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diff --git a/dom/localstorage/LSWriteOptimizer.h b/dom/localstorage/LSWriteOptimizer.h
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+++ b/dom/localstorage/LSWriteOptimizer.h
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+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_dom_localstorage_LSWriteOptimizer_h
+#define mozilla_dom_localstorage_LSWriteOptimizer_h
+
+#include <cstdint>
+#include <utility>
+#include "mozilla/Assertions.h"
+#include "mozilla/CheckedInt.h"
+#include "mozilla/UniquePtr.h"
+#include "nsClassHashtable.h"
+#include "nsHashKeys.h"
+#include "nsISupports.h"
+#include "nsStringFwd.h"
+#include "nsTArrayForwardDeclare.h"
+
+namespace mozilla {
+namespace dom {
+
+/**
+ * Base class for coalescing manipulation queue.
+ */
+class LSWriteOptimizerBase {
+  class WriteInfoComparator;
+
+ protected:
+  class WriteInfo;
+  class DeleteItemInfo;
+  class TruncateInfo;
+
+  UniquePtr<WriteInfo> mTruncateInfo;
+  nsClassHashtable<nsStringHashKey, WriteInfo> mWriteInfos;
+  CheckedUint64 mLastSerialNumber;
+  int64_t mTotalDelta;
+
+  NS_DECL_OWNINGTHREAD
+
+ public:
+  LSWriteOptimizerBase() : mLastSerialNumber(0), mTotalDelta(0) {}
+
+  LSWriteOptimizerBase(LSWriteOptimizerBase&& aWriteOptimizer)
+      : mTruncateInfo(std::move(aWriteOptimizer.mTruncateInfo)) {
+    AssertIsOnOwningThread();
+    MOZ_ASSERT(&aWriteOptimizer != this);
+
+    mWriteInfos.SwapElements(aWriteOptimizer.mWriteInfos);
+    mTotalDelta = aWriteOptimizer.mTotalDelta;
+    aWriteOptimizer.mTotalDelta = 0;
+  }
+
+  void AssertIsOnOwningThread() const {
+    NS_ASSERT_OWNINGTHREAD(LSWriteOptimizerBase);
+  }
+
+  void DeleteItem(const nsAString& aKey, int64_t aDelta = 0);
+
+  void Truncate(int64_t aDelta = 0);
+
+  bool HasWrites() const {
+    AssertIsOnOwningThread();
+
+    return mTruncateInfo || !mWriteInfos.IsEmpty();
+  }
+
+  void Reset() {
+    AssertIsOnOwningThread();
+
+    mTruncateInfo = nullptr;
+    mWriteInfos.Clear();
+  }
+
+ protected:
+  uint64_t NextSerialNumber() {
+    AssertIsOnOwningThread();
+
+    mLastSerialNumber++;
+
+    MOZ_ASSERT(mLastSerialNumber.isValid());
+
+    return mLastSerialNumber.value();
+  }
+
+  /**
+   * This method can be used by derived classes to get a sorted list of write
+   * infos. Write infos are sorted by the serial number.
+   */
+  void GetSortedWriteInfos(nsTArray<NotNull<WriteInfo*>>& aWriteInfos);
+};
+
+/**
+ * Base class for specific mutations.
+ */
+class LSWriteOptimizerBase::WriteInfo {
+  uint64_t mSerialNumber;
+
+ public:
+  WriteInfo(uint64_t aSerialNumber) : mSerialNumber(aSerialNumber) {}
+
+  virtual ~WriteInfo() = default;
+
+  uint64_t SerialNumber() const { return mSerialNumber; }
+
+  enum Type { InsertItem = 0, UpdateItem, DeleteItem, Truncate };
+
+  virtual Type GetType() const = 0;
+};
+
+class LSWriteOptimizerBase::DeleteItemInfo final : public WriteInfo {
+  nsString mKey;
+
+ public:
+  DeleteItemInfo(uint64_t aSerialNumber, const nsAString& aKey)
+      : WriteInfo(aSerialNumber), mKey(aKey) {}
+
+  const nsAString& GetKey() const { return mKey; }
+
+ private:
+  Type GetType() const override { return DeleteItem; }
+};
+
+/**
+ * Truncate mutation.
+ */
+class LSWriteOptimizerBase::TruncateInfo final : public WriteInfo {
+ public:
+  explicit TruncateInfo(uint64_t aSerialNumber) : WriteInfo(aSerialNumber) {}
+
+ private:
+  Type GetType() const override { return Truncate; }
+};
+
+/**
+ * Coalescing manipulation queue.
+ */
+template <typename T, typename U = T>
+class LSWriteOptimizer;
+
+template <typename T, typename U>
+class LSWriteOptimizer : public LSWriteOptimizerBase {
+ protected:
+  class InsertItemInfo;
+  class UpdateItemInfo;
+
+ public:
+  void InsertItem(const nsAString& aKey, const T& aValue, int64_t aDelta = 0);
+
+  void UpdateItem(const nsAString& aKey, const T& aValue, int64_t aDelta = 0);
+};
+
+/**
+ * Insert mutation (the key did not previously exist).
+ */
+template <typename T, typename U>
+class LSWriteOptimizer<T, U>::InsertItemInfo : public WriteInfo {
+  nsString mKey;
+  U mValue;
+
+ public:
+  InsertItemInfo(uint64_t aSerialNumber, const nsAString& aKey, const T& aValue)
+      : WriteInfo(aSerialNumber), mKey(aKey), mValue(aValue) {}
+
+  const nsAString& GetKey() const { return mKey; }
+
+  const T& GetValue() const { return mValue; }
+
+ private:
+  WriteInfo::Type GetType() const override { return InsertItem; }
+};
+
+/**
+ * Update mutation (the key already existed).
+ */
+template <typename T, typename U>
+class LSWriteOptimizer<T, U>::UpdateItemInfo final : public InsertItemInfo {
+  bool mUpdateWithMove;
+
+ public:
+  UpdateItemInfo(uint64_t aSerialNumber, const nsAString& aKey, const T& aValue,
+                 bool aUpdateWithMove)
+      : InsertItemInfo(aSerialNumber, aKey, aValue),
+        mUpdateWithMove(aUpdateWithMove) {}
+
+  bool UpdateWithMove() const { return mUpdateWithMove; }
+
+ private:
+  WriteInfo::Type GetType() const override { return WriteInfo::UpdateItem; }
+};
+
+}  // namespace dom
+}  // namespace mozilla
+
+#endif  // mozilla_dom_localstorage_LSWriteOptimizer_h