ref: https://www.okb-shelf.work/entry/2020/01/19/114713

aggregater_test.exs

defmodule AggregaterTest do
  use ExUnit.Case
  doctest Aggregater

  describe "test for Aggregater.counter" do
    test "シンプルなケース" do
      assert Aggregater.counter(["A", "B", "C", "D"]) == %{"A" => 1, "B" => 1, "C" => 1, "D" => 1}
    end

    test "重複した値がうまくカウントされているか" do
      assert Aggregater.counter(["A", "B", "A", "C"]) == %{"A" => 2, "B" => 1, "C" => 1}
    end

    test "リストが空の場合" do
      assert Aggregater.counter([]) == %{}
    end

    test "複数の値が重複する場合" do
      assert Aggregater.counter(["A", "B", "A", "C", "D", "C", "E", "F", "D", "F", "G"]) == %{
               "A" => 2,
               "B" => 1,
               "C" => 2,
               "D" => 2,
               "E" => 1,
               "F" => 2,
               "G" => 1
             }
    end

    test "重複の発生が3回以上" do
      assert Aggregater.counter(["A", "A", "A"]) == %{"A" => 3}
    end
  end
end