Question

Hi I have a program that needs to be fix. Here is the problem:

For example I have :

Free credit of 2 and regular credit of 5.

For example I want to buy an item for 3 credits.

I want to deduct 2 free credits and 1 regular credits.

so the remaining balance will be 0 free credits and 4 regular credits.

Free credits will be the first to deduct before regular credit deducted.

In my existing program if I buy 1 item for 3 credits

here is the balance come out:
-1 free credits
5 regular credits

Which is not be like that.

I want to have like this:
0 free credits
4 regular credits

I hope someone can help me on this issue.
Thank you.

Answer 1

$free_credits -= $cost;
if($free_credits < 0) {
    $regular_credits += $free_credits;
    $free_credits = 0;
}

Or preferably in the SQL directly:

START TRANSACTION;
    SELECT @ok := (free_credits + regular_credits) > :cost
        FROM user_credits WHERE id = :id  FOR UPDATE;

    IF @ok THEN
        UPDATE user_credits
            SET
                free_credits = free_credits - :cost,
                regular_credits = regular_credits + IF(free_credits < 0, free_credits, 0)
            WHERE id = :id
    ELSE;
        /* something to cause the commit to fail */
    END IF;
COMMIT;

As a technicality, for this to work the datatype for free_credits needs to be signed.