Implemented BankingModelService to filter out duplicates in retrieved AccountTransactions

BankingClient/src/commonMain/kotlin/net/codinux/banking/client/service/BankingModelService.kt

@@ -0,0 +1,24 @@
+package net.codinux.banking.client.service
+
+import net.codinux.banking.client.model.AccountTransaction
+
+open class BankingModelService {
+
+    /**
+     * It's not possible to retrieve only new transactions from bank server (almost no bank implements HKKAN job). So
+     * for updating account transactions we start at the date of latest account transactions retrieval time (e.g.
+     * transactions have at last been fetched at 01. September 12:00, then there may have been some other transactions
+     * been booked on September 1st after 12:00 o'clock).
+     *
+     * Therefore retrieved account transactions may contain transactions that we already have locally. This method filters
+     * from [retrievedTransactions] those already in [existingTransactions] and returns only that ones, that are not in
+     * [existingTransactions].
+     */
+    open fun findNewTransactions(retrievedTransactions: List<AccountTransaction>, existingTransactions: List<AccountTransaction>): List<AccountTransaction> {
+        val existingTransactionsByIdentifier = existingTransactions.associateBy { it.identifier }
+        val existingTransactionsIdentifiers = existingTransactionsByIdentifier.keys
+
+        return retrievedTransactions.filter { transaction -> existingTransactionsIdentifiers.contains(transaction.identifier) == false }
+    }
+
+}