Ordered User Keys

import UIKit

struct CommentModel {
    var postId: String?
    var userUid: String?
    var postText: String?
    var postDate: Double?
    init(dictionary: [String: Any]) {
        postId = dictionary["postId"] as? String
        userUid = dictionary["userUid"] as? String
        postText = dictionary["postText"] as? String
        postDate = dictionary["postDate"] as? Double
    }
}

// Returns a random date between the start and end times,
// truncated to the nearest second.
func randomDate(between start: Date, and end: Date) -> Date {
    let delta = end.timeIntervalSince(start)
    let randomDelta = arc4random_uniform(UInt32(delta))
    return start.addingTimeInterval(TimeInterval(randomDelta))
}

func randomDateInPast24Hours() -> Date {
    // TimeInterval is measured in seconds
    let twentyFourHours: TimeInterval = 24 /* hours */ * 60 /* minutes */ * 60 /* seconds */
    let now = Date()
    let twentyFourHoursAgo = now.addingTimeInterval(-1 * twentyFourHours)
    return randomDate(between: twentyFourHoursAgo, and: now)
}

func generateMockComment(userUid: String?) -> CommentModel {
    let dictionary: [String: Any] = [
        "postId": UUID().uuidString,
        "userUid": userUid ?? UUID().uuidString,
        "postText": "Hello World!",
        "postDate": randomDateInPast24Hours().timeIntervalSince1970
    ]
    return CommentModel(dictionary: dictionary)
}


let aliceUid = "9F463FC9-8729-47D0-8AAA-1B43F6D4121F"
let bobUid = "A92F5217-FA19-49B4-A79F-72EF07CB96D3"
let charlieUid = "932CAA5A-DFFF-4BD6-9AA2-79A439069049"

/**
 # Step 1: Collect the comments
 */

let allComments = [
    generateMockComment(userUid: aliceUid),
    generateMockComment(userUid: bobUid),
    generateMockComment(userUid: charlieUid),
    generateMockComment(userUid: aliceUid),
    generateMockComment(userUid: bobUid),
    generateMockComment(userUid: charlieUid),
    generateMockComment(userUid: aliceUid),
    generateMockComment(userUid: bobUid),
    generateMockComment(userUid: charlieUid),
]

/**
 # Step 2: Sort the comments
 */

let sortedComments = allComments.sorted { (lhs, rhs) -> Bool in
    return lhs.postDate ?? 0 > rhs.postDate ?? 0
}

/**
 # Step 3: Get the unique users, ordered by post date
 */

var orderedUsers = [String]()

for comment in sortedComments {
    guard let userUid = comment.userUid else { continue }

    // If the ordered users array *does not* contain the UID
    // add it to the array
    if !orderedUsers.contains(userUid) {
        orderedUsers.append(userUid)
    }
}

print("orderedUsers", orderedUsers)

/**
 # Step 4: Map the comments using the index of this user
 */

// I created a wrapper struct for this step so I wouldn't have
// to modify the core CommentModel struct.
struct UserKeyedCommentModel {
    var userOrder: Int
    var comment: CommentModel

    init(userOrder: Int, comment: CommentModel) {
        self.userOrder = userOrder
        self.comment = comment
    }
}

// Uses compact map so posts with a nil userUid get skipped.
let mappedComments: [UserKeyedCommentModel] = sortedComments.compactMap { (comment) -> UserKeyedCommentModel? in
    guard let userUid = comment.userUid else { return nil }
    guard let index = orderedUsers.firstIndex(of: userUid) else { return nil }
    return UserKeyedCommentModel(userOrder: index, comment: comment)
}

/**
 # Step 5: Display these comments in the table view
 */

mappedComments