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StringTable.h (3585B)

      1 /* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
      2 /* vim: set ts=8 sts=2 et sw=2 tw=80: */
      3 /* This Source Code Form is subject to the terms of the Mozilla Public
      4 * License, v. 2.0. If a copy of the MPL was not distributed with this
      5 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
      6 
      7 #ifndef dom_ipc_StringTable_h
      8 #define dom_ipc_StringTable_h
      9 
     10 #include "mozilla/RangedPtr.h"
     11 #include "nsTHashMap.h"
     12 
     13 /**
     14 * This file contains helper classes for creating and accessing compact string
     15 * tables, which can be used as the building blocks of shared memory databases.
     16 * Each string table a de-duplicated set of strings which can be referenced
     17 * using their character offsets within a data block and their lengths. The
     18 * string tables, once created, cannot be modified, and are primarily useful in
     19 * read-only shared memory or memory mapped files.
     20 */
     21 
     22 namespace mozilla::dom::ipc {
     23 
     24 /**
     25 * Contains the character offset and character length of an entry in a string
     26 * table. This may be used for either 8-bit or 16-bit strings, and is required
     27 * to retrieve an entry from a string table.
     28 */
     29 struct StringTableEntry {
     30  uint32_t mOffset;
     31  uint32_t mLength;
     32 
     33  // Ignore mLength. It must be the same for any two strings with the same
     34  // offset.
     35  uint32_t Hash() const { return mOffset; }
     36 
     37  bool operator==(const StringTableEntry& aOther) const {
     38    return mOffset == aOther.mOffset;
     39  }
     40 };
     41 
     42 template <typename StringType>
     43 class StringTable {
     44  using ElemType = typename StringType::char_type;
     45 
     46 public:
     47  MOZ_IMPLICIT StringTable(const RangedPtr<uint8_t>& aBuffer)
     48      : mBuffer(aBuffer.ReinterpretCast<ElemType>()) {
     49    MOZ_ASSERT(uintptr_t(aBuffer.get()) % alignof(ElemType) == 0,
     50               "Got misalinged buffer");
     51  }
     52 
     53  StringType Get(const StringTableEntry& aEntry) const {
     54    StringType res;
     55    res.AssignLiteral(GetBare(aEntry), aEntry.mLength);
     56    return res;
     57  }
     58 
     59  const ElemType* GetBare(const StringTableEntry& aEntry) const {
     60    return &mBuffer[aEntry.mOffset];
     61  }
     62 
     63 private:
     64  RangedPtr<ElemType> mBuffer;
     65 };
     66 
     67 template <typename KeyType, typename StringType>
     68 class StringTableBuilder {
     69 public:
     70  using ElemType = typename StringType::char_type;
     71 
     72  StringTableEntry Add(const StringType& aKey) {
     73    return mEntries.WithEntryHandle(aKey,
     74                                    [&](auto&& entry) -> StringTableEntry {
     75                                      auto length = uint32_t(aKey.Length());
     76                                      entry.OrInsertWith([&]() {
     77                                        Entry newEntry{mSize, aKey};
     78                                        mSize += length + 1;
     79 
     80                                        return newEntry;
     81                                      });
     82 
     83                                      return {entry->mOffset, length};
     84                                    });
     85  }
     86 
     87  void Write(const RangedPtr<uint8_t>& aBuffer) {
     88    auto buffer = aBuffer.ReinterpretCast<ElemType>();
     89 
     90    for (const auto& entry : mEntries.Values()) {
     91      memcpy(&buffer[entry.mOffset], entry.mValue.BeginReading(),
     92             sizeof(ElemType) * (entry.mValue.Length() + 1));
     93    }
     94  }
     95 
     96  uint32_t Count() const { return mEntries.Count(); }
     97 
     98  uint32_t Size() const { return mSize * sizeof(ElemType); }
     99 
    100  void Clear() { mEntries.Clear(); }
    101 
    102  static constexpr size_t Alignment() { return alignof(ElemType); }
    103 
    104 private:
    105  struct Entry {
    106    uint32_t mOffset;
    107    StringType mValue;
    108  };
    109 
    110  nsTHashMap<KeyType, Entry> mEntries;
    111  uint32_t mSize = 0;
    112 };
    113 
    114 }  // namespace mozilla::dom::ipc
    115 
    116 #endif