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diff --git a/Firebase/Database/FPriorityIndex.m b/Firebase/Database/FPriorityIndex.m
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+++ b/Firebase/Database/FPriorityIndex.m
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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 "FPriorityIndex.h"
+
+#import "FNode.h"
+#import "FUtilities.h"
+#import "FNamedNode.h"
+#import "FEmptyNode.h"
+#import "FLeafNode.h"
+#import "FMaxNode.h"
+
+// TODO: Abstract into some common base class?
+
+@implementation FPriorityIndex
+
+- (NSComparisonResult) compareKey:(NSString *)key1
+                          andNode:(id<FNode>)node1
+                       toOtherKey:(NSString *)key2
+                          andNode:(id<FNode>)node2
+{
+    id<FNode> child1 = [node1 getPriority];
+    id<FNode> child2 = [node2 getPriority];
+    NSComparisonResult indexCmp = [child1 compare:child2];
+    if (indexCmp == NSOrderedSame) {
+        return [FUtilities compareKey:key1 toKey:key2];
+    } else {
+        return indexCmp;
+    }
+}
+
+- (NSComparisonResult) compareKey:(NSString *)key1
+                          andNode:(id<FNode>)node1
+                       toOtherKey:(NSString *)key2
+                          andNode:(id<FNode>)node2
+                          reverse:(BOOL)reverse
+{
+    if (reverse) {
+        return [self compareKey:key2 andNode:node2 toOtherKey:key1 andNode:node1];
+    } else {
+        return [self compareKey:key1 andNode:node1 toOtherKey:key2 andNode:node2];
+    }
+}
+
+- (NSComparisonResult) compareNamedNode:(FNamedNode *)namedNode1 toNamedNode:(FNamedNode *)namedNode2
+{
+    return [self compareKey:namedNode1.name andNode:namedNode1.node toOtherKey:namedNode2.name andNode:namedNode2.node];
+}
+
+- (BOOL)isDefinedOn:(id <FNode>)node {
+    return !node.getPriority.isEmpty;
+}
+
+- (BOOL)indexedValueChangedBetween:(id <FNode>)oldNode and:(id <FNode>)newNode {
+    id<FNode> oldValue = [oldNode getPriority];
+    id<FNode> newValue = [newNode getPriority];
+    return ![oldValue isEqual:newValue];
+}
+
+- (FNamedNode *)minPost {
+    return FNamedNode.min;
+}
+
+- (FNamedNode *)maxPost {
+    return [self makePost:[FMaxNode maxNode] name:[FUtilities maxName]];
+}
+
+- (FNamedNode*)makePost:(id<FNode>)indexValue name:(NSString*)name {
+    id<FNode> node = [[FLeafNode alloc] initWithValue:@"[PRIORITY-POST]" withPriority:indexValue];
+    return [[FNamedNode alloc] initWithName:name andNode:node];
+}
+
+- (NSString *)queryDefinition {
+    return @".priority";
+}
+
+- (NSString *)description {
+    return @"FPriorityIndex";
+}
+
+- (id)copyWithZone:(NSZone *)zone {
+    // Safe since we're immutable.
+    return self;
+}
+
+- (BOOL) isEqual:(id)other {
+    return [other isKindOfClass:[FPriorityIndex class]];
+}
+
+- (NSUInteger) hash {
+    // chosen by a fair dice roll. Guaranteed to be random
+    return 3155577;
+}
+
++ (id<FIndex>) priorityIndex {
+    static id<FIndex> index;
+    static dispatch_once_t once;
+    dispatch_once(&once, ^{
+        index = [[FPriorityIndex alloc] init];
+    });
+
+    return index;
+}
+
+@end