Swift implementation of a bidirectional dictionary

BidirectionalDictionary

struct BidirectionalDictionary<KeyType:Hashable, ValueType:Hashable>{
    var keyToValue = [KeyType:ValueType]()
    var valueToKey = [ValueType:KeyType]()
    
    subscript(key:KeyType) -> ValueType? {
        get {
            return self.keyToValue[key]
        }
        set {
            self.keyToValue[key] = newValue
            if let aValue = newValue {
                self.valueToKey[aValue] = key
            }
        }
    }
    
    subscript(value:ValueType) -> KeyType? {
        get {
            return self.valueToKey[value]
        }
        set(key) {
            self.valueToKey[value] = key
            if let aKey = key {
                self.keyToValue[aKey] = value
            }
        }
    }
}