canonical way to randomize an NSArray in Objective-C

My utility library defines this category on NSMutableArray to do it:

@interface NSMutableArray (ArchUtils_Shuffle)- (void)shuffle;@end// Chooses a random integer below n without bias.// Computes m, a power of two slightly above n, and takes random() modulo m,// then throws away the random number if it's between n and m.// (More naive techniques, like taking random() modulo n, introduce a bias // towards smaller numbers in the range.)static NSUInteger random_below(NSUInteger n) {    NSUInteger m = 1;    // Compute smallest power of two greater than n.    // There's probably a faster solution than this loop, but bit-twiddling    // isn't my specialty.    do {        m <<= 1;    } while(m < n);    NSUInteger ret;    do {        ret = random() % m;    } while(ret >= n);    return ret;}@implementation NSMutableArray (ArchUtils_Shuffle)- (void)shuffle {    // http://en.wikipedia.org/wiki/Knuth_shuffle    for(NSUInteger i = [self count]; i > 1; i--) {        NSUInteger j = random_below(i);        [self exchangeObjectAtIndex:i-1 withObjectAtIndex:j];    }}@end

Make sure you seed the random number generator (with e.g. srandom(time(NULL))) sometime before you call it; otherwise the output won't be very random.

Here it is!

- (NSArray*)shuffleArray:(NSArray*)array {    NSMutableArray *temp = [[NSMutableArray alloc] initWithArray:array];    for(NSUInteger i = [array count]; i > 1; i--) {        NSUInteger j = arc4random_uniform(i);        [temp exchangeObjectAtIndex:i-1 withObjectAtIndex:j];    }    return [NSArray arrayWithArray:temp];}

if ([array count] > 1) {    for (NSUInteger shuffleIndex = [array count] - 1; shuffleIndex > 0; shuffleIndex--)        [array exchangeObjectAtIndex:shuffleIndex withObjectAtIndex:random() % (shuffleIndex + 1)];}

Make sure to seed the random() function with either srandomdev() or srandom().