feat(investment): interest schedule API for Interest Entry

Add GET /v1/investment/purchases/{id}/interest-schedule. For a holding it
lists every half-yearly coupon and every 31 Mar / 30 Sep closing provision
from purchase to maturity, worked out on the face value still held:
provision = interest earned since the last coupon (30/360), and the coupon
clears what is already booked (purchased accrued interest or the earlier
provision). Lines up to the as-on date are marked due. Calculation only;
nothing is posted.

The day-count and interest arithmetic move into InvestmentInterestCalculator,
shared with Purchase Entry, and are covered by unit tests using Jitendra
Sir's worked example. Duplicate-code errors on the masters now keep the
original exception as the cause.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

fineract-provider/src/test/java/org/apache/fineract/portfolio/investment/service/InvestmentInterestCalculatorTest.java

@@ -0,0 +1,173 @@
+/**
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements. See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership. The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License. You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied. See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+package org.apache.fineract.portfolio.investment.service;
+
+import static org.apache.fineract.portfolio.investment.service.InvestmentInterestCalculator.TYPE_COUPON;
+import static org.apache.fineract.portfolio.investment.service.InvestmentInterestCalculator.TYPE_PROVISION;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import java.math.BigDecimal;
+import java.time.LocalDate;
+import java.util.List;
+import org.apache.fineract.portfolio.investment.service.InvestmentInterestCalculator.ScheduleRow;
+import org.junit.jupiter.api.Test;
+
+/**
+ * Figures from Jitendra Sir's worked example: 7.50% GOI 2034, face value 1 crore, coupons 10 Feb / 10 Aug, bought on
+ * 08-10-2026.
+ */
+class InvestmentInterestCalculatorTest {
+
+    private static final BigDecimal ONE_CRORE = new BigDecimal("10000000");
+    private static final BigDecimal RATE = new BigDecimal("7.50");
+    private static final LocalDate MATURITY = LocalDate.of(2034, 8, 10);
+    private static final LocalDate LAST_COUPON = LocalDate.of(2026, 8, 10);
+    private static final LocalDate DEAL = LocalDate.of(2026, 10, 8);
+
+    @Test
+    void days360CountsEveryMonthAsThirtyDays() {
+        assertEquals(50, InvestmentInterestCalculator.days360(LAST_COUPON, LocalDate.of(2026, 9, 30)));
+        assertEquals(58, InvestmentInterestCalculator.days360(LAST_COUPON, DEAL));
+        assertEquals(90, InvestmentInterestCalculator.days360(LocalDate.of(2026, 6, 30), LocalDate.of(2026, 9, 30)));
+        assertEquals(50, InvestmentInterestCalculator.days360(LocalDate.of(2027, 2, 10), LocalDate.of(2027, 3, 31)));
+        assertEquals(180, InvestmentInterestCalculator.days360(LAST_COUPON, LocalDate.of(2027, 2, 10)));
+        assertEquals(360, InvestmentInterestCalculator.days360(LAST_COUPON, LocalDate.of(2027, 8, 10)));
+    }
+
+    @Test
+    void days360IsZeroWhenDatesAreMissingOrReversed() {
+        assertEquals(0, InvestmentInterestCalculator.days360(null, DEAL));
+        assertEquals(0, InvestmentInterestCalculator.days360(DEAL, null));
+        assertEquals(0, InvestmentInterestCalculator.days360(DEAL, DEAL));
+        assertEquals(0, InvestmentInterestCalculator.days360(DEAL, LAST_COUPON));
+    }
+
+    @Test
+    void interestForDaysRoundsToTheRupee() {
+        assertEquals(new BigDecimal("120833"), InvestmentInterestCalculator.interestForDays(ONE_CRORE, RATE, 58));
+        assertEquals(new BigDecimal("104167"), InvestmentInterestCalculator.interestForDays(ONE_CRORE, RATE, 50));
+        assertEquals(new BigDecimal("375000"), InvestmentInterestCalculator.interestForDays(ONE_CRORE, RATE, 180));
+    }
+
+    @Test
+    void halfYearCouponIsFaceValueTimesRateOverTwo() {
+        assertEquals(new BigDecimal("375000"), InvestmentInterestCalculator.halfYearCoupon(ONE_CRORE, RATE));
+        assertEquals(new BigDecimal("362500"), InvestmentInterestCalculator.halfYearCoupon(ONE_CRORE, new BigDecimal("7.25")));
+    }
+
+    @Test
+    void couponDatesRunSixMonthlyFromMaturity() {
+        assertEquals(LAST_COUPON, InvestmentInterestCalculator.lastCouponOnOrBefore(MATURITY, DEAL));
+        assertEquals(LAST_COUPON, InvestmentInterestCalculator.lastCouponOnOrBefore(MATURITY, LAST_COUPON));
+
+        final List<LocalDate> coupons = InvestmentInterestCalculator.couponDatesAfter(MATURITY, LAST_COUPON);
+        assertEquals(16, coupons.size());
+        assertEquals(LocalDate.of(2027, 2, 10), coupons.get(0));
+        assertEquals(LocalDate.of(2027, 8, 10), coupons.get(1));
+        assertEquals(MATURITY, coupons.get(15));
+    }
+
+    @Test
+    void monthEndMaturityKeepsMonthEndCoupons() {
+        final LocalDate maturity = LocalDate.of(2030, 8, 31);
+        final List<LocalDate> coupons = InvestmentInterestCalculator.couponDatesAfter(maturity, LocalDate.of(2029, 8, 31));
+        assertEquals(List.of(LocalDate.of(2030, 2, 28), maturity), coupons);
+    }
+
+    @Test
+    void closingDatesAreMarchAndSeptemberEnds() {
+        assertEquals(List.of(LocalDate.of(2027, 3, 31), LocalDate.of(2027, 9, 30)),
+                InvestmentInterestCalculator.closingDates(DEAL, LocalDate.of(2028, 3, 30)));
+        assertEquals(List.of(LocalDate.of(2026, 9, 30)),
+                InvestmentInterestCalculator.closingDates(LocalDate.of(2026, 9, 30), LocalDate.of(2026, 9, 30)));
+    }
+
+    @Test
+    void scheduleClearsPurchasedInterestAndProvisionsAtEachClosing() {
+        final List<ScheduleRow> rows = InvestmentInterestCalculator.schedule(ONE_CRORE, RATE, MATURITY, LAST_COUPON, DEAL,
+                new BigDecimal("120833"));
+
+        // 16 coupons + 15 closings (Mar 2027 … Mar 2034); 30-09-2026 is before the deal, so it is the seller's.
+        assertEquals(31, rows.size());
+
+        // First coupon: 1,20,833 paid to the seller is recovered, only the rest is our income.
+        assertRow(rows.get(0), TYPE_COUPON, LocalDate.of(2027, 2, 10), LocalDate.of(2026, 8, 11), 180, "375000", "120833", "254167");
+        // 31 March closing: 50 days earned since 10 Feb, nothing booked yet.
+        assertRow(rows.get(1), TYPE_PROVISION, LocalDate.of(2027, 3, 31), LocalDate.of(2027, 2, 11), 50, "104167", "0", "104167");
+        // August coupon clears the March provision.
+        assertRow(rows.get(2), TYPE_COUPON, LocalDate.of(2027, 8, 10), LocalDate.of(2027, 2, 11), 180, "375000", "104167", "270833");
+        assertRow(rows.get(3), TYPE_PROVISION, LocalDate.of(2027, 9, 30), LocalDate.of(2027, 8, 11), 50, "104167", "0", "104167");
+
+        final ScheduleRow last = rows.get(rows.size() - 1);
+        assertEquals(TYPE_COUPON, last.type());
+        assertEquals(MATURITY, last.date());
+
+        // Over the life, income booked = all coupons − interest bought from the seller.
+        final BigDecimal totalIncome = rows.stream().map(ScheduleRow::amount).reduce(BigDecimal.ZERO, BigDecimal::add);
+        assertEquals(new BigDecimal("375000").multiply(BigDecimal.valueOf(16)).subtract(new BigDecimal("120833")), totalIncome);
+    }
+
+    @Test
+    void closingOnACouponDateIsNotProvisionedTwice() {
+        // Coupons on 31 Mar / 30 Sep: every closing is a coupon day, so there are no provision lines.
+        final List<ScheduleRow> rows = InvestmentInterestCalculator.schedule(ONE_CRORE, RATE, LocalDate.of(2028, 3, 31),
+                LocalDate.of(2026, 9, 30), DEAL, BigDecimal.ZERO);
+        assertEquals(3, rows.size());
+        assertTrue(rows.stream().allMatch(r -> TYPE_COUPON.equals(r.type())));
+    }
+
+    @Test
+    void scheduleIsEmptyWithoutRateOrDates() {
+        assertTrue(InvestmentInterestCalculator.schedule(ONE_CRORE, BigDecimal.ZERO, MATURITY, LAST_COUPON, DEAL, null).isEmpty());
+        assertTrue(InvestmentInterestCalculator.schedule(ONE_CRORE, RATE, null, LAST_COUPON, DEAL, null).isEmpty());
+        assertTrue(InvestmentInterestCalculator.schedule(ONE_CRORE, RATE, MATURITY, null, DEAL, null).isEmpty());
+    }
+
+    @Test
+    void dealNumberFollowsTheAprilToMarchFinancialYear() {
+        assertEquals("202627", InvestmentPurchaseWritePlatformServiceImpl.dealNoPrefix(LocalDate.of(2026, 4, 1)));
+        assertEquals("202627", InvestmentPurchaseWritePlatformServiceImpl.dealNoPrefix(DEAL));
+        assertEquals("202627", InvestmentPurchaseWritePlatformServiceImpl.dealNoPrefix(LocalDate.of(2027, 3, 31)));
+        assertEquals("202728", InvestmentPurchaseWritePlatformServiceImpl.dealNoPrefix(LocalDate.of(2027, 4, 1)));
+        assertEquals("209900", InvestmentPurchaseWritePlatformServiceImpl.dealNoPrefix(LocalDate.of(2099, 12, 1)));
+    }
+
+    @Test
+    void approxYtmRisesWhenBoughtAtADiscount() {
+        final LocalDate issue = LocalDate.of(2024, 8, 10);
+        assertEquals(new BigDecimal("7.59"),
+                InvestmentPurchaseWritePlatformServiceImpl.approxYtm(RATE, new BigDecimal("99.50"), issue, MATURITY));
+        assertEquals(new BigDecimal("7.50"),
+                InvestmentPurchaseWritePlatformServiceImpl.approxYtm(RATE, new BigDecimal("100"), issue, MATURITY));
+        assertEquals(RATE, InvestmentPurchaseWritePlatformServiceImpl.approxYtm(RATE, new BigDecimal("99.50"), null, MATURITY));
+    }
+
+    private static void assertRow(final ScheduleRow row, final String type, final LocalDate date, final LocalDate from, final int days,
+            final String gross, final String alreadyBooked, final String amount) {
+        assertEquals(type, row.type());
+        assertEquals(date, row.date());
+        assertEquals(from, row.fromDate());
+        assertEquals(date, row.toDate());
+        assertEquals(days, row.days());
+        assertEquals(new BigDecimal(gross), row.grossInterest());
+        assertEquals(new BigDecimal(alreadyBooked), row.alreadyBooked());
+        assertEquals(new BigDecimal(amount), row.amount());
+    }
+}