MessageDecoder

MessageDecoder.swift

import Foundation

final class MessagePrinter {
    private let decoder = MessageDecoder()

    func printEncodedMessage(_ encodedMessage: [Int]) {
        decoder.decodeMessage(encodedMessage) { message in
            self.printDecodedMessage(message)
        }
    }

    private func printDecodedMessage(_ message: String) {
        print("Decoded message is:")
        print(message)
    }
}

final class MessageDecoder {

    func decodeMessage(_ encodedMessage: [Int], completion: @escaping (String) -> Void) {
        /// Task:
        ///
        /// decode message asynchronously
        /// using provided internal implementation
        let group = DispatchGroup()
        var preResult = [(index: Int, result: String)]()
        encodedMessage.enumerated().forEach { currentIndex, mapIndex in
            group.enter()

            decodePhrase(at: mapIndex) { value in
                let tuple = (index: currentIndex, result: value)
                preResult.append(tuple)
                group.leave()
            }
        }
        group.notify(queue: DispatchQueue.main) {
            let result: String = preResult.sorted(by: { $0.index < $1.index })
                                          .reduce("", {
                                              return $0 + $1.result
                                          })
            completion(result)
        }
    }

    private static let phrases = ["problem.", "First,", "the", "solve", "Then,", "code.", "the", "write"]

    private func decodePhrase(at index: Int, completion: @escaping (String) -> Void) {
        let delay = Double.random(in: 0..<2)
        DispatchQueue.global().asyncAfter(deadline: .now() + delay) {
            let phrase = Self.phrases[index]
            DispatchQueue.main.async {
                completion(phrase)
            }
        }
    }
}

MessagePrinter().printEncodedMessage([1, 3, 2, 0, 4, 7, 6, 5])