Dictionary Extension: Map Keys

Dictionary+MapKeys.swift

extension Dictionary {
    /// Returns a new dictionary containing the keys of this dictionary transformed
    /// by the given closure with the existing values.
    ///
    /// - Parameter transform: A closure that transforms a key. `transform`
    ///   accepts each key of the dictionary as its parameter and returns a
    ///   transformed key of the same or of a different type.
    /// - Returns: A dictionary containing the transformed keys and values of
    ///   this dictionary.
    ///
    /// - Complexity: O(*n*), where *n* is the length of the dictionary.
    @inlinable func mapKeys<NewKey>(_ transform: (Key) throws -> NewKey) rethrows -> [NewKey: Value] {
        try reduce(into: [NewKey: Value]()) { dictionary, keyValue in
            let key = try transform(keyValue.key)
            dictionary[key] = keyValue.value
        }
    }
}