Concurrent map for NSArray

gistfile1.m

- (NSArray *)mapWithOptions:(NSEnumerationOptions)options usingBlock:(id (^)(id obj))block {
	NSUInteger count = [self count];
	NSArray *result = nil;

	if (options & NSEnumerationConcurrent) {
		volatile id * restrict objects = malloc(sizeof(id) * count);
		if (!objects) {
			return nil;
		}

		@onExit {
			free((void *)objects);
		};

		[self enumerateObjectsWithOptions:options usingBlock:^(id obj, NSUInteger index, BOOL *stop){
			obj = block(obj);
			if (!obj)
				obj = [EXTNil null];

			if (options & NSEnumerationReverse)
				index = count - index - 1;

			// retain each object, in case a concurrent autorelease pool drain
			// would screw us up below
			objects[index] = [obj retain];
		}];

		@onExit {
			// balance the -retain calls from within the block
			for (NSUInteger i = 0;i < count;++i)
				[objects[i] release];
		};
		
		// force a consistent view of the 'objects' array
		OSMemoryBarrier();

		// discard 'volatile' on these next two operations
		result = [[self class] arrayWithObjects:(const id *)objects count:count];
	} else {
		id * restrict objects = malloc(sizeof(id) * count);
		if (!objects) {
			return nil;
		}

		@onExit {
			free(objects);
		};

		[self enumerateObjectsWithOptions:options usingBlock:^(id obj, NSUInteger index, BOOL *stop){
			obj = block(obj);
			if (!obj)
				obj = [EXTNil null];

			if (options & NSEnumerationReverse)
				index = count - index - 1;

			objects[index] = obj;
		}];

		result = [[self class] arrayWithObjects:objects count:count];
	}

	return result;
}