fixes overpayment interest calculation

src/test/app/LendingHelpers_test.cpp

@@ -0,0 +1,74 @@
+#include <xrpl/beast/unit_test/suite.h>
+//
+#include <test/jtx.h>
+#include <test/jtx/mpt.h>
+
+#include <xrpld/app/misc/LendingHelpers.h>
+#include <xrpld/app/misc/LoadFeeTrack.h>
+#include <xrpld/app/tx/detail/Batch.h>
+#include <xrpld/app/tx/detail/LoanSet.h>
+
+#include <xrpl/beast/xor_shift_engine.h>
+#include <xrpl/protocol/SField.h>
+
+#include "test/jtx/amount.h"
+
+namespace ripple {
+namespace test {
+
+class LendingHelpers_test : public beast::unit_test::suite
+{
+    void
+    testComputeOverpaymentComponents()
+    {
+        testcase("computeOverpaymentComponents");
+        using namespace jtx;
+        using namespace ripple::detail;
+
+        Account const issuer{"issuer"};
+        PrettyAsset const IOU = issuer["IOU"];
+        int32_t const loanScale = 1;
+        auto const overpayment = Number{1'000};
+        auto const overpaymentInterestRate = TenthBips32{10'000};  // 10%
+        auto const overpaymentFeeRate = TenthBips32{10'000};       // 10%
+        auto const managementFeeRate = TenthBips16{10'000};        // 10%
+
+        auto const expectedOverpaymentFee = Number{100};  // 10% of 1,000
+        auto const expectedOverpaymentInterestGross =
+            Number{100};  // 10% of 1,000
+        auto const expectedOverpaymentInterestNet =
+            Number{90};  // 100 - 10% of 100
+        auto const expectedOverpaymentManagementFee = Number{10};  // 10% of
+        auto const expectedPrincipalPortion = Number{800};  // 1,000 - 100 - 100
+
+        auto const components = detail::computeOverpaymentComponents(
+            IOU,
+            loanScale,
+            overpayment,
+            overpaymentInterestRate,
+            overpaymentFeeRate,
+            managementFeeRate);
+
+        BEAST_EXPECT(
+            components.untrackedManagementFee == expectedOverpaymentFee);
+        BEAST_EXPECT(
+            components.untrackedInterest == expectedOverpaymentInterestNet);
+        BEAST_EXPECT(
+            components.trackedManagementFeeDelta ==
+            expectedOverpaymentManagementFee);
+        BEAST_EXPECT(
+            components.trackedPrincipalDelta == expectedPrincipalPortion);
+    }
+
+public:
+    void
+    run() override
+    {
+        testComputeOverpaymentComponents();
+    }
+};
+
+BEAST_DEFINE_TESTSUITE(LendingHelpers, app, ripple);
+
+}  // namespace test
+}  // namespace ripple