Sorting custom objects array by another array with a specific property in Swift

array-sorting-extension.swift

/// These implementations of sorting based on a specific property of custom object. For example: 
///     struct Person {
///       var age: Int
///       var name: String
///     }
///
///     // Case 1:
///     let people = [Person(age: 10, name: "Tom"), Person(age: 12, name: "John"), Person(age: 20, name: "Peter"), Person(age: 1, name: "Nancy")]
///     let orderedPeople = [Person(age: 12, name: "John"), Person(age: 10, name: "Tom")]
///     
///     let result = people.reorder(by: orderedPeople, keyPath: \.age))
///     // Prints: [Person(age: 12, name: "John"), Person(age: 10, name: "Tom"), Person(age: 20, name: "Peter"), Person(age: 1, name: "Nancy")]
///     
///     // Case 2:
///     let people = the same as above
///     let orderedAges = [12, 10]
///
///     let result = people.reorder(by: orderedAges, keyPath: \.age))
///     // Prints: the same as above
extension Array {
    func reorder<T: Equatable>(by orderedArray: [T], keyPath: KeyPath<Element, T>) -> [Element] {
        return self.sorted { (lhs, rhs) -> Bool in
            guard let aIndex = orderedArray.firstIndex(of: lhs[keyPath: keyPath]) else { return false }
            guard let bIndex = orderedArray.firstIndex(of: rhs[keyPath: keyPath]) else { return true }
            return aIndex < bIndex
        }
    }
    
    func reorder<T: Equatable>(by orderedArray: [Element], keyPath: KeyPath<Element, T>) -> [Element] {
        let comp = orderedArray.map { e in return e[keyPath: keyPath] }
        return self.sorted { (lhs, rhs) -> Bool in
            guard let aIndex = comp.firstIndex(of: lhs[keyPath: keyPath]) else { return false }
            guard let bIndex = comp.firstIndex(of: rhs[keyPath: keyPath]) else { return true }
            return aIndex < bIndex
        }
    }
}