1 files changed, 116 insertions, 0 deletions
diff --git a/dom/ipc/StringTable.h b/dom/ipc/StringTable.h
new file mode 100644
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+++ b/dom/ipc/StringTable.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 dom_ipc_StringTable_h
+#define dom_ipc_StringTable_h
+
+#include "mozilla/RangedPtr.h"
+#include "nsTHashMap.h"
+
+/**
+ * This file contains helper classes for creating and accessing compact string
+ * tables, which can be used as the building blocks of shared memory databases.
+ * Each string table a de-duplicated set of strings which can be referenced
+ * using their character offsets within a data block and their lengths. The
+ * string tables, once created, cannot be modified, and are primarily useful in
+ * read-only shared memory or memory mapped files.
+ */
+
+namespace mozilla::dom::ipc {
+
+/**
+ * Contains the character offset and character length of an entry in a string
+ * table. This may be used for either 8-bit or 16-bit strings, and is required
+ * to retrieve an entry from a string table.
+ */
+struct StringTableEntry {
+  uint32_t mOffset;
+  uint32_t mLength;
+
+  // Ignore mLength. It must be the same for any two strings with the same
+  // offset.
+  uint32_t Hash() const { return mOffset; }
+
+  bool operator==(const StringTableEntry& aOther) const {
+    return mOffset == aOther.mOffset;
+  }
+};
+
+template <typename StringType>
+class StringTable {
+  using ElemType = typename StringType::char_type;
+
+ public:
+  MOZ_IMPLICIT StringTable(const RangedPtr<uint8_t>& aBuffer)
+      : mBuffer(aBuffer.ReinterpretCast<ElemType>()) {
+    MOZ_ASSERT(uintptr_t(aBuffer.get()) % alignof(ElemType) == 0,
+               "Got misalinged buffer");
+  }
+
+  StringType Get(const StringTableEntry& aEntry) const {
+    StringType res;
+    res.AssignLiteral(GetBare(aEntry), aEntry.mLength);
+    return res;
+  }
+
+  const ElemType* GetBare(const StringTableEntry& aEntry) const {
+    return &mBuffer[aEntry.mOffset];
+  }
+
+ private:
+  RangedPtr<ElemType> mBuffer;
+};
+
+template <typename KeyType, typename StringType>
+class StringTableBuilder {
+ public:
+  using ElemType = typename StringType::char_type;
+
+  StringTableEntry Add(const StringType& aKey) {
+    return mEntries.WithEntryHandle(aKey,
+                                    [&](auto&& entry) -> StringTableEntry {
+                                      auto length = uint32_t(aKey.Length());
+                                      entry.OrInsertWith([&]() {
+                                        Entry newEntry{mSize, aKey};
+                                        mSize += length + 1;
+
+                                        return newEntry;
+                                      });
+
+                                      return {entry->mOffset, length};
+                                    });
+  }
+
+  void Write(const RangedPtr<uint8_t>& aBuffer) {
+    auto buffer = aBuffer.ReinterpretCast<ElemType>();
+
+    for (const auto& entry : mEntries.Values()) {
+      memcpy(&buffer[entry.mOffset], entry.mValue.BeginReading(),
+             sizeof(ElemType) * (entry.mValue.Length() + 1));
+    }
+  }
+
+  uint32_t Count() const { return mEntries.Count(); }
+
+  uint32_t Size() const { return mSize * sizeof(ElemType); }
+
+  void Clear() { mEntries.Clear(); }
+
+  static constexpr size_t Alignment() { return alignof(ElemType); }
+
+ private:
+  struct Entry {
+    uint32_t mOffset;
+    StringType mValue;
+  };
+
+  nsTHashMap<KeyType, Entry> mEntries;
+  uint32_t mSize = 0;
+};
+
+}  // namespace mozilla::dom::ipc
+
+#endif