Are those ghosts stupid or what?

Munchkin was released in 1981 for the Magnavox Odyssey 2(a.k.a Philips Videopac G7000) console, and was a very popular clone of Pac-Man.

There are a couple of features that sets Munchkin apart, for example:

• There are only  twelve"pills", Munchies, in the maze, and they move around - faster for each one eaten. Of these twelve, four are "power pills" that allows Munchkin to eat the ghosts, Munchers, for a period of time.
• All game objects can only change direction when at a junction in the maze. This goes for Munchkin, Munchers and Munchies alike.
• Near the center of the maze there is a three sided "box" that rotates (i.e. the open side moves around). This makes it possible to create mazes that are divided in sections, and the sections can only be reached via the center turnstile.
• Invisible mazes where the inner walls are visible only when Munchkin is not moving.
• A maze editor where custom mazes can be made by the player.

Munchers in the original game

When porting a game to a new platform, as is the case with Munchkin, You usually want to keep things as close to the original as possible.

For Pac-Man it is well known (and documented) how the ghosts are moving and why. The three Munchers have - to the best of my knowledge - not had their behavour documented in any way, and while playing the original game I could not really find any pattern to their movements.

Asking around, everyone seemed to agree that the Munchers move randomly around the maze, so that was the approach used in my port to Commodore 64.  Now, in code - and especially when dealing with older, limited, computers - random will always be pseudo random one way or another, and the random in my code will therefore be different from the random used in the original game.

As a result, the Munchers in the port will not behave exactly as in the original, and one thing I noticed was that Munchers needing to go to their "home base" to regenerate after being munched, could take forever. Top remedy this I added the condition that if they are next to the homebase, they will always try to enter it.

That's about it for porting the original game, though I still have a feeling that the Munchers in the original seem to have some sort of cunning plan going on.

The arcade version

Now, another idea I had when starting out with my port of the game was to add an extended version. The original game (as most games on that console) only gave the player one life - that was it. I wanted to expand on the original game to make an "arcade version":

• Three lifes to begin with, with the possibility to gain extra life every 100 point scored.
• Various mazes so that You could progress through distinct levels.
• "Boss levels" - levels where there are only enemies and regular Munchies, no "power" pills. These levels can also have more than the three original Munchers - up to a total of seven.
• And, mainly, some AI for the Munchers!

As it turned out I also added other challenges for the player such as bombs occuring randomly in the maze that Munchkin has to eat before they explode, keys to unlock areas of the maze, and a few more challenges along the way.

Making the Munchers scary

I decided to make the AI pretty straight forward:

• Locate the position of the player and go to the players current location - chasing the player.
• Locate the position the player is going and go to this location to try to intercept the player.
• When you can be eaten, try to move as far from the player as possible.
• When eaten, only try to find Your way back to the home base.
• Since no real path finding algorithm is used, Munchers can get "stuck" going back and forth so check if going back and forth too many times, and if so move randomly instead.
• Have some (adjustable) random element of movement to avoid being to apparent, and to be able to make the Munchers "smarter" at higher levels (less random).

Each Muncher use one of the two first strategies, and which Muncher using which strategy changes during the game play.

When the game begins, the Munchers are mostly moving randomly and only use its locating algorithm 25 % of the time they select which way to go. This percentage increase during the game until the locating algorithm is used 75 % of the times. Higher than that made the game both very difficult, but also the enemies a bit too artificial for my liking.

Now, while these rules are quite simplistic, they still make for some interesting behavour of the Munchers. For instance it occasionally appears that the Munchers co-operate to hunt down Munchkin with one Muncher hunting and another one intercepting - effectively cornering the player.

Another noticeable instance is when the player has died and is invincible for a few seconds when re-spawned at the center. The Munchers movement is totally random during this period, but as soon as the player is vulnerable again the Munchers starts their hunt/intercept pattern, and it looks like (depending on the probability) insects homing in on a target.

All in all, sometimes a relatively simple AI really is all it takes to give the player a good challenge.