sql::MetaTable.next_download_id, DownloadManager::GetNextId()

content/browser/download/download_id.h

+// Copyright (c) 2011 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef CHROME_BROWSER_DOWNLOAD_DOWNLOAD_ID_H_
+#define CHROME_BROWSER_DOWNLOAD_DOWNLOAD_ID_H_
+#pragma once
+
+#include <iostream>
+
+#include "base/hash_tables.h"
+
+class DownloadManager;
+
+// DownloadId combines per-profile Download ids with an indication of which
+// profile in order to be globally unique. DownloadIds are not persistent across
+// sessions, but their local() field is.
+class DownloadId {
+ public:
+  DownloadId(const DownloadManager* manager, int32 local_id)
+    : manager_(manager),
+      local_id_(local_id) {
+  }
+
+  // Return the per-profile and persistent part of this DownloadId.
+  int32 local() const { return local_id_; }
+
+  // Returns true if this DownloadId has been allocated and could possibly refer
+  // to a DownloadItem that exists.
+  bool IsValid() const { return ((manager_ != NULL) && (local_id_ >= 0)); }
+
+  // The following methods (operator==, hash(), copy, and assign) provide
+  // support for STL containers such as hash_map.
+
+  bool operator==(const DownloadId& that) const {
+    return ((that.local_id_ == local_id_) &&
+            (that.manager_ == manager_));
+  }
+  bool operator<(const DownloadId& that) const {
+    // Even though DownloadManager* < DownloadManager* is not well defined and
+    // GCC does not require it for hash_map, MSVC requires operator< for
+    // hash_map. We don't ifdef it out here because we will probably make a
+    // set<DownloadId> at some point, when GCC will require it.
+    return ((that.local_id_ < local_id_) &&
+            (that.manager_ < manager_));
+  }
+
+  size_t hash() const {
+    // The top half of manager is unlikely to be distinct, and the user is
+    // unlikely to have >64K downloads. If these assumptions are incorrect, then
+    // DownloadFileManager's hash_map might have a few collisions, but it will
+    // use operator== to safely disambiguate.
+    return reinterpret_cast<size_t>(manager_) +
+           (static_cast<size_t>(local_id_) << (4 * sizeof(size_t)));
+  }
+
+ private:
+  // DownloadId is used mostly off the UI thread, so manager's methods can't be
+  // called, but the pointer can be compared.
+  const DownloadManager* manager_;
+
+  int32 local_id_;
+
+  friend std::ostream& operator<<(std::ostream& out,
+                                  const DownloadId& global_id);
+
+  // Allow copy and assign.
+};
+
+// Allow logging DownloadIds. Looks like "0x01234567:42".
+std::ostream& operator<<(std::ostream& out, const DownloadId& global_id);
+
+// Allow using DownloadIds as keys in hash_maps.
+namespace BASE_HASH_NAMESPACE {
+#if defined(COMPILER_GCC)
+template<> struct hash<DownloadId> {
+  std::size_t operator()(const DownloadId& id) const {
+    return id.hash();
+  }
+};
+#elif defined(COMPILER_MSVC)
+inline size_t hash_value(const DownloadId& id) {
+  return id.hash();
+}
+#endif  // COMPILER
+}
+#endif  // CHROME_BROWSER_DOWNLOAD_DOWNLOAD_ID_H_