NSMutableArray category for Fisher–Yates style shuffling. See http://en.wikipedia.org/wiki/Fisher–Yates_shuffle

NSMutableArray+Shuffle.h

/**
 * NSMutableArray+Shuffle.h
 *
 * (c) Copyright 2011 David Wagner.
 *
 * Complain/commend: http://noiseandheat.com/
 *
 *
 * Licensed under the MIT license:
 * 
 *     http://www.opensource.org/licenses/mit-license.php
 * 
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 * 
 * The above copyright notice and this permission notice shall be included in
 * all copies or substantial portions of the Software.
 * 
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 * THE SOFTWARE.
 */

#import <Foundation/Foundation.h>

@interface NSMutableArray (NSMutableArray_Shuffle)

/**
 * Shuffle the array elements in place.
 */
- (void)shuffle;

@end

NSMutableArray+Shuffle.m

/**
 * NSMutableArray+Shuffle.m
 *
 * (c) Copyright 2011 David Wagner.
 *
 * Complain/commend: http://noiseandheat.com/
 *
 *
 * Licensed under the MIT license:
 * 
 *     http://www.opensource.org/licenses/mit-license.php
 * 
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 * 
 * The above copyright notice and this permission notice shall be included in
 * all copies or substantial portions of the Software.
 * 
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 * THE SOFTWARE.
 */

#import "NSMutableArray+Shuffle.h"

@implementation NSMutableArray (NSMutableArray_Shuffle)

/**
 * Shuffle using a modern Fisher-Yates shuffle: 
 *
 * See: http://en.wikipedia.org/wiki/Fisher-Yates_shuffle
 */
- (void)shuffle
{
    for (NSUInteger i = [self count] - 1; i >= 1; i--)
    {
        // arc4random_uniform(upper) returns an unsigned integer, j, 
        // such that:
        //    0 <= j < upper
        // 
        // So we want to add 1 to i in order to make it return a number:
        //    0 <= j <= i
        
        u_int32_t j = arc4random_uniform(i + 1);
        
        [self exchangeObjectAtIndex:j withObjectAtIndex:i];
    }
}

@end