Sortable Using Keypaths Example

import UIKit

protocol Sortable {}

extension Sequence where Element: Sortable {
    
    func sorted<Q>(byKeys keys: [KeyPath<Element, Q>]) -> [Element] where Q: Hashable & Comparable {
        
        guard let key = keys.first else {
            return self as! [Self.Element]
        }
        
        var buckets: [Q: [Element]] = Dictionary(grouping: self) { element in
            return element[keyPath: key]
        }
        
        let indexs = buckets.keys.sorted()
        
        buckets.forEach { index, body in
            var remainingKeys = keys
            remainingKeys.removeFirst()
            
            let bucket = buckets[index]
            buckets[index] = bucket!.sorted(byKeys: remainingKeys)
        }
        
        let values = indexs.map { buckets[$0]! }
        let result = values.reduce([], +)
        
        return result
    }
}

struct Person {
    let firstName: String
    let lastName: String
}

extension Person: Sortable {}

let people = [
    Person(firstName: "James", lastName: "Campbell"),
    Person(firstName: "Bruce", lastName: "Wayne"),
    Person(firstName: "Bob", lastName: "Campbell")
]

let sorted = people.sorted(byKeys:[\.lastName, \.firstName])