I'd like to extend Apple implementation of NSMutableSet to include a
randomObject method. The purpose of this method is self-explanatory and its
potential use is I'm sure obvious to most of you.
For my first idea I thought about simply adding a category to NSMutableSet but
any obvious implementation of -randomObject would require accessing existing
instance variables or adding my own in order to implement the random access I
would need to for the set. I also don't know how objects are stored
internally. Is it an array? Is it a key/value pair? I don't know. How am I
going to correlate the random numbers I generate with the objects in the set?
Next, I thought about using inheritance with NSMutableArray. I even wrote some
code:
+ (void)initialize
{
if ( self == [MCRandomlyAccessedArray class] )
{
srandom(time(NULL));
}
}
- (id)randomObject
{
// If the array is empty, throw an exception. The caller should know
better.
// If there is only one, return object zero every time.
// If there are only two, alternate with every access.
// If there are three or more, randomize but never provide the same object
// twice in a row.
if ( [self count] )
{
NSUInteger index = 0;
if ( [self count] > 1 )
{
if ( 2 == [self count] )
{
index = (++lastObjectIndex % 2);
}
else
{
do
{
index = (random() % [self count]);
} while (index == lastObjectIndex);
}
lastObjectIndex = index;
}
return [self objectAtIndex:index];
}
else
{
[[NSException exceptionWithName:NSRangeException
reason:@"Array is empty"
userInfo:nil] raise];
}
return nil; // unreachable
}
Come to find out at runtime that this won't work either because NSMutableArray
is abstract!
*** Terminating app due to uncaught exception 'NSInvalidArgumentException',
reason: '*** -[NSMutableArray addObject:]: method only defined for abstract
class. Define -[MCRandomlyAccessedArray addObject:]!'
This would seem to indicate that I need to write a lot more code in order to
have a complete implementation. I'm not in the mood to re-invent the wheel.
I'm not lazy, I just have a lot of other code that needs to be written, which
pertains to the big picture.
Next, I thought maybe I could simply wrap-up an NSMutableArray in my own class
and let it do most of the heavy lifting when it comes to all of those existing
methods. The only problem is that I'm no expert on how to duplicate all of the
class and instance init methods and memory management code.
So, that brings me to you lovely people. What do you recommend? Do I bite the
bullet and re-implement NSMutableArray? If so, where can I find guidance on
this? I love the containers provided by Apple. They simply work and I don't
have to think about them. So much so, that I don't really know how they work.
My expertise is/was in C/C++ and STL.
Is there fourth alternative or something I missed in my brief examination as
described above?
Thanks.
-Michael
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