Swift Dictionary key mapping from https://forums.swift.org/t/mapping-dictionary-keys/15342/4

gistfile1.txt

extension Dictionary {

    /// Transforms dictionary keys without modifying values.
    /// Deduplicates transformed keys.
    ///
    /// Example:
    /// ```
    /// ["one": 1, "two": 2, "three": 3, "": 4].mapKeys({ $0.first }, uniquingKeysWith: { max($0, $1) })
    /// // [Optional("o"): 1, Optional("t"): 3, nil: 4]
    /// ```
    ///
    /// - Parameters:
    ///   - transform: A closure that accepts each key of the dictionary as
    ///   its parameter and returns a transformed key of the same or of a different type.
    ///   - combine:A closure that is called with the values for any duplicate
    ///   keys that are encountered. The closure returns the desired value for
    ///   the final dictionary.
    /// - Returns: A dictionary containing the transformed keys and values of this dictionary.
    func mapKeys<T>(_ transform: (Key) throws -> T, uniquingKeysWith combine: (Value, Value) throws -> Value) rethrows -> [T: Value] {
        try .init(map { (try transform($0.key), $0.value) }, uniquingKeysWith: combine)
    }

    /// Transforms dictionary keys without modifying values.
    /// Drops (key, value) pairs where the transform results in a nil key.
    /// Deduplicates transformed keys.
    ///
    /// Example:
    /// ```
    /// ["one": 1, "two": 2, "three": 3, "": 4].compactMapKeys({ $0.first }, uniquingKeysWith: { max($0, $1) })
    /// // ["o": 1, "t": 3]
    /// ```
    ///
    /// - Parameters:
    ///   - transform: A closure that accepts each key of the dictionary as
    ///   its parameter and returns an optional transformed key of the same or of a different type.
    ///   - combine: A closure that is called with the values for any duplicate
    ///   keys that are encountered. The closure returns the desired value for
    ///   the final dictionary.
    /// - Returns: A dictionary containing the non-nil transformed keys and values of this dictionary.
    func compactMapKeys<T>(_ transform: (Key) throws -> T?, uniquingKeysWith combine: (Value, Value) throws -> Value) rethrows -> [T: Value] {
        try .init(compactMap { (try transform($0.key), $0.value) as? (T, Value) }, uniquingKeysWith: combine)
    }

}