Re: A83: Galaxian algorithm


Re: A83: Galaxian algorithm

>I'm having a little trouble figuring out how does Galaxian handle the
>bullets, move so many sprites and keep track of them. I could figure out 
>source more easily if I knew the basic idea behind the code. Anyone care to
>help? Patrick Davidson, perhaps, if you're on this list? =)

  I haven't worked with Galaxian in awhile, but I'll try to remember. The 
idea is very very simple. A much simpler example of it can be found in my 
SubHunt game (the source code is less than 1/4 of Galaxian's size, and 
significantly less sophisticated in coding style). Basically, every aspect 
of each bullet and enemy is stored within a large temporary data area. In 
galaxian, the definitions are:
ybullets          =APD_BUF+9   ;30 bytes
ebuls             =APD_BUF+39  ;30 bytes
enemies           =APD_BUF+69  ;80 bytes
enemy_coords      =APD_BUF+256+1 ;40 bytes

  The data includes the X coordinate, Y coordinate, HP remaining, enemy 
type, etc. Collisions are detected by looping through the coordinates of the 
bullets and comparing each bullet X,Y coordinate with every enemy X,Y 
   When determining when and how to move an enemy, the enemy type is looked 
up in the data table. Each enemy is moved by altering the X and Y coords 
that are also stored. Actually, I think I have the order wrong. A internal 
timer tells when a specific enemy type should move, then the enemy movement 
routine looks up which enemies should be moved.
   Anyway, I reiterate that I think the idea is much more readable and 
understandable in the SubHunt source code.

  hope this helps,

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