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Old February 2, 2008, 14:10   #8
Join Date: Dec 2007
Posts: 167
roustk is on a distinguished road
Originally Posted by Zero View Post
This happened to me also. I believe it is because the price listed in the store display is an average for the stack. When you buy a staff, the number of charges you get is ceiling(stack_charges / stack_size), and the price you pay is not the price of an average staff, but the price for the specific staff you are buying. If the number of charges divided by the number of items in the stack does not equal an integer, then you'll pay more than the listed average.
That is *exactly* it. From 3.0.9 store.c, around line 1830, you select the item you want to purchase:
                /* Price of one */
                price = price_item(o_ptr, FALSE);

                /* Check if the player can afford any at all */
                if ((u32b)p_ptr->au < (u32b)price)
                        /* Tell the user */
                        msg_print("You do not have enough gold for this item.");
                        store_flags |= STORE_KEEP_PROMPT;

                        /* Abort now */
                        return FALSE;
Then you select the quantity and, at line 1865, charges are allocated to the wand/staff you are actually buying:
         * XXX Stacking
         * If a rod or wand, allocate total maximum timeouts or charges
         * between those purchased and left on the shelf.
        reduce_charges(i_ptr, i_ptr->number - amt);
(reduce_charges() is in object2.c and uses integer division to calculate the number of charges to take away, so you always buy above-average charges and below-average charges are destroyed in combat.)

Finally, at about line 1888, the price is re-calculated and deducted, without a sanity check.
                /* Extract the price for the entire stack */
                price = price_item(i_ptr, FALSE) * i_ptr->number;
                /* Spend the money */
                p_ptr->au -= price;
Now that we know what is going wrong, we can debate the correct way to correct it. Because this is a special case, and the error is small, I'd recommend letting the purchase happen while simply forcing p_ptr->au to be non-negative for appearances. That is, after "p_ptr->au -= price;" add something like "if (p_ptr->au < 0) p_ptr->au = 0L;"

If we add another sanity check at this point, we would have to either (1) tell the player that we lied about the quantity he could afford or (2) we would have to (silently?) sell below-average items so that he could afford them.

I don't know what has happened to store.c since 3.0.9, but takkaria can find this code by looking for the store_purchase() function.

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