Category on NSArray to allow subscripting via an NSIndexPath.

NSArray+indexPathKeyedSubscript.m

//
// Category on NSArray to allow subscripting via an NSIndexPath.
//
// Adam Sharp
// May 27, 2013
//
// Given this array:
//
//     NSArray *matrix = @[
//         @[ @1, @2 ],
//         @[ @3, @4 ]
//     ];
//
// It can be subscripted like so:
//
//     NSIndexPath *indexPath = [NSIndexPath indexPathForRow:1 inSection:0];
//     matrix[indexPath]; // => @2
//

@interface NSArray (indexPathKeyedSubscript)

@end

@implementation NSArray (indexPathKeyedSubscript)

- (id)objectForKeyedSubscript:(id)key
{
    NSParameterAssert([key isKindOfClass:[NSIndexPath class]]);
    NSIndexPath *indexPath = (NSIndexPath *)key;

    NSUInteger length = indexPath.length;
    NSUInteger *indexes = malloc(sizeof(NSUInteger) * length);
    [indexPath getIndexes:indexes];

    id memo = self;
    for (int i = 0; i < length; i++) {
        NSAssert([memo respondsToSelector:@selector(objectAtIndexedSubscript:)],
                 @"expected child collection '%@' to respond to objectAtIndexedSubscript:", memo);
        memo = memo[indexes[i]];
    }

    free(indexes);

    return memo;
}

@end