feat: Add similarity score calculation for Day 1 Part 2

lib/advent_code2024.ex

@@ -53,4 +53,25 @@ defmodule AdventCode2024 do
     |> Enum.map(fn {a, b} -> abs(a - b) end)
     |> Enum.sum()
   end
+
+  @doc """
+  Calculates the similarity score between two lists.
+  For each number in the left list, multiplies it by the number of times it appears in the right list.
+  Returns the sum of all these products.
+
+  ## Examples
+
+      iex> AdventCode2024.calculate_similarity_score([3, 4, 2, 1, 3, 3], [4, 3, 5, 3, 9, 3])
+      31
+
+  """
+  def calculate_similarity_score(left_list, right_list) do
+    frequencies = Enum.frequencies(right_list)
+    
+    left_list
+    |> Enum.map(fn num -> 
+      num * Map.get(frequencies, num, 0)
+    end)
+    |> Enum.sum()
+  end
 end

test/advent_code2024_test.exs

@@ -31,4 +31,25 @@ defmodule AdventCode2024Test do
       assert AdventCode2024.calculate_total_distance([1], [3]) == 2
     end
   end
+
+  describe "Day 1 Part 2: Similarity Score" do
+    test "calculates similarity score using example input" do
+      left_list = [3, 4, 2, 1, 3, 3]
+      right_list = [4, 3, 5, 3, 9, 3]
+      
+      assert AdventCode2024.calculate_similarity_score(left_list, right_list) == 31
+    end
+
+    test "handles empty lists for similarity score" do
+      assert AdventCode2024.calculate_similarity_score([], []) == 0
+    end
+
+    test "handles lists with no matches" do
+      assert AdventCode2024.calculate_similarity_score([1, 2], [3, 4]) == 0
+    end
+
+    test "handles lists with all matches" do
+      assert AdventCode2024.calculate_similarity_score([1, 1], [1, 1]) == 4
+    end
+  end
 end