//
//  Copyright (c) 2019 Open Whisper Systems. All rights reserved.
//

import Foundation

@objc
public class ConversationMessageMapping: NSObject {
    private let viewName: String
    private let group: String?

    // The desired number of the items to load BEFORE the pivot (see below).
    @objc
    public var desiredLength: UInt

    typealias ItemId = String

    // The list of currently loaded items.
    private var itemIds = [ItemId]()

    // When we enter a conversation, we want to load up to N interactions. This
    // is the "initial load window".
    //
    // We subsequently expand the load window in two directions using two very
    // different behaviors.
    //
    // * We expand the load window "upwards" (backwards in time) only when
    //   loadMore() is called, in "pages".
    // * We auto-expand the load window "downwards" (forward in time) to include
    //   any new interactions created after the initial load.
    //
    // We define the "pivot" as the last item in the initial load window.  This
    // value is only set once.
    //
    // For example, if you enter a conversation with messages, 1..15:
    //
    // 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
    //
    // We initially load just the last 5 (if 5 is the initial desired length):
    //
    // 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
    //                      |      pivot ^ | <-- load window
    // pivot: 15, desired length=5.
    //
    // If a few more messages (16..18) are sent or received, we'll always load
    // them immediately (they're after the pivot):
    //
    // 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
    //                      |      pivot ^        | <-- load window
    // pivot: 15, desired length=5.
    //
    // To load an additional page of items (perhaps due to user scrolling
    // upward), we extend the desired length and thereby load more items
    // before the pivot.
    //
    // 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
    //           |                 pivot ^        | <-- load window
    // pivot: 15, desired length=10.
    //
    // To reiterate:
    //
    // * The pivot doesn't move.
    // * The desired length applies _before_ the pivot.
    // * Everything after the pivot is auto-loaded.
    //
    // One last optimization:
    //
    // After an update, we _can sometimes_ move the pivot (for perf
    // reasons), but we also adjust the "desired length" so that this
    // no effect on the load behavior.
    //
    // And note: we use the pivot's sort id, not its uniqueId, which works
    // even if the pivot itself is deleted.
    private var pivotSortId: UInt64?

    @objc
    public var canLoadMore = false

    @objc
    public required init(group: String?, desiredLength: UInt) {
        self.viewName = TSMessageDatabaseViewExtensionName
        self.group = group
        self.desiredLength = desiredLength
    }

    @objc
    public func loadedUniqueIds() -> [String] {
        return itemIds
    }

    @objc
    public func contains(uniqueId: String) -> Bool {
        return loadedUniqueIds().contains(uniqueId)
    }

    // This method can be used to extend the desired length
    // and update.
    @objc
    public func update(withDesiredLength desiredLength: UInt, transaction: YapDatabaseReadTransaction) {
        assert(desiredLength >= self.desiredLength)

        self.desiredLength = desiredLength

        update(transaction: transaction)
    }

    // This is the core method of the class. It updates the state to
    // reflect the latest database state & the current desired length.
    @objc
    public func update(transaction: YapDatabaseReadTransaction) {
        AssertIsOnMainThread()

        guard let view = transaction.ext(viewName) as? YapDatabaseAutoViewTransaction else {
            owsFailDebug("Could not load view.")
            return
        }
        guard let group = group else {
            owsFailDebug("No group.")
            return
        }

        // Deserializing interactions is expensive, so we only
        // do that when necessary.
        let sortIdForItemId: (String) -> UInt64? = { (itemId) in
            guard let interaction = TSInteraction.fetch(uniqueId: itemId, transaction: transaction) else {
                owsFailDebug("Could not load interaction.")
                return nil
            }
            return interaction.sortId
        }

        // If we have a "pivot", load all items AFTER the pivot and up to minDesiredLength items BEFORE the pivot.
        // If we do not have a "pivot", load up to minDesiredLength BEFORE the pivot.
        var newItemIds = [ItemId]()
        var canLoadMore = false
        let desiredLength = self.desiredLength
        // Not all items "count" towards the desired length. On an initial load, all items count.  Subsequently,
        // only items above the pivot count.
        var afterPivotCount: UInt = 0
        var beforePivotCount: UInt = 0
        // (void (^)(NSString *collection, NSString *key, id object, NSUInteger index, BOOL *stop))block;
        view.enumerateKeys(inGroup: group, with: NSEnumerationOptions.reverse) { (_, key, _, stop) in
            let itemId = key

            // Load "uncounted" items after the pivot if possible.
            //
            // As an optimization, we can skip this check (which requires
            // deserializing the interaction) if beforePivotCount is non-zero,
            // e.g. after we "pass" the pivot.
            if beforePivotCount == 0,
                let pivotSortId = self.pivotSortId {
                if let sortId = sortIdForItemId(itemId) {
                    let isAfterPivot = sortId > pivotSortId
                    if isAfterPivot {
                        newItemIds.append(itemId)
                        afterPivotCount += 1
                        return
                    }
                } else {
                    owsFailDebug("Could not determine sort id for interaction: \(itemId)")
                }
            }

            // Load "counted" items unless the load window overflows.
            if beforePivotCount >= desiredLength {
                // Overflow
                canLoadMore = true
                stop.pointee = true
            } else {
                newItemIds.append(itemId)
                beforePivotCount += 1
            }
        }

        // The items need to be reversed, since we load them in reverse order.
        self.itemIds = Array(newItemIds.reversed())
        self.canLoadMore = canLoadMore

        // Establish the pivot, if necessary and possible.
        //
        // Deserializing interactions is expensive. We only need to deserialize
        // interactions that are "after" the pivot.  So there would be performance
        // benefits to moving the pivot after each update to the last loaded item.
        //
        // However, this would undesirable side effects. The desired length for
        // conversations with very short disappearing message durations would
        // continuously grow as messages appeared and disappeared.
        //
        // Therefore, we only move the pivot when we've accumulated N items after
        // the pivot.  This puts an upper bound on the number of interactions we
        // have to deserialize while minimizing "load window size creep".
        let kMaxItemCountAfterPivot = 32
        let shouldSetPivot = (self.pivotSortId == nil ||
            afterPivotCount > kMaxItemCountAfterPivot)
        if shouldSetPivot {
            if let newLastItemId = newItemIds.first {
                // newItemIds is in reverse order, so its "first" element is actually last.
                if let sortId = sortIdForItemId(newLastItemId) {
                    // Update the pivot.
                    if self.pivotSortId != nil {
                        self.desiredLength += afterPivotCount
                    }
                    self.pivotSortId = sortId
                } else {
                    owsFailDebug("Could not determine sort id for interaction: \(newLastItemId)")
                }
            }
        }
    }

    // Tries to ensure that the load window includes a given item.
    // On success, returns the index path of that item.
    // On failure, returns nil.
    @objc(ensureLoadWindowContainsUniqueId:transaction:)
    public func ensureLoadWindowContains(uniqueId: String,
                                         transaction: YapDatabaseReadTransaction) -> IndexPath? {
        if let oldIndex = loadedUniqueIds().firstIndex(of: uniqueId) {
            return IndexPath(row: oldIndex, section: 0)
        }
        guard let view = transaction.ext(viewName) as? YapDatabaseAutoViewTransaction else {
            owsFailDebug("Could not load view.")
            return nil
        }
        guard let group = group else {
            owsFailDebug("No group.")
            return nil
        }

        let indexPtr: UnsafeMutablePointer<UInt> = UnsafeMutablePointer<UInt>.allocate(capacity: 1)
        let wasFound = view.getGroup(nil, index: indexPtr, forKey: uniqueId, inCollection: TSInteraction.collection())
        guard wasFound else {
            owsFailDebug("Could not find interaction.")
            return nil
        }
        let index = indexPtr.pointee
        let threadInteractionCount = view.numberOfItems(inGroup: group)
        guard index < threadInteractionCount else {
            owsFailDebug("Invalid index.")
            return nil
        }
        // This math doesn't take into account the number of items loaded _after_ the pivot.
        // That's fine; it's okay to load too many interactions here.
        let desiredWindowSize: UInt = threadInteractionCount - index
        self.update(withDesiredLength: desiredWindowSize, transaction: transaction)

        guard let newIndex = loadedUniqueIds().firstIndex(of: uniqueId) else {
            owsFailDebug("Couldn't find interaction.")
            return nil
        }
        return IndexPath(row: newIndex, section: 0)
    }

    @objc
    public class ConversationMessageMappingDiff: NSObject {
        @objc
        public let addedItemIds: Set<String>
        @objc
        public let removedItemIds: Set<String>
        @objc
        public let updatedItemIds: Set<String>

        init(addedItemIds: Set<String>, removedItemIds: Set<String>, updatedItemIds: Set<String>) {
            self.addedItemIds = addedItemIds
            self.removedItemIds = removedItemIds
            self.updatedItemIds = updatedItemIds
        }
    }

    // Updates and then calculates which items were inserted, removed or modified.
    @objc
    public func updateAndCalculateDiff(transaction: YapDatabaseReadTransaction,
                                       notifications: [NSNotification]) -> ConversationMessageMappingDiff? {
        let oldItemIds = Set(self.itemIds)
        self.update(transaction: transaction)
        let newItemIds = Set(self.itemIds)

        let removedItemIds = oldItemIds.subtracting(newItemIds)
        let addedItemIds = newItemIds.subtracting(oldItemIds)
        // We only notify for updated items that a) were previously loaded b) weren't also inserted or removed.
        let updatedItemIds = (self.updatedItemIds(for: notifications)
            .subtracting(addedItemIds)
            .subtracting(removedItemIds)
            .intersection(oldItemIds))

        return ConversationMessageMappingDiff(addedItemIds: addedItemIds,
                                              removedItemIds: removedItemIds,
                                              updatedItemIds: updatedItemIds)
    }

    // For performance reasons, the database modification notifications are used
    // to determine which items were modified.  If YapDatabase ever changes the
    // structure or semantics of these notifications, we'll need to update this
    // code to reflect that.
    private func updatedItemIds(for notifications: [NSNotification]) -> Set<String> {
        var updatedItemIds = Set<String>()
        for notification in notifications {
            // Unpack the YDB notification, looking for row changes.
            guard let userInfo =
                notification.userInfo else {
                    owsFailDebug("Missing userInfo.")
                    continue
            }
            guard let viewChangesets =
                userInfo[YapDatabaseExtensionsKey] as? NSDictionary else {
                    // No changes for any views, skip.
                    continue
            }
            guard let changeset =
                viewChangesets[viewName] as? NSDictionary else {
                    // No changes for this view, skip.
                    continue
            }
            // This constant matches a private constant in YDB.
            let changeset_key_changes: String = "changes"
            guard let changesetChanges = changeset[changeset_key_changes] as? [Any] else {
                owsFailDebug("Missing changeset changes.")
                continue
            }
            for change in changesetChanges {
                if change as? YapDatabaseViewSectionChange != nil {
                    // Ignore.
                } else if let rowChange = change as? YapDatabaseViewRowChange {
                    updatedItemIds.insert(rowChange.collectionKey.key)
                } else {
                    owsFailDebug("Invalid change: \(type(of: change)).")
                    continue
                }
            }
        }

        return updatedItemIds
    }
}