dom/ipc/StringTable.h

   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