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117 lines
3.5 KiB
C++
117 lines
3.5 KiB
C++
/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
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/* vim: set ts=8 sts=2 et sw=2 tw=80: */
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/* This Source Code Form is subject to the terms of the Mozilla Public
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* License, v. 2.0. If a copy of the MPL was not distributed with this
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* file, You can obtain one at http://mozilla.org/MPL/2.0/. */
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#ifndef dom_ipc_StringTable_h
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#define dom_ipc_StringTable_h
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#include "mozilla/RangedPtr.h"
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#include "nsTHashMap.h"
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/**
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* This file contains helper classes for creating and accessing compact string
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* tables, which can be used as the building blocks of shared memory databases.
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* Each string table a de-duplicated set of strings which can be referenced
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* using their character offsets within a data block and their lengths. The
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* string tables, once created, cannot be modified, and are primarily useful in
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* read-only shared memory or memory mapped files.
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*/
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namespace mozilla::dom::ipc {
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/**
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* Contains the character offset and character length of an entry in a string
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* table. This may be used for either 8-bit or 16-bit strings, and is required
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* to retrieve an entry from a string table.
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*/
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struct StringTableEntry {
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uint32_t mOffset;
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uint32_t mLength;
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// Ignore mLength. It must be the same for any two strings with the same
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// offset.
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uint32_t Hash() const { return mOffset; }
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bool operator==(const StringTableEntry& aOther) const {
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return mOffset == aOther.mOffset;
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}
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};
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template <typename StringType>
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class StringTable {
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using ElemType = typename StringType::char_type;
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public:
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MOZ_IMPLICIT StringTable(const RangedPtr<uint8_t>& aBuffer)
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: mBuffer(aBuffer.ReinterpretCast<ElemType>()) {
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MOZ_ASSERT(uintptr_t(aBuffer.get()) % alignof(ElemType) == 0,
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"Got misalinged buffer");
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}
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StringType Get(const StringTableEntry& aEntry) const {
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StringType res;
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res.AssignLiteral(GetBare(aEntry), aEntry.mLength);
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return res;
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}
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const ElemType* GetBare(const StringTableEntry& aEntry) const {
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return &mBuffer[aEntry.mOffset];
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}
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private:
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RangedPtr<ElemType> mBuffer;
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};
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template <typename KeyType, typename StringType>
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class StringTableBuilder {
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public:
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using ElemType = typename StringType::char_type;
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StringTableEntry Add(const StringType& aKey) {
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return mEntries.WithEntryHandle(aKey,
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[&](auto&& entry) -> StringTableEntry {
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auto length = uint32_t(aKey.Length());
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entry.OrInsertWith([&]() {
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Entry newEntry{mSize, aKey};
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mSize += length + 1;
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return newEntry;
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});
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return {entry->mOffset, length};
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});
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}
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void Write(const RangedPtr<uint8_t>& aBuffer) {
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auto buffer = aBuffer.ReinterpretCast<ElemType>();
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for (const auto& entry : mEntries.Values()) {
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memcpy(&buffer[entry.mOffset], entry.mValue.BeginReading(),
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sizeof(ElemType) * (entry.mValue.Length() + 1));
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}
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}
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uint32_t Count() const { return mEntries.Count(); }
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uint32_t Size() const { return mSize * sizeof(ElemType); }
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void Clear() { mEntries.Clear(); }
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static constexpr size_t Alignment() { return alignof(ElemType); }
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private:
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struct Entry {
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uint32_t mOffset;
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StringType mValue;
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};
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nsTHashMap<KeyType, Entry> mEntries;
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uint32_t mSize = 0;
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};
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} // namespace mozilla::dom::ipc
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#endif
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