firebase-ios-sdk/Firebase/Database/Core/FWriteTree.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 "FWriteTree.h"
#import "FImmutableTree.h"
#import "FPath.h"
#import "FNode.h"
#import "FWriteTreeRef.h"
#import "FChildrenNode.h"
#import "FNamedNode.h"
#import "FWriteRecord.h"
#import "FEmptyNode.h"
#import "FIndex.h"
#import "FCompoundWrite.h"
#import "FCacheNode.h"

@interface FWriteTree ()
/**
* A tree tracking the results of applying all visible writes. This does not include transactions with
* applyLocally=false or writes that are completely shadowed by other writes.
* Contains id<FNode> as values.
*/
@property (nonatomic, strong) FCompoundWrite *visibleWrites;
/**
* A list of pending writes, regardless of visibility and shadowed-ness. Used to calcuate arbitrary
* sets of the changed data, such as hidden writes (from transactions) or changes with certain writes excluded (also
* used by transactions).
* Contains FWriteRecords.
*/
@property (nonatomic, strong) NSMutableArray *allWrites;
@property (nonatomic) NSInteger lastWriteId;
@end

/**
* FWriteTree tracks all pending user-initiated writes and has methods to calcuate the result of merging them with
* underlying server data (to create "event cache" data). Pending writes are added with addOverwriteAtPath: and
* addMergeAtPath: and removed with removeWriteId:.
*/
@implementation FWriteTree

@synthesize allWrites;
@synthesize lastWriteId;

- (id) init {
    self = [super init];
    if (self) {
        self.visibleWrites = [FCompoundWrite emptyWrite];
        self.allWrites = [[NSMutableArray alloc] init];
        self.lastWriteId = -1;
    }
    return self;
}

/**
* Create a new WriteTreeRef for the given path. For use with a new sync point at the given path.
*/
- (FWriteTreeRef *) childWritesForPath:(FPath *)path {
    return [[FWriteTreeRef alloc] initWithPath:path writeTree:self];
}

/**
* Record a new overwrite from user code.
* @param visible Is set to false by some transactions. It should be excluded from event caches.
*/
- (void) addOverwriteAtPath:(FPath *)path newData:(id <FNode>)newData writeId:(NSInteger)writeId isVisible:(BOOL)visible {
    NSAssert(writeId > self.lastWriteId, @"Stacking an older write on top of a newer one");
    FWriteRecord *record = [[FWriteRecord alloc] initWithPath:path overwrite:newData writeId:writeId visible:visible];
    [self.allWrites addObject:record];

    if (visible) {
        self.visibleWrites = [self.visibleWrites addWrite:newData atPath:path];
    }

    self.lastWriteId = writeId;
}

/**
* Record a new merge from user code.
* @param changedChildren maps NSString -> id<FNode>
*/
- (void) addMergeAtPath:(FPath *)path changedChildren:(FCompoundWrite *)changedChildren writeId:(NSInteger)writeId {
    NSAssert(writeId > self.lastWriteId, @"Stacking an older merge on top of newer one");
    FWriteRecord *record = [[FWriteRecord alloc] initWithPath:path merge:changedChildren writeId:writeId];
    [self.allWrites addObject:record];

    self.visibleWrites = [self.visibleWrites addCompoundWrite:changedChildren atPath:path];
    self.lastWriteId = writeId;
}

- (FWriteRecord *)writeForId:(NSInteger)writeId {
    NSUInteger index = [self.allWrites indexOfObjectPassingTest:^BOOL(FWriteRecord *write, NSUInteger idx, BOOL *stop) {
        return write.writeId == writeId;
    }];
    return (index == NSNotFound) ? nil : self.allWrites[index];
}

/**
* Remove a write (either an overwrite or merge) that has been successfully acknowledged by the server. Recalculates the
* tree if necessary. We return the path of the write and whether it may have been visible, meaning views need to
* reevaluate.
*
* @return YES if the write may have been visible (meaning we'll need to reevaluate / raise events as a result).
*/
- (BOOL) removeWriteId:(NSInteger)writeId {
    NSUInteger index = [self.allWrites indexOfObjectPassingTest:^BOOL(FWriteRecord *record, NSUInteger idx, BOOL *stop) {
        if (record.writeId == writeId) {
            return YES;
        } else {
            return NO;
        }
    }];
    NSAssert(index != NSNotFound, @"[FWriteTree removeWriteId:] called with nonexistent writeId.");
    FWriteRecord *writeToRemove = self.allWrites[index];
    [self.allWrites removeObjectAtIndex:index];

    BOOL removedWriteWasVisible = writeToRemove.visible;
    BOOL removedWriteOverlapsWithOtherWrites = NO;
    NSInteger i = [self.allWrites count] - 1;

    while (removedWriteWasVisible && i >= 0) {
        FWriteRecord *currentWrite = [self.allWrites objectAtIndex:i];
        if (currentWrite.visible) {
            if (i >= index && [self record:currentWrite containsPath:writeToRemove.path]) {
                // The removed write was completely shadowed by a subsequent write.
                removedWriteWasVisible = NO;
            } else if ([writeToRemove.path contains:currentWrite.path]) {
                // Either we're covering some writes or they're covering part of us (depending on which came first).
                removedWriteOverlapsWithOtherWrites = YES;
            }
        }
        i--;
    }

    if (!removedWriteWasVisible) {
        return NO;
    } else if (removedWriteOverlapsWithOtherWrites) {
        // There's some shadowing going on. Just rebuild the visible writes from scratch.
        [self resetTree];
        return YES;
    } else {
        // There's no shadowing.  We can safely just remove the write(s) from visibleWrites.
        if ([writeToRemove isOverwrite]) {
            self.visibleWrites = [self.visibleWrites removeWriteAtPath:writeToRemove.path];
        } else {
            FCompoundWrite *merge = writeToRemove.merge;
            [merge enumerateWrites:^(FPath *path, id<FNode> node, BOOL *stop) {
                self.visibleWrites = [self.visibleWrites removeWriteAtPath:[writeToRemove.path child:path]];
            }];
        }
        return YES;
    }
}

- (NSArray *) removeAllWrites {
    NSArray *writes = self.allWrites;
    self.visibleWrites = [FCompoundWrite emptyWrite];
    self.allWrites = [NSMutableArray array];
    return writes;
}

/**
* @return A complete snapshot for the given path if there's visible write data at that path, else nil.
* No server data is considered.
*/
- (id <FNode>) completeWriteDataAtPath:(FPath *)path {
    return [self.visibleWrites completeNodeAtPath:path];
}

/**
* Given optional, underlying server data, and an optional set of constraints (exclude some sets, include hidden
* writes), attempt to calculate a complete snapshot for the given path
* @param includeHiddenWrites Defaults to false, whether or not to layer on writes with visible set to false
*/
- (id <FNode>) calculateCompleteEventCacheAtPath:(FPath *)treePath completeServerCache:(id <FNode>)completeServerCache
                                 excludeWriteIds:(NSArray *)writeIdsToExclude includeHiddenWrites:(BOOL)includeHiddenWrites {
    if (writeIdsToExclude == nil && !includeHiddenWrites) {
        id<FNode> shadowingNode = [self.visibleWrites completeNodeAtPath:treePath];
        if (shadowingNode != nil) {
            return shadowingNode;
        } else {
            // No cache here. Can't claim complete knowledge.
            FCompoundWrite *subMerge = [self.visibleWrites childCompoundWriteAtPath:treePath];
            if (subMerge.isEmpty) {
                return completeServerCache;
            } else if (completeServerCache == nil && ![subMerge hasCompleteWriteAtPath:[FPath empty]]) {
                // We wouldn't have a complete snapshot since there's no underlying data and no complete shadow
                return nil;
            } else {
                id<FNode> layeredCache = completeServerCache != nil ? completeServerCache : [FEmptyNode emptyNode];
                return [subMerge applyToNode:layeredCache];
            }
        }
    } else {
        FCompoundWrite *merge = [self.visibleWrites childCompoundWriteAtPath:treePath];
        if (!includeHiddenWrites && merge.isEmpty) {
            return completeServerCache;
        } else {
            // If the server cache is null and we don't have a complete cache, we need to return nil
            if (!includeHiddenWrites && completeServerCache == nil && ![merge hasCompleteWriteAtPath:[FPath empty]]) {
                return nil;
            } else {
                BOOL (^filter) (FWriteRecord *) = ^(FWriteRecord *record) {
                    return (BOOL) ((record.visible || includeHiddenWrites) &&
                            (writeIdsToExclude == nil || ![writeIdsToExclude containsObject:[NSNumber numberWithInteger:record.writeId]]) &&
                            ([record.path contains:treePath] || [treePath contains:record.path]));
                };
                FCompoundWrite *mergeAtPath = [FWriteTree layerTreeFromWrites:self.allWrites filter:filter treeRoot:treePath];
                id<FNode> layeredCache = completeServerCache ? completeServerCache : [FEmptyNode emptyNode];
                return [mergeAtPath applyToNode:layeredCache];
            }
        }
    }
}

/**
* With optional, underlying server data, attempt to return a children node of children that we have complete data for.
* Used when creating new views, to pre-fill their complete event children snapshot.
*/
- (FChildrenNode *) calculateCompleteEventChildrenAtPath:(FPath *)treePath
                                  completeServerChildren:(id<FNode>)completeServerChildren {
    __block id<FNode> completeChildren = [FEmptyNode emptyNode];
    id<FNode> topLevelSet = [self.visibleWrites completeNodeAtPath:treePath];
    if (topLevelSet != nil) {
        if (![topLevelSet isLeafNode]) {
            // We're shadowing everything. Return the children.
            FChildrenNode *topChildrenNode = topLevelSet;
            [topChildrenNode enumerateChildrenUsingBlock:^(NSString *key, id<FNode> node, BOOL *stop) {
                completeChildren = [completeChildren updateImmediateChild:key withNewChild:node];
            }];
        }
        return completeChildren;
    } else {
        // Layer any children we have on top of this
        // We know we don't have a top-level set, so just enumerate existing children, and apply any updates
        FCompoundWrite *merge = [self.visibleWrites childCompoundWriteAtPath:treePath];
        [completeServerChildren enumerateChildrenUsingBlock:^(NSString *key, id<FNode> node, BOOL *stop) {
            FCompoundWrite *childMerge = [merge childCompoundWriteAtPath:[[FPath alloc] initWith:key]];
            id<FNode> newChildNode = [childMerge applyToNode:node];
            completeChildren = [completeChildren updateImmediateChild:key withNewChild:newChildNode];
        }];
        // Add any complete children we have from the set.
        for (FNamedNode *node in merge.completeChildren) {
            completeChildren = [completeChildren updateImmediateChild:node.name withNewChild:node.node];
        }
        return completeChildren;
    }
}

/**
* Given that the underlying server data has updated, determine what, if anything, needs to be applied to the event cache.
*
* Possibilities
*
* 1. No write are shadowing. Events should be raised, the snap to be applied comes from the server data.
*
* 2. Some write is completely shadowing. No events to be raised.
*
* 3. Is partially shadowed. Events ..
*
* Either existingEventSnap or existingServerSnap must exist.
*/
- (id <FNode>) calculateEventCacheAfterServerOverwriteAtPath:(FPath *)treePath childPath:(FPath *)childPath existingEventSnap:(id <FNode>)existingEventSnap existingServerSnap:(id <FNode>)existingServerSnap {
    NSAssert(existingEventSnap != nil || existingServerSnap != nil,
            @"Either existingEventSnap or existingServerSanp must exist.");

    FPath *path = [treePath child:childPath];
    if ([self.visibleWrites hasCompleteWriteAtPath:path]) {
        // At this point we can probably guarantee that we're in case 2, meaning no events
        // May need to check visibility while doing the findRootMostValueAndPath call
        return nil;
    } else {
        // This could be more efficient if the serverNode + updates doesn't change the eventSnap
        // However this is tricky to find out, since user updates don't necessary change the server
        // snap, e.g. priority updates on empty nodes, or deep deletes. Another special case is if the server
        // adds nodes, but doesn't change any existing writes. It is therefore not enough to
        // only check if the updates change the serverNode.
        // Maybe check if the merge tree contains these special cases and only do a full overwrite in that case?
        FCompoundWrite *childMerge = [self.visibleWrites childCompoundWriteAtPath:path];
        if (childMerge.isEmpty) {
            // We're not shadowing at all. Case 1
            return [existingServerSnap getChild:childPath];
        } else {
            return [childMerge applyToNode:[existingServerSnap getChild:childPath]];
        }
    }
}

/**
* Returns a complete child for a given server snap after applying all user writes or nil if there is no complete child
* for this child key.
*/
- (id<FNode>) calculateCompleteChildAtPath:(FPath *)treePath childKey:(NSString *)childKey cache:(FCacheNode *)existingServerCache {
    FPath *path = [treePath childFromString:childKey];
    id<FNode> shadowingNode = [self.visibleWrites completeNodeAtPath:path];
    if (shadowingNode != nil) {
        return shadowingNode;
    } else {
        if ([existingServerCache isCompleteForChild:childKey]) {
            FCompoundWrite *childMerge = [self.visibleWrites childCompoundWriteAtPath:path];
            return [childMerge applyToNode:[existingServerCache.node getImmediateChild:childKey]];
        } else {
            return nil;
        }
    }
}

/**
* Returns a node if there is a complete overwrite for this path. More specifically, if there is a write at
* a higher path, this will return the child of that write relative to the write and this path.
* Returns null if there is no write at this path.
*/
- (id<FNode>) shadowingWriteAtPath:(FPath *)path {
    return [self.visibleWrites completeNodeAtPath:path];
}

/**
* This method is used when processing child remove events on a query. If we can, we pull in children that were outside
* the window, but may now be in the window.
*/
- (FNamedNode *)calculateNextNodeAfterPost:(FNamedNode *)post
                                    atPath:(FPath *)treePath
                        completeServerData:(id<FNode>)completeServerData
                                   reverse:(BOOL)reverse
                                     index:(id<FIndex>)index
{
    __block id<FNode> toIterate;
    FCompoundWrite *merge = [self.visibleWrites childCompoundWriteAtPath:treePath];
    id<FNode> shadowingNode = [merge completeNodeAtPath:[FPath empty]];
    if (shadowingNode != nil) {
        toIterate = shadowingNode;
    } else if (completeServerData != nil) {
        toIterate = [merge applyToNode:completeServerData];
    } else {
        return nil;
    }

    __block NSString *currentNextKey = nil;
    __block id<FNode> currentNextNode = nil;
    [toIterate enumerateChildrenUsingBlock:^(NSString *key, id<FNode> node, BOOL *stop) {
        if ([index compareKey:key andNode:node toOtherKey:post.name andNode:post.node reverse:reverse] > NSOrderedSame &&
            (!currentNextKey || [index compareKey:key andNode:node toOtherKey:currentNextKey andNode:currentNextNode reverse:reverse] < NSOrderedSame)) {
            currentNextKey = key;
            currentNextNode = node;
        }
    }];

    if (currentNextKey != nil) {
        return [FNamedNode nodeWithName:currentNextKey node:currentNextNode];
    } else {
        return nil;
    }
}

#pragma mark -
#pragma mark Private Methods

- (BOOL) record:(FWriteRecord *)record containsPath:(FPath *)path {
    if ([record isOverwrite]) {
        return [record.path contains:path];
    } else {
        __block BOOL contains = NO;
        [record.merge enumerateWrites:^(FPath *childPath, id<FNode> node, BOOL *stop) {
            contains = [[record.path child:childPath] contains:path];
            *stop = contains;
        }];
        return contains;
    }
}

/**
* Re-layer the writes and merges into a tree so we can efficiently calculate event snapshots
*/
- (void) resetTree {
    self.visibleWrites = [FWriteTree layerTreeFromWrites:self.allWrites filter:[FWriteTree defaultFilter] treeRoot:[FPath empty]];
    if ([self.allWrites count] > 0) {
        FWriteRecord *lastRecord = self.allWrites[[self.allWrites count] - 1];
        self.lastWriteId = lastRecord.writeId;
    } else {
        self.lastWriteId = -1;
    }
}

/**
* The default filter used when constructing the tree. Keep everything that's visible.
*/
+ (BOOL (^)(FWriteRecord *record)) defaultFilter {
    static BOOL (^filter)(FWriteRecord *);
    static dispatch_once_t filterToken;
    dispatch_once(&filterToken, ^{
        filter = ^(FWriteRecord *record) {
            return YES;
        };
    });
    return filter;
}

/**
* Static method. Given an array of WriteRecords, a filter for which ones to include, and a path, construct a merge
* at that path
* @return An FImmutableTree of id<FNode>s.
*/
+ (FCompoundWrite *) layerTreeFromWrites:(NSArray *)writes filter:(BOOL (^)(FWriteRecord *record))filter treeRoot:(FPath *)treeRoot {
    __block FCompoundWrite *compoundWrite = [FCompoundWrite emptyWrite];
    [writes enumerateObjectsUsingBlock:^(FWriteRecord *record, NSUInteger idx, BOOL *stop) {
        // Theory, a later set will either:
        // a) abort a relevant transaction, so no need to worry about excluding it from calculating that transaction
        // b) not be relevant to a transaction (separate branch), so again will not affect the data for that transaction
        if (filter(record)) {
            FPath *writePath = record.path;
            if ([record isOverwrite]) {
                if ([treeRoot contains:writePath]) {
                    FPath *relativePath = [FPath relativePathFrom:treeRoot to:writePath];
                    compoundWrite = [compoundWrite addWrite:record.overwrite atPath:relativePath];
                } else if ([writePath contains:treeRoot]) {
                    id<FNode> child = [record.overwrite getChild:[FPath relativePathFrom:writePath to:treeRoot]];
                    compoundWrite = [compoundWrite addWrite:child atPath:[FPath empty]];
                } else {
                    // There is no overlap between root path and write path, ignore write
                }
            } else {
                if ([treeRoot contains:writePath]) {
                    FPath *relativePath = [FPath relativePathFrom:treeRoot to:writePath];
                    compoundWrite = [compoundWrite addCompoundWrite:record.merge atPath:relativePath];
                } else if ([writePath contains:treeRoot]) {
                    FPath *relativePath = [FPath relativePathFrom:writePath to:treeRoot];
                    if (relativePath.isEmpty) {
                        compoundWrite = [compoundWrite addCompoundWrite:record.merge atPath:[FPath empty]];
                    } else {
                        id<FNode> child = [record.merge completeNodeAtPath:relativePath];
                        if (child != nil) {
                            // There exists a child in this node that matches the root path
                            id<FNode> deepNode = [child getChild:[relativePath popFront]];
                            compoundWrite = [compoundWrite addWrite:deepNode atPath:[FPath empty]];
                        }
                    }
                } else {
                    // There is no overlap between root path and write path, ignore write
                }
            }
        }
    }];
    return compoundWrite;
}

@end