I’m looking to take the community temperature about adding CountedSet to Swift Foundation/Standard Library. This is a Swift native version of NSCountedSet with the following preliminary design:
/// An unordered collection of unique hashable elements that
/// may appear more than once in the collection.
///
/// Each distinct value inserted into an `CountedSet`
/// has an associated counter. `CountedSet` keeps
/// track of the number of times each value is inserted.
/// That value must be removed the same number
/// of times.
///
/// Like `Set`, each `CountedSet` contains one entry for
/// each value, even when a value has been added multiple
/// times. The `count` method for both `Set` and `CountedSet`
/// returns the total number of values stored in each set.
/// A `CountedSet` does _not_ return the total number of times
/// each value is represented in the set.
///
/// New counted sets can be constructed from arrays.
/// Assign an array literal to a variable or constant
/// of the `CountedSet` type.
///
/// let quiver: CountedSet<Arrow> = [Arrow.iron, Arrow.elven, Arrow. elven]
/// if let count = quiver[.iron] {
/// print("There are \(count) iron arrows in your quiver")
/// }
Counted sets (also called bags, see CFBag) are often used in game development for tracking player inventories. They are useful for any task where one keeps count of unique values in a collection. They provide support for standard set operations (subsets, contains, insert, remove, union, intersection, etc). They can be iterated over (without guarantees of order), providing (value, count) tuples. Some function, likely a subscript, returns the number of times a value has been inserted into the set.
The type is easily implemented by wrapping a dictionary.
What are your thoughts about adding this type to Swift?
