bag.ex

defmodule Bag do
  defstruct store: %{}

  def new do
    %Bag{}
  end

  def put(%Bag{store: store}, thing) do
    %Bag{store: Map.update(store, thing, 1, &( &1 + 1))}
  end

  def to_map(%Bag{store: store}) do
    store
  end
end

defimpl Collectable, for: Bag do
  def empty(_dict) do
    Bag.new
  end

  def into(original) do
    # not sure what this means, just copied the HashSet impl
    {original, fn
      bag, {:cont, x} -> Bag.put(bag, x)
      bag, :done -> bag
      _, :halt -> :ok
    end}
  end
end

word_stats_test.exs

defmodule WordStats do
  def letter_frequency words do
    words
    |> Stream.flat_map(&String.graphemes/1)
    |> Enum.into(Bag.new)
    |> Bag.to_map
  end
end

ExUnit.start

defmodule TaskPlaygroundTest do
  use ExUnit.Case

  test "the truth" do
    assert 1 + 1 == 2
    #words = File.read!("/usr/share/dict/words") |> String.split("\n")
    words = [
      "cat",
      "bag"
    ]
    assert %{"c" => 1, "a" => 2, "t" => 1, "b" => 1, "g" => 1} == WordStats.letter_frequency(words)
  end
end