Objective-C Literals: Negative Array Subscripts

NSArray+NegativeIndexes.m

//
//  NSArray+NegativeIndexes.m
//  Allow Negative Array Literals
//
//  Created by Matthew Robinson on 25th October 2012.
//  Copyright (c) 2012 Matthew Robinson. All rights reserved.
//
//  WARNING: This is a proof of concept and should not be used
//           in production code.  This could break at anytime.

#import <Foundation/Foundation.h>

#if __has_feature(objc_subscripting)

#import <objc/runtime.h>

@implementation NSArray (NegativeIndexes)

+ (void)load {
    method_exchangeImplementations(
        class_getInstanceMethod(self, @selector(objectAtIndexedSubscript:)),
        class_getInstanceMethod(self, @selector(BLC_objectAtIndexedSubscript:))
    );
}

- (id)BLC_objectAtIndexedSubscript:(NSInteger)idx {
    if (idx < 0) {
        idx = [self count] + idx;
    }

    return [self BLC_objectAtIndexedSubscript:idx];
}

@end

@implementation NSMutableArray (NegativeIndexes)

+ (void)load {
    method_exchangeImplementations(
        class_getInstanceMethod(self, @selector(setObject:atIndexedSubscript:)),
        class_getInstanceMethod(self, @selector(BLC_setObject:atIndexedSubscript:))
    );
}

- (void)BLC_setObject:(id)anObject atIndexedSubscript:(NSInteger)idx {
    if (idx < 0) {
        idx = [self count] + idx;
    }

    [self BLC_setObject:anObject atIndexedSubscript:idx];
}

@end

#endif

Great stuff. This inspired me to https://gist.github.com/mickeyl/d266e567da2e3af2ac0b