Bitwise

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Lua: Bitwise

Description:	Most people don't see the potential of bitwise in Lua or even know it exists in Lua. This article aims to explain a bit about bitwise and how to use it.
Original Author:	Butt Face
Created:	22th April 2008

Contents 1 What is a bitwise? 2 Available Operators 3 Example 4 Real world example 4.1 How does it work? 5 Additional Notes

What is a bitwise?

• Bitwise is a set of operators that act on the binary level of a number.

Available Operators

|	OR
&	AND
<<	Bit shift left
>>	Bit shift right

Example

local a, b, c, d, e = 1, 2, 4, 8, 16;
local sum = b | e; // 18
local value = e;
 
if( sum & value == value )then
	// Our value is in the sum.
end
 

• This example may be very overwhelming for bitwise newcomers. But it is only meant to show a very basic use of bitwise operators.

Real world example

• You ask "When the hell would I ever use this in Lua?", well, you might use it to check if a player has access to do a certain thing.

 
// Each of these must be a power of 2
local kick = 1;
local ban = 2;
local pee = 4;
local weapons = 8;
 
local user_flags = kick | weapons; // 9
 
// Check if a user has access to kick
if( user_flags & kick == kick )then
	// User has this flag.
end
 

How does it work?

Here's what happens when you use the OR operator in the previous example kick | weapons :

00000001 //This is 1 in binary
00001000 //And this is 8 in binary
-------- //Using the OR operator, if any of the bits are 1 they will be 1 in the result.
00001001 //This is 9 in binary.

And now for user_flags & kick :

00001001 //This is user_flags( 9 ) in binay
00000001 //And this is kick( 1 ) in binary
-------- //Using the AND operator, if both of the bits are 1 they will be in the result.
00000001 //This is the result of the operation (1) and it is equal to kick which was also 1.

Additional Notes

• Bitwise operators are only available in the Garry's Mod Lua environment, although there are modules that add bitwise functions.
• Hammer uses bitwise to store and retrieve spawnflags. To set them use ent:Fire( "spawnflags", "num" ) where "num" is the value of the flags you want.
• More information on bitwise operators can be found at http://en.wikipedia.org/wiki/Bitwise_operation
• A very helpful tutorial on bitwise (directed towards PHP, but can be easily applied to Lua as well) can be found at http://www.litfuel.net/tutorials/bitwise.htm
• The bitshift, << and >>, operators only seem to work with constants. To get around this, you can use the RunString function.
  • This may not be true anymore, with the recent update(November 3rd), they now work with variables.