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diff --git a/Firestore/Source/Local/FSTEagerGarbageCollector.mm b/Firestore/Source/Local/FSTEagerGarbageCollector.mm
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+/*
+ * 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 "Firestore/Source/Local/FSTEagerGarbageCollector.h"
+
+#import "Firestore/Source/Model/FSTDocumentKey.h"
+
+NS_ASSUME_NONNULL_BEGIN
+
+#pragma mark - FSTMultiReferenceSet
+
+@interface FSTEagerGarbageCollector ()
+
+/** The garbage collectible sources to double-check during garbage collection. */
+@property(nonatomic, strong, readonly) NSMutableArray<id<FSTGarbageSource>> *sources;
+
+/** A set of potentially garbage keys. */
+@property(nonatomic, strong, readonly) NSMutableSet<FSTDocumentKey *> *potentialGarbage;
+
+@end
+
+@implementation FSTEagerGarbageCollector
+
+- (instancetype)init {
+  self = [super init];
+  if (self) {
+    _sources = [NSMutableArray array];
+    _potentialGarbage = [[NSMutableSet alloc] init];
+  }
+  return self;
+}
+
+- (BOOL)isEager {
+  return YES;
+}
+
+- (void)addGarbageSource:(id<FSTGarbageSource>)garbageSource {
+  [self.sources addObject:garbageSource];
+  garbageSource.garbageCollector = self;
+}
+
+- (void)removeGarbageSource:(id<FSTGarbageSource>)garbageSource {
+  [self.sources removeObject:garbageSource];
+  garbageSource.garbageCollector = nil;
+}
+
+- (void)addPotentialGarbageKey:(FSTDocumentKey *)key {
+  [self.potentialGarbage addObject:key];
+}
+
+- (NSMutableSet<FSTDocumentKey *> *)collectGarbage {
+  NSMutableArray<id<FSTGarbageSource>> *sources = self.sources;
+
+  NSMutableSet<FSTDocumentKey *> *actualGarbage = [NSMutableSet set];
+  for (FSTDocumentKey *key in self.potentialGarbage) {
+    BOOL isGarbage = YES;
+    for (id<FSTGarbageSource> source in sources) {
+      if ([source containsKey:key]) {
+        isGarbage = NO;
+        break;
+      }
+    }
+
+    if (isGarbage) {
+      [actualGarbage addObject:key];
+    }
+  }
+
+  // Clear locally retained potential keys and returned confirmed garbage.
+  [self.potentialGarbage removeAllObjects];
+  return actualGarbage;
+}
+
+@end
+
+NS_ASSUME_NONNULL_END