implement zesa currency swap feature

src/main/java/zw/qantra/tm/domain/services/TransactionService.java

@@ -3,6 +3,8 @@ package zw.qantra.tm.domain.services;
 import lombok.Getter;
 import lombok.RequiredArgsConstructor;
 
+import java.math.BigDecimal;
+import java.math.RoundingMode;
 import java.util.List;
 import java.util.UUID;
 
@@ -15,6 +17,7 @@ import org.springframework.stereotype.Service;
 
 import zw.qantra.tm.domain.enums.RequestType;
 import zw.qantra.tm.domain.models.AdditionalData;
+import zw.qantra.tm.domain.models.Currency;
 import zw.qantra.tm.domain.models.Transaction;
 import zw.qantra.tm.domain.repositories.TransactionRepository;
 import zw.qantra.tm.exceptions.ApiException;
@@ -28,6 +31,21 @@ public class TransactionService {
     @Getter
     private final TransactionRepository transactionRepository;
     private final AdditionalDataService additionalDataService;
+    private final CurrencyService currencyService;
+
+    public BigDecimal calculateCreditAmount(Transaction transaction) {
+        if (transaction.getCreditCurrency() == null) {
+            return BigDecimal.ZERO;
+        }
+
+        Currency currency = currencyService.findCurrencyByIsoCode(transaction.getCreditCurrency().name());
+
+        BigDecimal bankRate = new BigDecimal(currency.getBankRate());
+        BigDecimal creditAmount = transaction.getAmount().multiply(bankRate).setScale(4, RoundingMode.HALF_EVEN);
+
+        transaction.setCreditAmount(creditAmount);
+        return creditAmount;
+    }
 
     public void reassignTransactions(String oldUid, String newUid){
         List<Transaction> transactions = transactionRepository.findAllByUserId(oldUid);