NSMutableArray (Shuffle)

NSMutableArray+Shuffle.h

@interface NSMutableArray (Shuffle)
    - (void)shuffle;
@end

NSMutableArray+Shuffle.m

#import "NSMutableArray+Shuffle.h"

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