test: add basic 'Helpers' unit tests

test/unit/concrete/helpers/computeNumberOfPayments.t.sol

@@ -0,0 +1,51 @@
+// SPDX-License-Identifier: UNLICENSED
+pragma solidity ^0.8.26;
+
+import { Base_Test } from "../../../Base.t.sol";
+import { Helpers } from "./../../../../src/modules/invoice-module/libraries/Helpers.sol";
+import { Types } from "./../../../../src/modules/invoice-module/libraries/Types.sol";
+
+contract ComputeNumberOfPayments_Helpers_Test is Base_Test {
+    function setUp() public virtual override {
+        Base_Test.setUp();
+    }
+
+    function test_ComputeNumberOfPayments_Weekly() external view {
+        // Create an interval of 11 weeks
+        uint40 startTime = uint40(block.timestamp);
+        uint40 endTime = uint40(block.timestamp + 11 weeks);
+
+        // Run the test
+        uint40 numberOfPayments =
+            Helpers.computeNumberOfPayments({ recurrence: Types.Recurrence.Weekly, interval: endTime - startTime });
+
+        // Assert the actual and expected number of payments
+        assertEq(numberOfPayments, 11);
+    }
+
+    function test_ComputeNumberOfPayments_Monthly() external view {
+        // Create an interval of 2 months
+        uint40 startTime = uint40(block.timestamp);
+        uint40 endTime = uint40(block.timestamp + 2 * 4 weeks);
+
+        // Run the test
+        uint40 numberOfPayments =
+            Helpers.computeNumberOfPayments({ recurrence: Types.Recurrence.Monthly, interval: endTime - startTime });
+
+        // Assert the actual and expected number of payments
+        assertEq(numberOfPayments, 2);
+    }
+
+    function test_ComputeNumberOfPayments_Yearly() external view {
+        // Create an interval of 3 years
+        uint40 startTime = uint40(block.timestamp);
+        uint40 endTime = uint40(block.timestamp + 3 * 48 weeks);
+
+        // Run the test
+        uint40 numberOfPayments =
+            Helpers.computeNumberOfPayments({ recurrence: Types.Recurrence.Yearly, interval: endTime - startTime });
+
+        // Assert the actual and expected number of payments
+        assertEq(numberOfPayments, 3);
+    }
+}