Commons and EDRPOU basic functionality (parser (but complete only), checksum with test coverage)

test/ukraine_taxid_ex/edrpou/check_sum_test.exs

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+defmodule UkraineTaxidEx.Edrpou.CheckSumTest do
+  use ExUnit.Case
+  alias UkraineTaxidEx.Edrpou.CheckSum
+
+  describe "check_sum/1" do
+    test "calculates correct checksum for EDRPOU with base weights (< 30M) short EDRPOU with leading zeros" do
+      # Example EDRPOU: 0003212[9] (where 2 is the check digit)
+      digits = [0, 0, 0, 3, 2, 1, 2, 9]
+      assert CheckSum.check_sum(digits) == 9
+    end
+
+    test "calculates correct checksum for EDRPOU with base weights (< 30M)" do
+      # Example EDRPOU: 1436050[6] (where 2 is the check digit)
+      digits = [1, 4, 3, 6, 0, 5, 0, 6]
+      assert CheckSum.check_sum(digits) == 6
+    end
+
+    test "calculates correct checksum for EDRPOU with base weights (> 60M)" do
+      # Example EDRPOU: 6543217[6]
+      digits = [6, 5, 4, 3, 2, 1, 7, 6]
+      assert CheckSum.check_sum(digits) == 6
+    end
+
+    test "calculates correct checksum for EDRPOU with alternative weights (30M-60M)" do
+      # Example EDRPOU: 3145193[2]
+      digits = [3, 1, 4, 5, 1, 9, 3, 2]
+      assert CheckSum.check_sum(digits) == 2
+    end
+
+    test "calculates correct checksum for EDRPOU with alternative weights (30M-60M) when first calculation >= 10" do
+      # Example EDRPOU: 3702668[4]
+      digits = [3, 7, 0, 2, 6, 6, 8, 4]
+      assert CheckSum.check_sum(digits) == 4
+    end
+
+    test "calculates correct checksum for EDRPOU with base weights when first calculation >= 10" do
+      # Example EDRPOU: 2113335[2]
+      digits = [2, 1, 1, 3, 3, 3, 5, 2]
+      assert CheckSum.check_sum(digits) == 2
+    end
+
+    test "handles edge case at 30M boundary" do
+      # Just below 30M
+      digits = [2, 9, 9, 9, 9, 9, 9]
+      result_below = CheckSum.check_sum(digits)
+
+      # Just at 30M
+      digits = [3, 0, 0, 0, 0, 0, 0]
+      result_at = CheckSum.check_sum(digits)
+
+      assert result_below != result_at
+    end
+
+    test "handles edge case at 60M boundary" do
+      # Just below 60M
+      digits = [5, 9, 9, 9, 9, 9, 9]
+      result_below = CheckSum.check_sum(digits)
+
+      # Just at 60M
+      digits = [6, 0, 0, 0, 0, 0, 0]
+      result_at = CheckSum.check_sum(digits)
+
+      assert result_below != result_at
+    end
+  end
+end