Although the game Pizza Tycoon – known as Pizza Connection in Europe – probably doesn’t ring a bell for many folk, this 1994 DOS title is special enough for [cowomaly] to write an open source engine to bring it into the modern age as Pizza Legacy. Along the way, some questions popped up, such as how to animate the little cars that you see driving around in the simulated city and how the heck this was done back in the day on a 25 MHz 386 CPU.
On today’s GHz+, multi-core CPUs, we can just brute-force shovel pixels, sprites, and even 3D models around without a second thought while dedicating an entire core to pathfinding and other algorithms. Naturally, the original game developers had no such luxury. To understand how this animation was originally achieved, [cowomaly] had to dive into the assembly code of the original game.
The original algorithm was very simple: each road tile has at least one direction associated with it, so that a car that is on such a tile knows which direction it can travel, essentially creating a grid of one-way roads. When there’s a crossing, a random direction is picked, with the extra rule that you cannot do two consecutive turns in the same direction, presumably to keep cars from going around in circles.
Meanwhile, collision detection is simply a matter of checking the list of cars for a potential collision and not moving said car if it’s the case. This check is also optimized to take the road directions and one-way nature into account, with a 10-tick wait if there’s a block. Amusingly, this seems to enable the formation of brief traffic jams to add to that feeling of realism.
Although not a perfect algorithm and with some small bugs due to unchecked conditions with collisions, it’s hard to deny that the effect is very natural car movement, something that games like Sim City likely used as well.