cart.exs

# from ounce of elixir 
#https://www.meetup.com/Women-Who-Code-Tampa/events/233014583/

ExUnit.start


defmodule CartTest do
    use ExUnit.Case
    test "starts with empty count" do
        assert Cart.count_items([]) == 0
    end
    test "with one add event then count items is one" do
        events = [{:add, "book"}] 
        assert Cart.count_items(events) == 1
    end
    test "we should have zero items if we added and removed" do
        events = [{:add, "book"}, {:remove, "book"}] 
        assert Cart.count_items(events) == 0
    end
    test "we should have 2 items when we add two items" do
        events = [{:add, "book"}, {:add, "book"}] 
        assert Cart.count_items(events) == 2
    end

    test "we should raise an error when we remove from an empty cart" do
        events = [{:remove, "book"}]
        assert_raise Cart.InvalidOperation, fn -> Cart.count_items(events) end

        events = [{:remove, "book"}, {:add, "book"}]
        assert_raise Cart.InvalidOperation, fn -> Cart.count_items(events) end
        
        events = [{:add, "book"}, {:remove, "book"}, {:remove, "book"}]
        assert_raise Cart.InvalidOperation, fn -> Cart.count_items(events) end
    end

    test "keeps track type of items for each operation" do
        events = [{:add, "banana"}, {:add, "book"}]
        assert Cart.count_items(events, "banana") == 1
        events = [{:add, "banana"}, {:add, "banana"}]
        assert Cart.count_items(events, "banana") == 2
        events = [{:add, "banana"}, {:add, "banana"}, {:remove, "banana"}]
        assert Cart.count_items(events, "banana") == 1
    end


end

defmodule Cart.InvalidOperation do
    defexception message: "Cannot do this"
end

defmodule Cart do

    def count_items([{:remove, _item} | _tail]) do
        raise Cart.InvalidOperation
    end

    def count_items(events) do
        Enum.reduce events, 0, fn(event, count) -> 
            case {event, count} do
                {{:add, _item}, _count} -> count + 1
                {{:remove, _item}, count} when count <= 0 -> raise Cart.InvalidOperation
                {{:remove, _item}, _count} -> count - 1                
            end
        end
    end

    def count_items(events, product) do
        Enum.filter(events, filter_events_by_product_type(product))
        |> count_items 
    end

    def filter_events_by_product_type(product) do
        fn({_action, event_product}) -> event_product == product end
    end
end