[Galactigov-devel] Bitmasks

[Alastair M. <a.g...@sm...>]

The bitmask is just a convienient way of representing all the boolean values 
for if a player can see a system. It can be coded both in PHP and directly in 
MySQL queries. The visibleTo field is an integer and only means something 
when expressed in its binary form. So, if only player three should be able to 
see a system, you'd need the bitmask

00100000

For players two, five and seven to see a system, you'd have bitmask

01001010

To avoid dealing with the decimal integers (which is unwieldy and defeats the 
point of having them) the bitwise operators |, ^ and & need to be used. Now, 
I'll probably make a hash of explaining this, so refer to PHP or mySQL docs 
for a better explanation of how these operators work:

The | operator - bitwise OR:
The result is the equivalent of an OR operation between each bit.

0010010 | 0110001 = 0110011

This would probably be what you would use to register that a system had been 
explored by someone - you would just OR the visibleTo field with the integer 
2 to the power of the player's id. This can be done directly with a  MySQL 
query.

The & operator - bitwise AND
The ^ operator - bitwise XOR

The beauty is that several masks can be easily OR'd or AND'ed or XOR'ed 
together, for example when determining which systems can be seen due to map 
sharing treaties.