defmodule Mune do
  @moduledoc """
  A collection of useful functions that aren't found in `Enum`,
  but are cumbersome or inconvenient to write your own optimized
  version of.
  """

  @type enum :: Enum.t()

  @doc """
  Reverse the enum, applying `mapper` to each element

      iex> Mune.map_reverse([1, 2, 3, 4, 5, 6], &Kernel.*(&1, 2))
      [12, 10, 8, 6, 4, 2]
  """
  @spec map_reverse(enum, (term -> var)) :: list(var) when var: term
  def map_reverse(enum, mapper) do
    Enum.reduce(enum, [], fn i, acc ->
      [mapper.(i) | acc]
    end)
  end

  @doc """
  Flatten the enum and reverse all elements, applying `mapper` to each element.

      iex> Mune.flat_map_reverse([[1, 2], [3, 4], [5, 6]], &Kernel.*(&1, 2))
      [12, 10, 8, 6, 4, 2]
  """
  @spec flat_map_reverse(enum, (term -> var)) :: list(var) when var: term
  def flat_map_reverse(enum, mapper) do
    Enum.reduce(enum, [], fn inner, acc ->
      Enum.reduce(inner, acc, fn i, acc ->
        [mapper.(i) | acc]
      end)
    end)
  end

  @doc """
  Flatten the enum and reverse all elements.


      iex> Mune.flatten_reverse([[1, 2], [3, 4], [5, 6]])
      [6, 5, 4, 3, 2, 1]
  """
  @spec flatten_reverse(enum) :: list
  def flatten_reverse(enum) do
    Enum.reduce(enum, [], fn inner, acc ->
      Enum.reduce(inner, acc, fn i, acc ->
        [i | acc]
      end)
    end)
  end
end