NSCountedSet

Someone in a group chat previously discussed how to calculate slot machine prize levels: the machine has four columns, and each column has four symbols. If all four are the same, that is level 1; if three are the same, that is level 2; and so on.

Using if else was said to be too cumbersome, so the question was whether there was a faster and more efficient method. The first thing I thought of was using KVC to remove duplicate data. For example:

NSArray *array = @[@2, @2, @2, @1];
NSArray *result = [array valueForKeyPath:@"@distinctUnionOfObjects.self"];
NSLog(@"%@", result);

This gives an array count of 2, which happens to be level 2. It looks reasonable. But if the array is NSArray *array = @[@2, @1, @2, @1];, the calculated array count is also 2, while the correct result should be level 3. Later I remembered a class that is used very rarely: NSCountedSet.

NSArray *array = @[@1, @2, @2, @1];
NSCountedSet *set = [[NSCountedSet alloc]initWithArray:array];

[set enumerateObjectsUsingBlock:^(id obj, BOOL *stop) {
	NSLog(@"%@ => %d", obj, [set countForObject:obj]);
}];

Output

2014-05-29 00:35:22.741 CoreData[3235:60b] 1 => 2
2014-05-29 00:35:22.742 CoreData[3235:60b] 2 => 2

You may notice that this class’s superclass is NSMutableSet. Huh? Isn’t NSMutableSet supposed to be unable to store duplicate objects? In fact, NSCountedSet also cannot store duplicate objects. Apple’s documentation describes this class with the following sentence:

Each distinct object inserted into an NSCountedSet object has a counter associated with it. Each distinct object inserted into an NSCountedSet object has an associated counter (Google translation).

In other words, when a duplicate object is added, that object’s counter is incremented by 1. So this class provides a method called - (NSUInteger)countForObject:(id)object; to count duplicate objects.