Just a quick update for those following along – I had a bug when running The Lurking Horror that would put the game into an endless loop upon pressing one of the buttons on the microwave.

The issue was an honest problem with one of my algorithms. Objects in z-machine games can have properties that tell the game things about them (things like descriptions and exits). Well, I wrote my code to assume that all objects have at least 1 property, when it truth they don’t necessarily, and it bit me in this case. The loop ran past where the object ended (since it was looking for properties where none existed), and general mayhem ensued. Fixed now. I can now cook things in the microwave.

First off, I’m renaming the DrupalZM module to the “Gruepal” module (ala Grue from Zork mixed with Drupal), thanks to Seth Cohn for the idea. Subsequent posts with be prefixed with Gruepal.

Also, a few bugs squashed, two of these took a while to track down, a lot of sloshing through zcode assembly –

Planetfall Endless Loop

While Planetfall would start fine, whenever I tried to do anything (move, look, wait, anything), the game would go into an endless loop. Upon tracing through an execution dump, I noticed that it kept comparing variables that were initialized from the random number opcode. For the loop to end, it needed to NOT match a range of numbers, but it was always matching, so the loop continued forever.

It turned out that the RANDOM z-machine opcode is inclusive in its range. I misread this originally, I thought if you specified an upper bound of, let’s say, 8, it would generate a random number from 1 to 7 (e.g. up to 8), which is the C programmer in me, everything is 0 based in an exclusive range (e.g. 8 would be 0-7). I did the see the part about it starting at 1, but not about including the upper bound of the range. In the Planetfall case, it was passing in an upper bound of 7, but I only generated 1-6, and the code was looking for a 7 to exit out of the loop, so it never did. I increased the range to be inclusive, and life was good. Which is good, because Planetfall rules, and I found myself playing quite a bit when I was supposed to be debugging. 😉

Take All Endless Loop

Another issue I noticed was in any game, when I typed “take all”, it would go into an endless loop. This was a REAL tricky one, because the loop wasn’t a tight one, it covered about 100 operations before looping. I nailed it down that the loop wasn’t exiting out when comparing the return value from a function that reported back the index number of the item number in the room to take (e.g. if there were 5 items, it would loop through 1 through 5, taking each item. But in my case, it would just get stuck on keep trying to take 2 over and over again because the iteration counter wasn’t increasing).

The reason is, there’s a little gotcha in the standard CALL opcode which, again, I did know about, but only implemented half of (by mistake). Normally a call statement is for calling a new subroutine – it saves all the information of the current call onto the stack, then (in my implementation’s case), creates a “call record” which has the return address once the subroutine exits and some other info.

Well, normally CALL has the address of the subroutine to set the PC to. However, there’s a gotcha that if its called with 0, it simply returns FALSE and doesn’t actually perform a call. This is to make certain operations in the game easier I imagine, when you need to dynamically call subroutines, and 0 indicates end of list – the false would stop the loop from going. Well, while I did return false, I unfortunately still overwrote part of the call record (which would happen if it was a normal call). But it overwrote the current call record instead of a new one, which could have caused any amount of havoc – in this case it just caused a loop. But I fixed that, and loop is gone.

Kill Me Bug

This one was easy. When I typed “kill me”, it would say “What do you want to kill the me?” instead of the correct “What do you want to kill the me with?” – e.g. it left the “with” out. Leaving words off I’m sure came up in other places, this was just how I found it. The issue is – there are a few opcodes that print to the screen, maybe 5 or so. 1 of them was passing the array with the output buffer by value instead of reference, so even though it was adding the word “with” to the output buffer, when the function exited that would be lost. Just a typo of one character, after adding a & for pass by reference, life was good. This was the bug I thought was having to do with zscii conversion, but it wasn’t!

New Found Bugs

A few more bugs to squash:

1. In lurking horror, using the buttons on the microwave puts the game into an endless loop. I have no idea what’s causing this yet without looking through an execution trace.

2. In Moonmist, when asking the user for their favorite color, it puts a weird symbol before the color repeated back to the user. I’m guessing this is a zscii/zchar issue.

Fun fun!

Unfortunately I came down with a nasty cold yesterday, but did manage to finish up the AJAX functionality tonight, including the ability for the page to redirect instead of displaying content (in the case of when the game is restarted/quit – in these cases the browser redirects to another URL which resets the game and sends the user back to the game description screen if they’re quitting).

Also, I implemented the status bar and got that working with both normal page refreshes and via AJAX. Additionally, the current room, score, and moves is now stored in the machine state database entry, which means that its possible to see where everyone online currently is and what the high scores are for all games.

Here’s a quick screenshot of a Zork I screen with status bar. You can see the beginnings of the save/load block as well, though more functionality and cleanup will be going into that.

Just a quick post, I have most the AJAX functionality done. I need to clean up a few little things like making sure the output window is always scrolled to the bottom, and handling when the page needs to redirect (in the case of game restarts/quits), but otherwise it works well.

Things have been a bit hectic lately – with the massive cooking and cleanup of Thanksgiving, and preparing for the TPUG WoC expo (and driving to and from Toronto to get there), I haven’t had a ton of time to do much coding. I did manage to slip in a couple hours last night and finished up the save/restore code.

It was actually pretty easy – what’s nice about DrupalZM is since it’s being driven from the web, it needs to save the state of the z-machine to the DB every page refresh. The save/restore code simply copies the current state of the machine to a save slot. Specifically, the table that stores the state of the zmachine now has a “slot” field associated with each record. Slot 0 indicates the current state, while slots 1-9 are saved games. Anytime a game is saved, DrupalZM simply copies the record in slot 0 to the desired slot, and restoring does vice versa.

I’m implementing a simple block to allow for easy saving/restoring. Classically one would type “save” or “restore” during gameplay, but typing these commands out in DrupalZM instructs the player to use the functionality in the block. The block will make saving/restoring much faster, plus an added bonus is the screen buffer is completely stored during the save process, so when a user restores a game, it shows them all the text on their screen exactly how it was at the time of saving.

To Do

Most of the zmachine is functional for versions 1-3 of zcode, but the following is still left to do (not necessarily in order of how I’ll tackle it):

1. Debug current issues (picking up “all” items seems to fail, zcharacter->ascii translator is still missing small bits of functionality, out of the 3 games I’ve tried, 2 work but 1 gets caught in an endless loop processing upon issuing the first command)
2. Finish all opcodes for versions 4-8 of the zmachine
3. Implement an AJAX interface
4. Finish the save/restore block
5. Implement room location/moves status bar (as well as exporting this with scores to be used elsewhere on site (e.g. location of currently logged in users, high scores per game, etc)
6. Experiment with extra functionality (quasi-multiplayer, automapping, integrating zmachine objects with drupal nodes/fields)