Chapter 11 - Chat App/ChatAppFinished/Pods/FirebaseDatabase/Firebase/Database/Core/FSyncTree.m
/*
 * Copyright 2017 Google
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

#import <FirebaseCore/FIRLogger.h>
#import "FSyncTree.h"
#import "FListenProvider.h"
#import "FWriteTree.h"
#import "FNode.h"
#import "FPath.h"
#import "FEventRegistration.h"
#import "FImmutableTree.h"
#import "FOperation.h"
#import "FWriteTreeRef.h"
#import "FOverwrite.h"
#import "FOperationSource.h"
#import "FMerge.h"
#import "FAckUserWrite.h"
#import "FView.h"
#import "FSyncPoint.h"
#import "FEmptyNode.h"
#import "FQueryParams.h"
#import "FQuerySpec.h"
#import "FSnapshotHolder.h"
#import "FChildrenNode.h"
#import "FTupleRemovedQueriesEvents.h"
#import "FAtomicNumber.h"
#import "FEventRaiser.h"
#import "FListenComplete.h"
#import "FSnapshotUtilities.h"
#import "FCacheNode.h"
#import "FUtilities.h"
#import "FCompoundWrite.h"
#import "FWriteRecord.h"
#import "FPersistenceManager.h"
#import "FKeepSyncedEventRegistration.h"
#import "FServerValues.h"
#import "FCompoundHash.h"
#import "FRangeMerge.h"

// Size after which we start including the compound hash
static const NSUInteger kFSizeThresholdForCompoundHash = 1024;

@interface FListenContainer : NSObject<FSyncTreeHash>

@property (nonatomic, strong) FView *view;
@property (nonatomic, copy) fbt_nsarray_nsstring onComplete;

@end

@implementation FListenContainer

- (instancetype)initWithView:(FView *)view onComplete:(fbt_nsarray_nsstring)onComplete {
    self = [super init];
    if (self != nil) {
        self->_view = view;
        self->_onComplete = onComplete;
    }
    return self;
}

- (id<FNode>)serverCache {
    return self.view.serverCache;
}

- (FCompoundHash *)compoundHash {
    return [FCompoundHash fromNode:[self serverCache]];
}

- (NSString *)simpleHash {
    return [[self serverCache] dataHash];
}

- (BOOL)includeCompoundHash {
    return [FSnapshotUtilities estimateSerializedNodeSize:[self serverCache]] > kFSizeThresholdForCompoundHash;
}

@end

@interface FSyncTree ()

/**
* Tree of SyncPoints. There's a SyncPoint at any location that has 1 or more views.
*/
@property (nonatomic, strong) FImmutableTree *syncPointTree;

/**
* A tree of all pending user writes (user-initiated set, transactions, updates, etc)
*/
@property (nonatomic, strong) FWriteTree *pendingWriteTree;

/**
* Maps tagId -> FTuplePathQueryParams
*/
@property (nonatomic, strong) NSMutableDictionary *tagToQueryMap;
@property (nonatomic, strong) NSMutableDictionary *queryToTagMap;
@property (nonatomic, strong) FListenProvider *listenProvider;
@property (nonatomic, strong) FPersistenceManager *persistenceManager;
@property (nonatomic, strong) FAtomicNumber *queryTagCounter;
@property (nonatomic, strong) NSMutableSet *keepSyncedQueries;

@end

/**
* SyncTree is the central class for managing event callback registration, data caching, views
* (query processing), and event generation.  There are typically two SyncTree instances for
* each Repo, one for the normal Firebase data, and one for the .info data.
*
* It has a number of responsibilities, including:
*  - Tracking all user event callbacks (registered via addEventRegistration: and removeEventRegistration:).
*  - Applying and caching data changes for user setValue:, runTransactionBlock:, and updateChildValues: calls
*    (applyUserOverwriteAtPath:, applyUserMergeAtPath:).
*  - Applying and caching data changes for server data changes (applyServerOverwriteAtPath:,
*    applyServerMergeAtPath:).
*  - Generating user-facing events for server and user changes (all of the apply* methods
*    return the set of events that need to be raised as a result).
*  - Maintaining the appropriate set of server listens to ensure we are always subscribed
*    to the correct set of paths and queries to satisfy the current set of user event
*    callbacks (listens are started/stopped using the provided listenProvider).
*
* NOTE: Although SyncTree tracks event callbacks and calculates events to raise, the actual
* events are returned to the caller rather than raised synchronously.
*/
@implementation FSyncTree

- (id) initWithListenProvider:(FListenProvider *)provider {
    return [self initWithPersistenceManager:nil listenProvider:provider];
}

- (id) initWithPersistenceManager:(FPersistenceManager *)persistenceManager listenProvider:(FListenProvider *)provider {
    self = [super init];
    if (self) {
        self.syncPointTree = [FImmutableTree empty];
        self.pendingWriteTree = [[FWriteTree alloc] init];
        self.tagToQueryMap = [[NSMutableDictionary alloc] init];
        self.queryToTagMap = [[NSMutableDictionary alloc] init];
        self.listenProvider = provider;
        self.persistenceManager = persistenceManager;
        self.queryTagCounter = [[FAtomicNumber alloc] init];
        self.keepSyncedQueries = [NSMutableSet set];
    }
    return self;
}

#pragma mark -
#pragma mark Apply Operations

/**
* Apply data changes for a user-generated setValue: runTransactionBlock: updateChildValues:, etc.
* @return NSArray of FEvent to raise.
*/
- (NSArray *) applyUserOverwriteAtPath:(FPath *)path newData:(id <FNode>)newData writeId:(NSInteger)writeId isVisible:(BOOL)visible {
    // Record pending write
    [self.pendingWriteTree addOverwriteAtPath:path newData:newData writeId:writeId isVisible:visible];
    if (!visible) {
        return @[];
    } else {
        FOverwrite *operation = [[FOverwrite alloc] initWithSource:[FOperationSource userInstance] path:path snap:newData];
        return [self applyOperationToSyncPoints:operation];
    }
}

/**
* Apply the data from a user-generated updateChildValues: call
* @return NSArray of FEvent to raise.
*/
- (NSArray *) applyUserMergeAtPath:(FPath *)path changedChildren:(FCompoundWrite *)changedChildren writeId:(NSInteger)writeId {
    // Record pending merge
    [self.pendingWriteTree addMergeAtPath:path changedChildren:changedChildren writeId:writeId];

    FMerge *operation = [[FMerge alloc] initWithSource:[FOperationSource userInstance] path:path children:changedChildren];
    return [self applyOperationToSyncPoints:operation];
}

/**
 * Acknowledge a pending user write that was previously registered with applyUserOverwriteAtPath: or applyUserMergeAtPath:
 * TODO[offline]: Taking a serverClock here is awkward, but server values are awkward. :-(
 * @return NSArray of FEvent to raise.
 */
- (NSArray *) ackUserWriteWithWriteId:(NSInteger)writeId revert:(BOOL)revert persist:(BOOL)persist clock:(id<FClock>)clock {
    FWriteRecord *write = [self.pendingWriteTree writeForId:writeId];
    BOOL needToReevaluate = [self.pendingWriteTree removeWriteId:writeId];
    if (write.visible) {
        if (persist) {
            [self.persistenceManager removeUserWrite:writeId];
        }
        if (!revert) {
            NSDictionary *serverValues = [FServerValues generateServerValues:clock];
            if ([write isOverwrite]) {
                id<FNode> resolvedNode = [FServerValues resolveDeferredValueSnapshot:write.overwrite withServerValues:serverValues];
                [self.persistenceManager applyUserWrite:resolvedNode toServerCacheAtPath:write.path];
            } else {
                FCompoundWrite *resolvedMerge = [FServerValues resolveDeferredValueCompoundWrite:write.merge withServerValues:serverValues];
                [self.persistenceManager applyUserMerge:resolvedMerge toServerCacheAtPath:write.path];
            }
        }
    }
    if (!needToReevaluate) {
        return @[];
    } else {
        __block FImmutableTree *affectedTree = [FImmutableTree empty];
        if (write.isOverwrite) {
            affectedTree = [affectedTree setValue:@YES atPath:[FPath empty]];
        } else {
            [write.merge enumerateWrites:^(FPath *path, id <FNode> node, BOOL *stop) {
                affectedTree = [affectedTree setValue:@YES atPath:path];
            }];
        }
        FAckUserWrite *operation = [[FAckUserWrite alloc] initWithPath:write.path affectedTree:affectedTree revert:revert];
        return [self applyOperationToSyncPoints:operation];
    }
}

/**
* Apply new server data for the specified path
* @return NSArray of FEvent to raise.
*/
- (NSArray *) applyServerOverwriteAtPath:(FPath *)path newData:(id <FNode>)newData {
    [self.persistenceManager updateServerCacheWithNode:newData forQuery:[FQuerySpec defaultQueryAtPath:path]];
    FOverwrite *operation = [[FOverwrite alloc] initWithSource:[FOperationSource serverInstance] path:path snap:newData];
    return [self applyOperationToSyncPoints:operation];
}

/**
* Applied new server data to be merged in at the specified path
* @return NSArray of FEvent to raise.
*/
- (NSArray *) applyServerMergeAtPath:(FPath *)path changedChildren:(FCompoundWrite *)changedChildren {
    [self.persistenceManager updateServerCacheWithMerge:changedChildren atPath:path];
    FMerge *operation = [[FMerge alloc] initWithSource:[FOperationSource serverInstance] path:path children:changedChildren];
    return [self applyOperationToSyncPoints:operation];
}

- (NSArray *) applyServerRangeMergeAtPath:(FPath *)path updates:(NSArray *)ranges {
    FSyncPoint *syncPoint = [self.syncPointTree valueAtPath:path];
    if (syncPoint == nil) {
        // Removed view, so it's safe to just ignore this update
        return @[];
    } else {
        // This could be for any "complete" (unfiltered) view, and if there is more than one complete view, they should
        // each have the same cache so it doesn't matter which one we use.
        FView *view = [syncPoint completeView];
        if (view != nil) {
            id<FNode> serverNode = [view serverCache];
            for (FRangeMerge *merge in ranges) {
                serverNode = [merge applyToNode:serverNode];
            }
            return [self applyServerOverwriteAtPath:path newData:serverNode];
        } else {
            // There doesn't exist a view for this update, so it was removed and it's safe to just ignore this range
            // merge
            return @[];
        }
    }
}

/**
* Apply a listen complete to a path
* @return NSArray of FEvent to raise.
*/
- (NSArray *) applyListenCompleteAtPath:(FPath *)path {
    [self.persistenceManager setQueryComplete:[FQuerySpec defaultQueryAtPath:path]];
    id<FOperation> operation = [[FListenComplete alloc] initWithSource:[FOperationSource serverInstance] path:path];
    return [self applyOperationToSyncPoints:operation];
}

/**
* Apply a listen complete to a path
* @return NSArray of FEvent to raise.
*/
- (NSArray *) applyTaggedListenCompleteAtPath:(FPath *)path tagId:(NSNumber *)tagId {
    FQuerySpec *query = [self queryForTag:tagId];
    if (query != nil) {
        [self.persistenceManager setQueryComplete:query];
        FPath *relativePath = [FPath relativePathFrom:query.path to:path];
        id<FOperation> op = [[FListenComplete alloc] initWithSource:[FOperationSource forServerTaggedQuery:query.params]
                                                               path:relativePath];
        return [self applyTaggedOperation:op atPath:query.path];
    } else {
        // We've already removed the query. No big deal, ignore the update.
        return @[];
    }
}

/**
* Internal helper method to apply tagged operation
*/
- (NSArray *) applyTaggedOperation:(id<FOperation>)operation atPath:(FPath *)path {
    FSyncPoint *syncPoint = [self.syncPointTree valueAtPath:path];
    NSAssert(syncPoint != nil, @"Missing sync point for query tag that we're tracking.");
    FWriteTreeRef *writesCache = [self.pendingWriteTree childWritesForPath:path];
    return [syncPoint applyOperation:operation writesCache:writesCache serverCache:nil];
}

/**
* Apply new server data for the specified tagged query
* @return NSArray of FEvent to raise.
*/
- (NSArray *) applyTaggedQueryOverwriteAtPath:(FPath *)path newData:(id <FNode>)newData tagId:(NSNumber *)tagId {
    FQuerySpec *query = [self queryForTag:tagId];
    if (query != nil) {
        FPath *relativePath = [FPath relativePathFrom:query.path to:path];
        FQuerySpec *queryToOverwrite = relativePath.isEmpty ? query : [FQuerySpec defaultQueryAtPath:path];
        [self.persistenceManager updateServerCacheWithNode:newData forQuery:queryToOverwrite];
        FOverwrite *operation = [[FOverwrite alloc] initWithSource:[FOperationSource forServerTaggedQuery:query.params]
                                                              path:relativePath snap:newData];
        return [self applyTaggedOperation:operation atPath:query.path];
    } else {
        // Query must have been removed already
        return @[];
    }
}

/**
* Apply server data to be merged in for the specified tagged query
* @return NSArray of FEvent to raise.
*/
- (NSArray *) applyTaggedQueryMergeAtPath:(FPath *)path changedChildren:(FCompoundWrite *)changedChildren tagId:(NSNumber *)tagId {
    FQuerySpec *query = [self queryForTag:tagId];
    if (query != nil) {
        FPath *relativePath = [FPath relativePathFrom:query.path to:path];
        [self.persistenceManager updateServerCacheWithMerge:changedChildren atPath:path];
        FMerge *operation = [[FMerge alloc] initWithSource:[FOperationSource forServerTaggedQuery:query.params]
                                                      path:relativePath
                                                  children:changedChildren];
        return [self applyTaggedOperation:operation atPath:query.path];
    } else {
        // We've already removed the query. No big deal, ignore the update.
        return @[];
    }
}

- (NSArray *) applyTaggedServerRangeMergeAtPath:(FPath *)path updates:(NSArray *)ranges tagId:(NSNumber *)tagId {
    FQuerySpec *query = [self queryForTag:tagId];
    if (query != nil) {
        NSAssert([path isEqual:query.path], @"Tagged update path and query path must match");
        FSyncPoint *syncPoint = [self.syncPointTree valueAtPath:path];
        NSAssert(syncPoint != nil, @"Missing sync point for query tag that we're tracking.");
        FView *view = [syncPoint viewForQuery:query];
        NSAssert(view != nil, @"Missing view for query tag that we're tracking");
        id<FNode> serverNode = [view serverCache];
        for (FRangeMerge *merge in ranges) {
            serverNode = [merge applyToNode:serverNode];
        }
        return [self applyTaggedQueryOverwriteAtPath:path newData:serverNode tagId:tagId];
    } else {
        // We've already removed the query. No big deal, ignore the update.
        return @[];
    }
}

/**
* Add an event callback for the specified query
* @return NSArray of FEvent to raise.
*/
- (NSArray *) addEventRegistration:(id<FEventRegistration>)eventRegistration forQuery:(FQuerySpec *)query {
    FPath *path = query.path;

    __block BOOL foundAncestorDefaultView = NO;
    [self.syncPointTree forEachOnPath:query.path whileBlock:^BOOL(FPath *pathToSyncPoint, FSyncPoint *syncPoint) {
        foundAncestorDefaultView = foundAncestorDefaultView || [syncPoint hasCompleteView];
        return !foundAncestorDefaultView;
    }];

    [self.persistenceManager setQueryActive:query];

    FSyncPoint *syncPoint = [self.syncPointTree valueAtPath:path];
    if (syncPoint == nil) {
        syncPoint = [[FSyncPoint alloc] initWithPersistenceManager:self.persistenceManager];
        self.syncPointTree = [self.syncPointTree setValue:syncPoint atPath:path];
    }

    BOOL viewAlreadyExists = [syncPoint viewExistsForQuery:query];
    NSArray *events;
    if (viewAlreadyExists) {
        events = [syncPoint addEventRegistration:eventRegistration forExistingViewForQuery:query];
    } else {
        if (![query loadsAllData]) {
            // We need to track a tag for this query
            NSAssert(self.queryToTagMap[query] == nil, @"View does not exist, but we have a tag");
            NSNumber *tagId = [self.queryTagCounter getAndIncrement];
            self.queryToTagMap[query] = tagId;
            self.tagToQueryMap[tagId] = query;
        }

        FWriteTreeRef *writesCache = [self.pendingWriteTree childWritesForPath:path];
        FCacheNode *serverCache = [self serverCacheForQuery:query];
        events = [syncPoint addEventRegistration:eventRegistration
                      forNonExistingViewForQuery:query
                                     writesCache:writesCache
                                     serverCache:serverCache];

        // There was no view and no default listen
        if (!foundAncestorDefaultView) {
            FView *view = [syncPoint viewForQuery:query];
            NSMutableArray *mutableEvents = [events mutableCopy];
            [mutableEvents addObjectsFromArray:[self setupListenerOnQuery:query view:view]];
            events = mutableEvents;
        }
    }

    return events;
}

- (FCacheNode *)serverCacheForQuery:(FQuerySpec *)query {
    __block id<FNode> serverCacheNode = nil;

    [self.syncPointTree forEachOnPath:query.path whileBlock:^BOOL(FPath *pathToSyncPoint, FSyncPoint *syncPoint) {
        FPath *relativePath = [FPath relativePathFrom:pathToSyncPoint to:query.path];
        serverCacheNode = [syncPoint completeServerCacheAtPath:relativePath];
        return serverCacheNode == nil;
    }];

    FCacheNode *serverCache;
    if (serverCacheNode != nil) {
        FIndexedNode *indexed = [FIndexedNode indexedNodeWithNode:serverCacheNode index:query.index];
        serverCache = [[FCacheNode alloc] initWithIndexedNode:indexed isFullyInitialized:YES isFiltered:NO];
    } else {
        FCacheNode *persistenceServerCache = [self.persistenceManager serverCacheForQuery:query];
        if (persistenceServerCache.isFullyInitialized) {
            serverCache = persistenceServerCache;
        } else {
            serverCacheNode = [FEmptyNode emptyNode];

            FImmutableTree *subtree = [self.syncPointTree subtreeAtPath:query.path];
            [subtree forEachChild:^(NSString *childKey, FSyncPoint *childSyncPoint) {
                id<FNode> completeCache = [childSyncPoint completeServerCacheAtPath:[FPath empty]];
                if (completeCache) {
                    serverCacheNode = [serverCacheNode updateImmediateChild:childKey withNewChild:completeCache];
                }
            }];
            // Fill the node with any available children we have
            [persistenceServerCache.node enumerateChildrenUsingBlock:^(NSString *key, id<FNode> node, BOOL *stop) {
                if (![serverCacheNode hasChild:key]) {
                    serverCacheNode = [serverCacheNode updateImmediateChild:key withNewChild:node];
                }
            }];
            FIndexedNode *indexed = [FIndexedNode indexedNodeWithNode:serverCacheNode index:query.index];
            serverCache = [[FCacheNode alloc] initWithIndexedNode:indexed isFullyInitialized:NO isFiltered:NO];
        }
    }

    return serverCache;
}

/**
* Remove event callback(s).
*
* If query is the default query, we'll check all queries for the specified eventRegistration.
* If eventRegistration is null, we'll remove all callbacks for the specified query/queries.
*
* @param eventRegistration if nil, all callbacks are removed
* @param cancelError If provided, appropriate cancel events will be returned
* @return NSArray of FEvent to raise.
*/
- (NSArray *) removeEventRegistration:(id <FEventRegistration>)eventRegistration
                             forQuery:(FQuerySpec *)query
                          cancelError:(NSError *)cancelError {
    // Find the syncPoint first. Then deal with whether or not it has matching listeners
    FPath *path = query.path;
    FSyncPoint *maybeSyncPoint = [self.syncPointTree valueAtPath:path];
    NSArray *cancelEvents = @[];

    // A removal on a default query affects all queries at that location. A removal on an indexed query, even one without
    // other query constraints, does *not* affect all queries at that location. So this check must be for 'default', and
    // not loadsAllData:
    if (maybeSyncPoint && ([query isDefault] || [maybeSyncPoint viewExistsForQuery:query])) {
        FTupleRemovedQueriesEvents *removedAndEvents = [maybeSyncPoint removeEventRegistration:eventRegistration forQuery:query cancelError:cancelError];
        if ([maybeSyncPoint isEmpty]) {
            self.syncPointTree = [self.syncPointTree removeValueAtPath:path];
        }
        NSArray *removed = removedAndEvents.removedQueries;
        cancelEvents = removedAndEvents.cancelEvents;

        // We may have just removed one of many listeners and can short-circuit this whole process
        // We may also not have removed a default listener, in which case all of the descendant listeners should already
        // be properly set up.
        //
        // Since indexed queries can shadow if they don't have other query constraints, check for loadsAllData: instead
        // of isDefault:
        NSUInteger defaultQueryIndex = [removed indexOfObjectPassingTest:^BOOL(FQuerySpec *q, NSUInteger idx, BOOL *stop) {
            return [q loadsAllData];
        }];
        BOOL removingDefault = defaultQueryIndex != NSNotFound;
        [removed enumerateObjectsUsingBlock:^(FQuerySpec *query, NSUInteger idx, BOOL *stop) {
            [self.persistenceManager setQueryInactive:query];
        }];
        NSNumber *covered = [self.syncPointTree findOnPath:path andApplyBlock:^id(FPath *relativePath, FSyncPoint *parentSyncPoint) {
            return [NSNumber numberWithBool:[parentSyncPoint hasCompleteView]];
        }];

        if (removingDefault && ![covered boolValue]) {
            FImmutableTree *subtree = [self.syncPointTree subtreeAtPath:path];
            // There are potentially child listeners. Determine what if any listens we need to send before executing
            // the removal
            if (![subtree isEmpty]) {
                // We need to fold over our subtree and collect the listeners to send
                NSArray *newViews = [self collectDistinctViewsForSubTree:subtree];

                // Ok, we've collected all the listens we need. Set them up.
                [newViews enumerateObjectsUsingBlock:^(FView *view, NSUInteger idx, BOOL *stop) {
                    FQuerySpec *newQuery = view.query;
                    FListenContainer *listenContainer = [self createListenerForView:view];
                    self.listenProvider.startListening([self queryForListening:newQuery], [self tagForQuery:newQuery],
                                                       listenContainer, listenContainer.onComplete);
                }];
            } else {
                // There's nothing below us, so nothing we need to start listening on
            }
        }

        // If we removed anything and we're not covered by a higher up listen, we need to stop listening on this query.
        // The above block has us covered in terms of making sure we're set up on listens lower in the tree.
        // Also, note that if we have a cancelError, it's already been removed at the provider level.
        if (![covered boolValue] && [removed count] > 0 && cancelError == nil) {
            // If we removed a default, then we weren't listening on any of the other queries here. Just cancel the one
            // default. Otherwise, we need to iterate through and cancel each individual query
            if (removingDefault) {
                // We don't tag default listeners
                self.listenProvider.stopListening([self queryForListening:query], nil);
            } else {
                [removed enumerateObjectsUsingBlock:^(FQuerySpec *queryToRemove, NSUInteger idx, BOOL *stop) {
                    NSNumber *tagToRemove = [self.queryToTagMap objectForKey:queryToRemove];
                    self.listenProvider.stopListening([self queryForListening:queryToRemove], tagToRemove);
                }];
            }
        }
        // Now, clear all the tags we're tracking for the removed listens.
        [self removeTags:removed];
    } else {
        // No-op, this listener must've been already removed
    }
    return cancelEvents;
}

- (void)keepQuery:(FQuerySpec *)query synced:(BOOL)keepSynced {
    // Only do something if we actually need to add/remove an event registration
    if (keepSynced && ![self.keepSyncedQueries containsObject:query]) {
        [self addEventRegistration:[FKeepSyncedEventRegistration instance] forQuery:query];
        [self.keepSyncedQueries addObject:query];
    } else if (!keepSynced && [self.keepSyncedQueries containsObject:query]) {
        [self removeEventRegistration:[FKeepSyncedEventRegistration instance] forQuery:query cancelError:nil];
        [self.keepSyncedQueries removeObject:query];
    }
}

- (NSArray *) removeAllWrites {
    [self.persistenceManager removeAllUserWrites];
    NSArray *removedWrites = [self.pendingWriteTree removeAllWrites];
    if (removedWrites.count > 0) {
        FImmutableTree *affectedTree = [[FImmutableTree empty] setValue:@YES atPath:[FPath empty]];
        return [self applyOperationToSyncPoints:[[FAckUserWrite alloc] initWithPath:[FPath empty]
                                                                       affectedTree:affectedTree revert:YES]];
    } else {
        return @[];
    }
}

/**
* Returns a complete cache, if we have one, of the data at a particular path. The location must have a listener above
* it, but as this is only used by transaction code, that should always be the case anyways.
*
* Note: this method will *include* hidden writes from transaction with applyLocally set to false.
* @param path The path to the data we want
* @param writeIdsToExclude A specific set to be excluded
*/
- (id <FNode>) calcCompleteEventCacheAtPath:(FPath *)path excludeWriteIds:(NSArray *)writeIdsToExclude {
    BOOL includeHiddenSets = YES;
    FWriteTree *writeTree = self.pendingWriteTree;
    id<FNode> serverCache = [self.syncPointTree findOnPath:path andApplyBlock:^id<FNode>(FPath *pathSoFar, FSyncPoint *syncPoint) {
        FPath *relativePath = [FPath relativePathFrom:pathSoFar to:path];
        id<FNode> serverCache = [syncPoint completeServerCacheAtPath:relativePath];
        if (serverCache) {
            return serverCache;
        } else {
            return nil;
        }
    }];
    return [writeTree calculateCompleteEventCacheAtPath:path completeServerCache:serverCache excludeWriteIds:writeIdsToExclude includeHiddenWrites:includeHiddenSets];
}

#pragma mark -
#pragma mark Private Methods
/**
* This collapses multiple unfiltered views into a single view, since we only need a single
* listener for them.
* @return NSArray of FView
*/
- (NSArray *) collectDistinctViewsForSubTree:(FImmutableTree *)subtree {
    return [subtree foldWithBlock:^NSArray *(FPath *relativePath, FSyncPoint *maybeChildSyncPoint, NSDictionary *childMap) {
        if (maybeChildSyncPoint && [maybeChildSyncPoint hasCompleteView]) {
            FView *completeView = [maybeChildSyncPoint completeView];
            return @[completeView];
        } else {
            // No complete view here, flatten any deeper listens into an array
            NSMutableArray *views = [[NSMutableArray alloc] init];
            if (maybeChildSyncPoint) {
                views = [[maybeChildSyncPoint queryViews] mutableCopy];
            }
            [childMap enumerateKeysAndObjectsUsingBlock:^(NSString *childKey, NSArray *childViews, BOOL *stop) {
                [views addObjectsFromArray:childViews];
            }];
            return views;
        }
    }];
}

/**
* @param queries NSArray of FQuerySpec
*/
- (void) removeTags:(NSArray *)queries {
    [queries enumerateObjectsUsingBlock:^(FQuerySpec *removedQuery, NSUInteger idx, BOOL *stop) {
        if (![removedQuery loadsAllData]) {
            // We should have a tag for this
            NSNumber *removedQueryTag = self.queryToTagMap[removedQuery];
            [self.queryToTagMap removeObjectForKey:removedQuery];
            [self.tagToQueryMap removeObjectForKey:removedQueryTag];
        }
    }];
}

- (FQuerySpec *) queryForListening:(FQuerySpec *)query {
    if (query.loadsAllData && !query.isDefault) {
        // We treat queries that load all data as default queries
        return [FQuerySpec defaultQueryAtPath:query.path];
    } else {
        return query;
    }
}

/**
* For a given new listen, manage the de-duplication of outstanding subscriptions.
* @return NSArray of FEvent events to support synchronous data sources
*/
- (NSArray *) setupListenerOnQuery:(FQuerySpec *)query view:(FView *)view {
    FPath *path = query.path;
    NSNumber *tagId = [self tagForQuery:query];
    FListenContainer *listenContainer = [self createListenerForView:view];

    NSArray *events = self.listenProvider.startListening([self queryForListening:query], tagId, listenContainer,
                                                         listenContainer.onComplete);

    FImmutableTree *subtree = [self.syncPointTree subtreeAtPath:path];
    // The root of this subtree has our query. We're here because we definitely need to send a listen for that, but we
    // may need to shadow other listens as well.
    if (tagId != nil) {
        NSAssert(![subtree.value hasCompleteView], @"If we're adding a query, it shouldn't be shadowed");
    } else {
        // Shadow everything at or below this location, this is a default listener.
        NSArray *queriesToStop = [subtree foldWithBlock:^id(FPath *relativePath, FSyncPoint *maybeChildSyncPoint, NSDictionary *childMap) {
            if (![relativePath isEmpty] && maybeChildSyncPoint != nil && [maybeChildSyncPoint hasCompleteView]) {
                return @[[maybeChildSyncPoint completeView].query];
            } else {
                // No default listener here, flatten any deeper queries into an array
                NSMutableArray *queries = [[NSMutableArray alloc] init];
                if (maybeChildSyncPoint != nil) {
                    for (FView *view in [maybeChildSyncPoint queryViews]) {
                        [queries addObject:view.query];
                    }
                }
                [childMap enumerateKeysAndObjectsUsingBlock:^(NSString *key, NSArray *childQueries, BOOL *stop) {
                    [queries addObjectsFromArray:childQueries];
                }];
                return queries;
            }
        }];
        for (FQuerySpec *queryToStop in queriesToStop) {
            self.listenProvider.stopListening([self queryForListening:queryToStop], [self tagForQuery:queryToStop]);
        }
    }
    return events;
}

- (FListenContainer *) createListenerForView:(FView *)view {
    FQuerySpec *query = view.query;
    NSNumber *tagId = [self tagForQuery:query];

    FListenContainer *listenContainer = [[FListenContainer alloc] initWithView:view
                                                                    onComplete:^(NSString *status) {
        if ([status isEqualToString:@"ok"]) {
            if (tagId != nil) {
                return [self applyTaggedListenCompleteAtPath:query.path tagId:tagId];
            } else {
                return [self applyListenCompleteAtPath:query.path];
            }
        } else {
            // If a listen failed, kill all of the listeners here, not just the one that triggered the error.
            // Note that this may need to be scoped to just this listener if we change permissions on filtered children
            NSError *error = [FUtilities errorForStatus:status andReason:nil];
            FFWarn(@"I-RDB038012", @"Listener at %@ failed: %@", query.path, status);
            return [self removeEventRegistration:nil forQuery:query cancelError:error];
        }
    }];

    return listenContainer;
}

/**
* @return The query associated with the given tag, if we have one
*/
- (FQuerySpec *) queryForTag:(NSNumber *)tagId {
    return self.tagToQueryMap[tagId];
}

/**
* @return The tag associated with the given query
*/
- (NSNumber *) tagForQuery:(FQuerySpec *)query {
    return self.queryToTagMap[query];
}

#pragma mark -
#pragma mark applyOperation Helpers

/**
* A helper method that visits all descendant and ancestor SyncPoints, applying the operation.
*
* NOTES:
* - Descendant SyncPoints will be visited first (since we raise events depth-first).

* - We call applyOperation: on each SyncPoint passing three things:
*   1. A version of the Operation that has been made relative to the SyncPoint location.
*   2. A WriteTreeRef of any writes we have cached at the SyncPoint location.
*   3. A snapshot Node with cached server data, if we have it.

* - We concatenate all of the events returned by each SyncPoint and return the result.
*
* @return Array of FEvent
*/
- (NSArray *) applyOperationToSyncPoints:(id<FOperation>)operation {
    return [self applyOperationHelper:operation syncPointTree:self.syncPointTree serverCache:nil
                          writesCache:[self.pendingWriteTree childWritesForPath:[FPath empty]]];
}

/**
* Recursive helper for applyOperationToSyncPoints_
*/
- (NSArray *) applyOperationHelper:(id<FOperation>)operation syncPointTree:(FImmutableTree *)syncPointTree
                       serverCache:(id<FNode>)serverCache writesCache:(FWriteTreeRef *)writesCache {
    if ([operation.path isEmpty]) {
        return [self applyOperationDescendantsHelper:operation syncPointTree:syncPointTree serverCache:serverCache writesCache:writesCache];
    } else {
        FSyncPoint *syncPoint = syncPointTree.value;

        // If we don't have cached server data, see if we can get it from this SyncPoint
        if (serverCache == nil && syncPoint != nil) {
            serverCache = [syncPoint completeServerCacheAtPath:[FPath empty]];
        }

        NSMutableArray *events = [[NSMutableArray alloc] init];
        NSString *childKey = [operation.path getFront];
        id<FOperation> childOperation = [operation operationForChild:childKey];
        FImmutableTree *childTree = [syncPointTree.children get:childKey];
        if (childTree != nil && childOperation != nil) {
            id<FNode> childServerCache = serverCache ? [serverCache getImmediateChild:childKey] : nil;
            FWriteTreeRef *childWritesCache = [writesCache childWriteTreeRef:childKey];
            [events addObjectsFromArray:[self applyOperationHelper:childOperation syncPointTree:childTree serverCache:childServerCache writesCache:childWritesCache]];
        }

        if (syncPoint) {
            [events addObjectsFromArray:[syncPoint applyOperation:operation writesCache:writesCache serverCache:serverCache]];
        }

        return events;
    }
}

/**
*  Recursive helper for applyOperationToSyncPoints:
*/
- (NSArray *) applyOperationDescendantsHelper:(id<FOperation>)operation syncPointTree:(FImmutableTree *)syncPointTree
                                  serverCache:(id<FNode>)serverCache writesCache:(FWriteTreeRef *)writesCache {
    FSyncPoint *syncPoint = syncPointTree.value;

    // If we don't have cached server data, see if we can get it from this SyncPoint
    id<FNode> resolvedServerCache;
    if (serverCache == nil & syncPoint != nil) {
        resolvedServerCache = [syncPoint completeServerCacheAtPath:[FPath empty]];
    } else {
        resolvedServerCache = serverCache;
    }

    NSMutableArray *events = [[NSMutableArray alloc] init];
    [syncPointTree.children enumerateKeysAndObjectsUsingBlock:^(NSString *childKey, FImmutableTree *childTree, BOOL *stop) {
        id<FNode> childServerCache = nil;
        if (resolvedServerCache != nil) {
            childServerCache = [resolvedServerCache getImmediateChild:childKey];
        }
        FWriteTreeRef *childWritesCache = [writesCache childWriteTreeRef:childKey];
        id<FOperation> childOperation = [operation operationForChild:childKey];
        if (childOperation != nil) {
            [events addObjectsFromArray:[self applyOperationDescendantsHelper:childOperation
                                                                syncPointTree:childTree
                                                                  serverCache:childServerCache
                                                                  writesCache:childWritesCache]];
        }
    }];

    if (syncPoint) {
        [events addObjectsFromArray:[syncPoint applyOperation:operation writesCache:writesCache serverCache:resolvedServerCache]];
    }

    return events;
}

@end
