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Well, I wrote a few UDFs that make the bitwise operators more readable
for using with flags. I've put my thoughts in the bottom of the udf....
<!----------------------------------------------------------------------------------------------------------->
<!---
Flags.cfm
Functions to select and deselect bit flags in an integer.
THIS ENABLES YOU TO RECORD MULTIPLE SELECTIONS OF ITEMS IN A
SINGLE INTEGER.
See notes at the bottom of the page for further info and example.
Dominic Watson, 2005
--->
<cfscript>
// int setFlag(int flag, int flagSet, boolean selected = true)
function setFlag(flag, flagSet)
{
var selected = true;
if(arrayLen(arguments) GT 2)
selected = arguments[3];
if(selected)
{
return bitOr(flag, flagSet);
}
return arguments.flagSet - bitAnd(arguments.flagSet,
arguments.flag);
}
// int toggleFlag(int flag, int flagSet)
function toggleFlag(flag, flagSet)
{
return bitXOr(flag,flagSet);
}
// boolean isFlagSet(int flag, int flagSet)
function isFlagSet(flag, flagSet)
{
if(arguments.flag EQ bitAnd(arguments.flagSet, arguments.flag))
{
return true;
}
return false;
}
</cfscript>
<!--- NOTES:
What's a bit flag?
------------------
An integer is made up of 32 bits (normally). Those bits look like this:
10100010 00000000 00000000 00000000
Each bit represents a number. The bit on the far left represents 1.
The next one along is 2, then 4,8,16,32,etc.
An integer number is calculated by adding all the selected bits up,
so, in the example above we have:
1 2 4 8 16 32 64 128, etc.
1 0 1 0 0 0 1 0 , etc.
1 + 4 + 64 = 69!
There is no other way of expressing 69 in binary other than:
10100010 00000000 00000000 00000000
SO WHAT?
Well, we can utilise this on-off nature of bits to set 'flags'.
Simply put, a flag is an option that you can either select or not
select.
So, if we say that each of the bits in our integer represents an
option that
we want to be able to select or deselect, we can store the status of
those options
in a single integer. Give each option an id that corresponds to a
bit and we can turn
the option on and off just by setting a single bit.
Now there are 32 bits in an integer, so thats a maximum of 32
options, which in most cases is more than plenty.
Other languages use long ints and double words that can store more
options, but I can't see that that is available in CF.
How can I use these functions?
------------------------------
The functions I have written make readable code of the bit functions
that come with CF that enable
you to utilise bit flags.
Below is an example of how they may be used.
<cfscript>
// Our options. Notice how the values correspond to the numbers
that bits represent from left to right.
cat=1; dog=2; snake=4; hampster=8; mouse=16;
// The variable that is going to store the multiple selections
(0 = 00000000 00000000 00000000 00000000, no flags are selected)
Pets=0;
// select snake, cat and hampster
pets = setFlag(snake,pets,true);
pets = setFlag(cat,pets);
// deselect cat
pets = setFlag(cat,pets,false);
// toggle the status of hampster
pets = toggleFlag(hampster,pets);
// check status of snake
if(isFlagSet(snake,pets))
{
// do stuff
}
</cfscript>
Why bother?
-----------
Put simply, it saves space. Not so much in the live running of the
application; but when it comes to storing
selections in a database it can save a lot of space (given a large
database).
I.e.
We have a customer record and we need to store their favourite pets
somewhere. There are 32 pets
to choose from. We create a seperate table that contains our 32 pets
and each one has an ID. Now we have at
least 3 options:
1. Create a 3rd table that contains the fields customerID and petID.
To get at the customer's
pet selections we simply query that table to select all pets
with that customer ID. A good
solution in that it enforces database integritry, but if we have
lots of these types of options
we have a lot of tables.
2. Add a string field to the customer table that will contain a list
of their selections.
This is ok, but if we have lots of options like this with lots
of customers we are
really wasting space because...
3. You can store the customer's selection in a single integer field.
If there are 8 or less options
this can be a 'tinyint'(in sql), 16 or less it's a 'smallint',
and 32 or less and it's an 'int'.
Well, thats why I would bother.
So that's it, a lot of babble for 3 functions that are just
packaging a couple of very simple bit operations.
--->
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