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diff --git a/dom/ipc/SharedStringMap.h b/dom/ipc/SharedStringMap.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_SharedStringMap_h
+#define dom_ipc_SharedStringMap_h
+
+#include "mozilla/AutoMemMap.h"
+#include "mozilla/Result.h"
+#include "mozilla/dom/ipc/StringTable.h"
+#include "nsTHashMap.h"
+
+namespace mozilla::dom::ipc {
+
+class SharedStringMapBuilder;
+
+/**
+ * This class provides a simple, read-only key-value string store, with all
+ * data packed into a single segment of memory, which can be shared between
+ * processes.
+ *
+ * Look-ups are performed by binary search of a static table in the mapped
+ * memory region, and all returned strings are literals which reference the
+ * mapped data. No copies are performed on instantiation or look-up.
+ *
+ * Important: The mapped memory created by this class is persistent. Once an
+ * instance has been initialized, the memory that it allocates can never be
+ * freed before process shutdown. Do not use it for short-lived mappings.
+ */
+class SharedStringMap {
+  using FileDescriptor = mozilla::ipc::FileDescriptor;
+
+ public:
+  /**
+   * The header at the beginning of the shared memory region describing its
+   * layout. The layout of the shared memory is as follows:
+   *
+   * - Header:
+   *   A Header struct describing the contents of the rest of the memory region.
+   *
+   * - Optional alignment padding for Header[].
+   *
+   * - Entry[header.mEntryCount]:
+   *   An array of Entry structs, one for each entry in the map. Entries are
+   *   lexocographically sorted by key.
+   *
+   * - StringTable<nsCString>:
+   *   A region of flat, null-terminated C strings. Entry key strings are
+   *   encoded as character offsets into this region.
+   *
+   * - Optional alignment padding for char16_t[]
+   *
+   * - StringTable<nsString>:
+   *   A region of flat, null-terminated UTF-16 strings. Entry value strings are
+   *   encoded as character (*not* byte) offsets into this region.
+   */
+  struct Header {
+    uint32_t mMagic;
+    // The number of entries in this map.
+    uint32_t mEntryCount;
+
+    // The raw byte offset of the beginning of the key string table, from the
+    // start of the shared memory region, and its size in bytes.
+    size_t mKeyStringsOffset;
+    size_t mKeyStringsSize;
+
+    // The raw byte offset of the beginning of the value string table, from the
+    // start of the shared memory region, and its size in bytes (*not*
+    // characters).
+    size_t mValueStringsOffset;
+    size_t mValueStringsSize;
+  };
+
+  /**
+   * Describes a value in the string map, as offsets into the key and value
+   * string tables.
+   */
+  struct Entry {
+    // The offset and size of the entry's UTF-8 key in the key string table.
+    StringTableEntry mKey;
+    // The offset and size of the entry's UTF-16 value in the value string
+    // table.
+    StringTableEntry mValue;
+  };
+
+  NS_INLINE_DECL_REFCOUNTING(SharedStringMap)
+
+  // Note: These constructors are infallible on the premise that this class
+  // is used primarily in cases where it is critical to platform
+  // functionality.
+  explicit SharedStringMap(const FileDescriptor&, size_t);
+  explicit SharedStringMap(SharedStringMapBuilder&&);
+
+  /**
+   * Searches for the given value in the map, and returns true if it exists.
+   */
+  bool Has(const nsCString& aKey);
+
+  /**
+   * Searches for the given value in the map, and, if it exists, returns true
+   * and places its value in aValue.
+   *
+   * The returned value is a literal string which references the mapped memory
+   * region.
+   */
+  bool Get(const nsCString& aKey, nsAString& aValue);
+
+ private:
+  /**
+   * Searches for an entry for the given key. If found, returns true, and
+   * places its index in the entry array in aIndex.
+   */
+  bool Find(const nsCString& aKey, size_t* aIndex);
+
+ public:
+  /**
+   * Returns the number of entries in the map.
+   */
+  uint32_t Count() const { return EntryCount(); }
+
+  /**
+   * Returns the string entry at the given index. Keys are guaranteed to be
+   * sorted lexographically.
+   *
+   * The given index *must* be less than the value returned by Count().
+   *
+   * The returned value is a literal string which references the mapped memory
+   * region.
+   */
+  nsCString GetKeyAt(uint32_t aIndex) const {
+    MOZ_ASSERT(aIndex < Count());
+    return KeyTable().Get(Entries()[aIndex].mKey);
+  }
+
+  /**
+   * Returns the string value for the entry at the given index.
+   *
+   * The given index *must* be less than the value returned by Count().
+   *
+   * The returned value is a literal string which references the mapped memory
+   * region.
+   */
+  nsString GetValueAt(uint32_t aIndex) const {
+    MOZ_ASSERT(aIndex < Count());
+    return ValueTable().Get(Entries()[aIndex].mValue);
+  }
+
+  /**
+   * Returns a copy of the read-only file descriptor which backs the shared
+   * memory region for this map. The file descriptor may be passed between
+   * processes, and used to construct new instances of SharedStringMap with
+   * the same data as this instance.
+   */
+  FileDescriptor CloneFileDescriptor() const;
+
+  size_t MapSize() const { return mMap.size(); }
+
+ protected:
+  ~SharedStringMap() = default;
+
+ private:
+  // Type-safe getters for values in the shared memory region:
+  const Header& GetHeader() const { return mMap.get<Header>()[0]; }
+
+  RangedPtr<const Entry> Entries() const {
+    return {reinterpret_cast<const Entry*>(&GetHeader() + 1), EntryCount()};
+  }
+
+  uint32_t EntryCount() const { return GetHeader().mEntryCount; }
+
+  StringTable<nsCString> KeyTable() const {
+    auto& header = GetHeader();
+    return {{&mMap.get<uint8_t>()[header.mKeyStringsOffset],
+             header.mKeyStringsSize}};
+  }
+
+  StringTable<nsString> ValueTable() const {
+    auto& header = GetHeader();
+    return {{&mMap.get<uint8_t>()[header.mValueStringsOffset],
+             header.mValueStringsSize}};
+  }
+
+  loader::AutoMemMap mMap;
+};
+
+/**
+ * A helper class which builds the contiguous look-up table used by
+ * SharedStringMap. Each key-value pair in the final map is added to the
+ * builder, before it is finalized and transformed into a snapshot.
+ */
+class MOZ_RAII SharedStringMapBuilder {
+ public:
+  SharedStringMapBuilder() = default;
+
+  /**
+   * Adds a key-value pair to the map.
+   */
+  void Add(const nsCString& aKey, const nsString& aValue);
+
+  /**
+   * Finalizes the binary representation of the map, writes it to a shared
+   * memory region, and then initializes the given AutoMemMap with a reference
+   * to the read-only copy of it.
+   */
+  Result<Ok, nsresult> Finalize(loader::AutoMemMap& aMap);
+
+ private:
+  using Entry = SharedStringMap::Entry;
+
+  StringTableBuilder<nsCStringHashKey, nsCString> mKeyTable;
+  StringTableBuilder<nsStringHashKey, nsString> mValueTable;
+
+  nsTHashMap<nsCStringHashKey, Entry> mEntries;
+};
+
+}  // namespace mozilla::dom::ipc
+
+#endif  // dom_ipc_SharedStringMap_h