aoc2020day3.ex

defmodule Day3 do
  def read_input do
    File.read!("input/day3.txt")
    |> String.split("\n", trim: true)
    |> Enum.map(&String.graphemes/1)
    |> Enum.map(&Enum.with_index/1)
    |> Enum.with_index
    |> Enum.flat_map(fn {line, row} -> Enum.map(line, fn {el, col} -> {{row, col}, (if el == "#", do: 1, else: 0) } end) end)
    |> Enum.into(%{})
  end

  def slope(dr, dc, column_count) do
    Stream.iterate({0, 0}, fn {row, col} -> {row + dr, rem(col + dc, column_count)} end)
  end

  def part1 do
    data = read_input()
    column_count = (data |> Map.keys |> Enum.map(fn {_row, col} -> col end) |> Enum.max) + 1
    row_count = (data |> Map.keys |> Enum.map(fn {row, _col} -> row end) |> Enum.max) + 1

    slope = slope(1, 3, column_count) |> Stream.take_while(fn {row, _col} -> row < row_count end) |> Enum.to_list

    data |> Map.take(slope) |> Map.values |> Enum.sum |> IO.inspect
  end

  def part2 do
    data = read_input()
    column_count = (data |> Map.keys |> Enum.map(fn {_row, col} -> col end) |> Enum.max) + 1
    row_count = (data |> Map.keys |> Enum.map(fn {row, _col} -> row end) |> Enum.max) + 1

    slopes = [
      {1, 1},
      {1, 3},
      {1, 5},
      {1, 7},
      {2, 1}]
      |> Enum.map(fn {dr, dc} -> slope(dr, dc, column_count) end)
      |> Enum.map(fn slope -> slope |> Stream.take_while(fn {row, _col} -> row < row_count end) |> Enum.to_list end)

    slopes
    |> Enum.map(fn slope -> (data |> Map.take(slope) |> Map.values |> Enum.sum) end)
    |> Enum.reduce(&(&1 * &2))
    |> IO.inspect
  end

end