spiral.ex

defmodule Spiral do
  def order(matrix) do
    bounds = %{
      xmin: 0,
      xmax: matrix |> hd() |> length(),
      ymin: 0,
      ymax: length(matrix)
    }

    matrix |> next({0, 0}, {1, 0}, bounds, []) |> Enum.reverse()
  end

  # Guard clause that checks if we've reached the center of the matrix
  defguardp is_center?(x, y, matrix)
            when matrix |> hd() |> length() |> div(2) |> floor() == x and
                   matrix |> length() |> div(2) |> floor() == y

  # We've reached the center, add it to the result and we're done
  defp next(matrix, {x, y}, _direction, _bounds, result) when is_center?(x, y, matrix) do
    [get(matrix, x, y) | result]
  end

  # When we get to the right edge, start moving downwards
  defp next(matrix, {x, y}, _direction, %{xmax: xmax, ymin: ymin} = bounds, result)
       when x >= xmax do
    next(matrix, {x - 1, y + 1}, {0, 1}, %{bounds | ymin: ymin + 1}, result)
  end

  # When we get to the bottom edge, start moving left
  defp next(matrix, {x, y}, _direction, %{ymax: ymax, xmax: xmax} = bounds, result)
       when y >= ymax do
    next(matrix, {x - 1, y - 1}, {-1, 0}, %{bounds | xmax: xmax - 1}, result)
  end

  # When we get to the left edge, start moving up
  defp next(matrix, {x, y}, _direction, %{xmin: xmin, ymax: ymax} = bounds, result)
       when x < xmin do
    next(matrix, {x + 1, y - 1}, {0, -1}, %{bounds | ymax: ymax - 1}, result)
  end

  # When we get to the top edge, start moving right
  defp next(matrix, {x, y}, _direction, %{ymin: ymin, xmin: xmin} = bounds, result)
       when y < ymin do
    next(matrix, {x + 1, y + 1}, {1, 0}, %{bounds | xmin: xmin + 1}, result)
  end

  # Process other cells normally
  defp next(matrix, {x, y}, {dx, dy} = direction, bounds, result) do
    next(matrix, {x + dx, y + dy}, direction, bounds, [get(matrix, x, y) | result])
  end

  # Returns the matrix element at (x, y)
  defp get(matrix, x, y), do: matrix |> Enum.at(y) |> Enum.at(x)
end

l = [
  [1, 2, 3],
  [4, 5, 6],
  [7, 8, 9]
]

IO.puts("Input: [
  [1, 2, 3],
  [4, 5, 6],
  [7, 8, 9]
]")
IO.puts("Output: #{l |> Spiral.order() |> inspect()}")

l = [
  [1, 2, 3, 4],
  [5, 6, 7, 8],
  [9, 10, 11, 12]
]

IO.puts("Input: [
  [1, 2, 3, 4],
  [5, 6, 7, 8],
  [9, 10, 11, 12]
]")
IO.puts("Output: #{l |> Spiral.order() |> inspect()}")