A handy Swift dictionary that returns a specified default value for non-existing keys.

DefaultDictionary.swift

public struct DefaultDictionary<Key : Hashable, Value>: CollectionType {

    private var _storage: [Key: Value]
    private var _defaultValue: () -> Value

    public typealias Element = (Key, Value)
    public typealias Index = DictionaryIndex<Key, Value>

    public var startIndex: Index {
        return _storage.startIndex
    }

    public var endIndex: Index {
        return _storage.endIndex
    }

    public func generate() -> DictionaryGenerator<Key, Value> {
        return _storage.generate()
    }

    public subscript (position: Index) -> Element {
        return _storage[position]
    }

    public subscript (key: Key) -> Value {
        get {
            if let value = _storage[key] {
                return value
            } else {
                return _defaultValue()
            }
        }
        set {
            _storage[key] = newValue
        }
    }

    init(defaultValue: () -> Value) {
        _storage = [:]
        _defaultValue = defaultValue
    }
}