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

src/xrpld/app/misc/LendingHelpers.h

@@ -387,6 +387,20 @@ struct LoanStateDeltas
     nonNegative();
 };
 
+std::pair<Number, Number>
+computeInterestAndFeeParts(
+    Number const& interest,
+    TenthBips16 managementFeeRate);
+
+ExtendedPaymentComponents
+computeOverpaymentComponents(
+    Asset const& asset,
+    int32_t const loanScale,
+    Number const& overpayment,
+    TenthBips32 const overpaymentInterestRate,
+    TenthBips32 const overpaymentFeeRate,
+    TenthBips16 const managementFeeRate);
+
 PaymentComponents
 computePaymentComponents(
     Asset const& asset,

src/xrpld/app/misc/detail/LendingHelpers.cpp

@@ -1225,19 +1225,14 @@ computeOverpaymentComponents(
     // This interest doesn't follow the normal amortization schedule - it's
     // a one-time charge for paying early.
     // Equation (20) and (21) from XLS-66 spec, Section A-2 Equation Glossary
-    auto const [rawOverpaymentInterest, _] = [&]() {
-        Number const interest =
-            tenthBipsOfValue(overpayment, overpaymentInterestRate);
-        return detail::computeInterestAndFeeParts(interest, managementFeeRate);
-    }();
-
-    // Round the penalty interest components to the loan scale
     auto const [roundedOverpaymentInterest, roundedOverpaymentManagementFee] =
         [&]() {
-            Number const interest =
-                roundToAsset(asset, rawOverpaymentInterest, loanScale);
+            auto const interest = roundToAsset(
+                asset,
+                tenthBipsOfValue(overpayment, overpaymentInterestRate),
+                loanScale);
             return detail::computeInterestAndFeeParts(
-                asset, interest, managementFeeRate, loanScale);
+                interest, managementFeeRate);
         }();
 
     auto const result = detail::ExtendedPaymentComponents{