DefaultedDictionary.swift

public struct DefaultedDictionary<Key: Hashable, Value> {
  var _data: [Key: Value]
  var _default: Value
  
  public init(default defaultValue: Value) {
    _data = [:]
    _default = defaultValue
  }
  
  public init(_ data: [Key: Value], default defaultValue: Value) {
    _data = data
    _default = defaultValue
  }
  
  public var count: Int {
    return _data.count
  }
  
  public subscript(key: Key) -> Value {
    get {
      return _data[key] ?? _default
    }
    set {
      _data[key] = newValue
    }
  }
  
  public mutating func removeValue(forKey key: Key) -> Value {
    return _data.removeValue(forKey: key) ?? _default
  }
}

extension DefaultedDictionary: Collection {
  public typealias Element = (key: Key, value: Value)
  public typealias Index = Dictionary<Key, Value>.Index
  
  public var startIndex: Index {
    return _data.startIndex
  }
  
  public var endIndex: Index {
    return _data.endIndex
  }
  
  public func index(after i: Index) -> Index {
    return _data.index(after: i)
  }
  
  public subscript(i: Index) -> (key: Key, value: Value) {
    return _data[i]
  }
}