[DOMStorage] Add reverse iteration optimization to blink::StorageAreaMap.

This makes the performance of blink::StorageAreaMap closer to that of
content::DOMStorageMap. In particular this makes any kind of reverse
iteration over a storage area less inefficient (not sure why people
would do so though).

Bug: 781870
Change-Id: I6f313c65868a21de8b0d2fbdff526a23895baa57
Reviewed-on: https://chromium-review.googlesource.com/1117206Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Daniel Murphy <dmurph@chromium.org>
Commit-Queue: Marijn Kruisselbrink <mek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#571017}

third_party/blink/renderer/modules/storage/storage_area_map.cc

@@ -27,15 +27,30 @@ String StorageAreaMap::GetKey(unsigned index) const {
   if (index >= GetLength())
     return String();
 
-  // TODO(mek): Figure out if we can support backwards iteration somewhat
-  // efficiently as well.
-  if (last_key_index_ > index) {
-    ResetKeyIterator();
+  // Decide if we should leave |key_iterator_| alone, or reset to either the
+  // beginning or end of the map for shortest iteration distance.
+  const unsigned distance_to_current = index > last_key_index_
+                                           ? index - last_key_index_
+                                           : last_key_index_ - index;
+  const unsigned distance_to_end = GetLength() - index;
+  if (index < distance_to_current && index < distance_to_end) {
+    // Distance from start is shortest, so reset iterator to begin.
+    last_key_index_ = 0;
+    key_iterator_ = keys_values_.begin();
+  } else if (distance_to_end < distance_to_current && distance_to_end < index) {
+    // Distance from end is shortest, so reset iterator to end.
+    last_key_index_ = GetLength();
+    key_iterator_ = keys_values_.end();
   }
-  while (last_key_index_ != index) {
+
+  while (last_key_index_ < index) {
     ++key_iterator_;
     ++last_key_index_;
   }
+  while (last_key_index_ > index) {
+    --key_iterator_;
+    --last_key_index_;
+  }
   return key_iterator_->key;
 }
 

third_party/blink/renderer/modules/storage/storage_area_map_test.cc

@@ -100,4 +100,33 @@ TEST(StorageAreaMapTest, EnforcesQuota) {
   EXPECT_EQ(kValue, map.GetItem(kKey2));
 }
 
+TEST(StorageAreaMapTest, Iteration) {
+  const int kNumTestItems = 100;
+  const size_t kQuota = 102400;  // 100K quota for this test.
+  StorageAreaMap map(kQuota);
+
+  // Fill the map with some data.
+  for (int i = 0; i < kNumTestItems; ++i)
+    EXPECT_TRUE(map.SetItem("key" + String::Number(i), "val", nullptr));
+  EXPECT_EQ(unsigned{kNumTestItems}, map.GetLength());
+
+  Vector<String> keys(kNumTestItems);
+  // Iterate over all keys.
+  for (int i = 0; i < kNumTestItems; ++i)
+    keys[i] = map.GetKey(i);
+
+  // Now iterate over some subsets, and make sure the right keys are returned.
+  for (int i = 5; i < 15; ++i)
+    EXPECT_EQ(keys[i], map.GetKey(i));
+  for (int i = kNumTestItems - 5; i >= kNumTestItems - 15; --i)
+    EXPECT_EQ(keys[i], map.GetKey(i));
+  for (int i = 20; i >= 10; --i)
+    EXPECT_EQ(keys[i], map.GetKey(i));
+  for (int i = 15; i < 20; ++i)
+    EXPECT_EQ(keys[i], map.GetKey(i));
+  for (int i = kNumTestItems - 1; i >= 0; --i)
+    EXPECT_EQ(keys[i], map.GetKey(i));
+  EXPECT_TRUE(map.GetKey(kNumTestItems).IsNull());
+}
+
 }  // namespace blink