> from_row
            from_row < to_row -> from_row + step
            true -> from_row - step
          end

        col =
          cond do
            from_col == to_col -> from_col
            from_col < to_col -> from_col + step
            true -> from_col - step
          end

        {row, col}
      end
    end)
    # |> IO.inspect()
    |> Enum.frequencies()
    # |> IO.inspect()
    |> Enum.count(fn {_point, count} -> count > 1 end)
  end
end

Day5.Part2.solve(my_input)
# |> IO.inspect()

20484