transfer-allowances (#143)

Reviewed-on: #143

backend/db.go

@@ -711,3 +711,71 @@ func (db *Db) AddAllowanceAmount(userId int, allowanceId int, request AddAllowan
 
 	return tx.Commit()
 }
+
+func (db *Db) TransferAllowance(fromId int, toId int, amount float64) error {
+	if fromId == toId {
+		return nil
+	}
+	amountCents := int(math.Round(amount * 100.0))
+	if amountCents <= 0 {
+		return fmt.Errorf("amount must be positive")
+	}
+
+	tx, err := db.db.Begin()
+	if err != nil {
+		return err
+	}
+	defer tx.MustRollback()
+
+	// Fetch from allowance (user_id, balance)
+	var fromUserId int
+	var fromBalance int
+	err = tx.Query("select user_id, balance from allowances where id = ?").Bind(fromId).ScanSingle(&fromUserId, &fromBalance)
+	if err != nil {
+		return err
+	}
+
+	// Fetch to allowance (user_id, target, balance)
+	var toUserId int
+	var toTarget int
+	var toBalance int
+	err = tx.Query("select user_id, target, balance from allowances where id = ?").Bind(toId).ScanSingle(&toUserId, &toTarget, &toBalance)
+	if err != nil {
+		return err
+	}
+
+	// Ensure same owner
+	if fromUserId != toUserId {
+		return fmt.Errorf(ErrDifferentUsers)
+	}
+
+	// Calculate how much the 'to' goal still needs
+	remainingTo := toTarget - toBalance
+	if remainingTo <= 0 {
+		// Nothing to transfer
+		return fmt.Errorf("target already reached")
+	}
+
+	// Limit transfer to what 'to' still needs
+	transfer := amountCents
+	if transfer > remainingTo {
+		transfer = remainingTo
+	}
+
+	// Ensure 'from' has enough balance
+	if fromBalance < transfer {
+		return fmt.Errorf(ErrInsufficientFunds)
+	}
+
+	// Perform updates
+	err = tx.Query("update allowances set balance = balance - ? where id = ? and user_id = ?").Bind(transfer, fromId, fromUserId).Exec()
+	if err != nil {
+		return err
+	}
+	err = tx.Query("update allowances set balance = balance + ? where id = ? and user_id = ?").Bind(transfer, toId, toUserId).Exec()
+	if err != nil {
+		return err
+	}
+
+	return tx.Commit()
+}