Cocoa array slicing

test.m

#import <Foundation/Foundation.h>

@interface Slice : NSObject
@property NSInteger from;
@property NSInteger to;
@property NSInteger step;
@end
@implementation Slice
@end

@interface NSNumber (SliceCreation)

- (Slice *):(NSInteger)to;
- (Slice *):(NSInteger)to :(NSInteger)step;

@end

@implementation NSNumber (SliceCreation)

- (Slice *):(NSInteger)to {
    return [self :to :1];
}

- (Slice *):(NSInteger)to :(NSInteger)step
{
    Slice *slice = [[Slice alloc] init];
    slice.from = [self integerValue];
    slice.to = to;
    slice.step = step;
    return slice;
}

@end

@interface NSArray (Slicing)

- (id)objectForKeyedSubscript: (id)subscript;

@end

@implementation NSArray (slicing)

- (id)objectForKeyedSubscript: (id)subscript
{
    Slice *slice = subscript;    
    
    NSMutableArray *subArray = [NSMutableArray array];
    NSInteger start = slice.from < 0 ? slice.from + self.count : slice.from;
    NSInteger end = slice.to < 0 ? slice.to + self.count : slice.to;

    for(NSInteger i = start; (i < end && slice.step > 0) || (i > end && slice.step < 0) ; i += slice.step) {
        [subArray addObject:[self objectAtIndex:i]];
    }
    
    return subArray;
}

@end

int main(int argc, char **argv)
{
    @autoreleasepool
    {
        NSMutableArray *array = [NSMutableArray array];
        for(int i = 0; i < 100; i++)
            [array addObject: @(i * i)];
        
        NSArray *sliced = array[[@-1:90:-1]];
        NSLog(@"%@", sliced);
    }
}

/*
 2012-07-10 15:17:32.163 NSArraySlice[4600:303] (
 9801,
 9604,
 9409,
 9216,
 9025,
 8836,
 8649,
 8464,
 8281
 )
 */