Require sender and receiver to not be equal in transferBankShards and add tests

src/main/kotlin/net/trueog/diamondbankog/balance/CachingBalanceManager.kt

@@ -93,6 +93,7 @@ internal class CachingBalanceManager private constructor() : BalanceManager {
     ): Result<Unit> {
         require(shardsToSubtractFromSender >= 0) { "shardsToSubtractFromSender must not be negative" }
         require(shardsToAddToReceiver >= 0) { "shardsToAddToReceiver must not be negative" }
+        require(sender != receiver) { "sender and receiver must not be equal" }
         if (economyDisabled) return Result.failure(EconomyDisabledException())
 
         return withLock(ShardType.BANK, sender, receiver) {

src/test/kotlin/net/trueog/diamondbankog/CachingBalanceManagerTest.kt

@@ -212,13 +212,19 @@ class CachingBalanceManagerTest {
     @Test
     @DisplayName("Transfer Bank Shards fails for negative shardsToSubtractFromSender")
     fun transferBankShardsNegativeSubtract() = runTest {
-        assertThrows<IllegalArgumentException> { manager.transferBankShards(playerUuid, playerUuid, -1, 5) }
+        assertThrows<IllegalArgumentException> { manager.transferBankShards(playerUuid, otherPlayerUuid, -1, 5) }
     }
 
     @Test
     @DisplayName("Transfer Bank Shards fails for negative shardsToAddToReceiver")
     fun transferBankShardsNegativeAdd() = runTest {
-        assertThrows<IllegalArgumentException> { manager.transferBankShards(playerUuid, playerUuid, 5, -1) }
+        assertThrows<IllegalArgumentException> { manager.transferBankShards(playerUuid, otherPlayerUuid, 5, -1) }
+    }
+
+    @Test
+    @DisplayName("Transfer Bank Shards fails for equal sender and receiver")
+    fun transferBankShardsEqualSenderReceiver() = runTest {
+        assertThrows<IllegalArgumentException> { manager.transferBankShards(playerUuid, playerUuid, 5, 5) }
     }
 
     @ParameterizedTest(name = "{0}")