Assigning a dictionary without re-hashing

[lukasa (Cory Benfield)]

@Alvae

extension Dictionary {
  mutating func setIfNotAlready(_ value: @autoclosure () -> Value, forKey: Key) { ... }
}

I think the way you get this to work is to force the Dictionary into the _modify accessor:

extension Dictionary {
    mutating func setIfNotAlready(_ value: @autoclosure () -> Value, forKey key: Key) {
        self[key].setIfNil(value)
    }
}

extension Optional {
    fileprivate mutating func setIfNil(_ newValue: () -> Wrapped) {
        if self == nil {
            self = .some(newValue())
        }
    }
}

You can see the code for the _modify accessor here, which includes only one call to mutatingFind which does the actual hashing. All other calls have already found a bucket, so no further hashing is required to perform the insertion.