Add a data structure that is structured in the same way as the UnackedPacketMap, in hope we can rep…

net/quic/core/packet_number_indexed_queue.h

Index: net/quic/core/packet_number_indexed_queue.h
diff --git a/net/quic/core/packet_number_indexed_queue.h b/net/quic/core/packet_number_indexed_queue.h
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+// Copyright (c) 2017 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 NET_QUIC_CORE_PACKET_NUMBER_INDEXED_QUEUE_H_
+#define NET_QUIC_CORE_PACKET_NUMBER_INDEXED_QUEUE_H_
+
+#include <deque>
+
+#include "base/logging.h"
+#include "net/quic/core/quic_types.h"
+
+namespace net {
+
+// PacketNumberIndexedQueue is a queue of mostly continuous numbered entries
+// which supports the following operations:
+// - adding elements to the end of the queue, or at some point past the end
+// - removing elements in any order
+// - retrieving elements
+// If all elements are inserted in order, all of the operations above are
+// amortized O(1) time.
+//
+// Internally, the data structure is a deque where each element is marked as
+// present or not.  The deque starts at the lowest present index.  Whenever an
+// element is removed, it's marked as not present, and the front of the deque is
+// cleared of elements that are not present.
+//
+// The tail of the queue is not cleared due to the assumption of entries being
+// inserted in order, though removing all elements of the queue will return it
+// to its initial state.
+//
+// Note that this data structure is inherently hazardous, since an addition of
+// just two entries will cause it to consume all of the memory available.
+// Because of that, it is not a general-purpose container and should not be used
+// as one.
+template <typename T>
+class PacketNumberIndexedQueue {
+ public:
+  PacketNumberIndexedQueue()
+      : number_of_present_entries_(0), first_packet_(0) {}
+
+  // Retrieve the entry associated with the packet number.  Returns the pointer
+  // to the entry in case of success, or nullptr if the entry does not exist.
+  T* GetEntry(QuicPacketNumber packet_number);
+
+  // Inserts data associated |packet_number| into (or past) the end of the
+  // queue, filling up the missing intermediate entries as necessary.  Returns
+  // true if the element has been inserted successfully, false if it was already
+  // in the queue or inserted out of order.
+  template <typename... Args>
+  bool Emplace(QuicPacketNumber packet_number, Args&&... args);
+
+  // Removes data associated with |packet_number| and frees the slots in the
+  // queue as necessary.
+  bool Remove(QuicPacketNumber packet_number);
+
+  bool IsEmpty() const { return number_of_present_entries_ == 0; }
+
+  // Returns the number of entries in the queue.
+  size_t number_of_present_entries() const {
+    return number_of_present_entries_;
+  }
+
+  // Returns the number of entries allocated in the underlying deque.  This is
+  // proportional to the memory usage of the queue.
+  size_t entry_slots_used() const { return entries_.size(); }
+
+  // Packet number of the first entry in the queue.  Zero if the queue is empty.
+  size_t first_packet() const { return first_packet_; }
+
+  // Packet number of the last entry ever inserted in the queue.  Note that the
+  // entry in question may have already been removed.  Zero if the queue is
+  // empty.
+  size_t last_packet() const {
+    if (IsEmpty()) {
+      return 0;
+    }
+    return first_packet_ + entries_.size() - 1;
+  }
+
+ private:
+  // Wrapper around T used to mark whether the entry is actually in the map.
+  struct EntryWrapper {
+    T data;
+    bool present;
+
+    EntryWrapper() : data(), present(false) {}
+
+    template <typename... Args>
+    explicit EntryWrapper(Args&&... args)
+        : data(std::forward<Args>(args)...), present(true) {}
+  };
+
+  // Cleans up unused slots in the front after removing an element.
+  void Cleanup();
+
+  EntryWrapper* GetEntryWrapper(QuicPacketNumber offset);
+
+  std::deque<EntryWrapper> entries_;
+  size_t number_of_present_entries_;
+  QuicPacketNumber first_packet_;
+};
+
+template <typename T>
+T* PacketNumberIndexedQueue<T>::GetEntry(QuicPacketNumber packet_number) {
+  EntryWrapper* entry = GetEntryWrapper(packet_number);
+  if (entry == nullptr) {
+    return nullptr;
+  }
+  return &entry->data;
+}
+
+template <typename T>
+template <typename... Args>
+bool PacketNumberIndexedQueue<T>::Emplace(QuicPacketNumber packet_number,
+                                          Args&&... args) {
+  if (IsEmpty()) {
+    DCHECK(entries_.empty());
+    DCHECK_EQ(0u, first_packet_);
+
+    entries_.emplace_back(std::forward<Args>(args)...);
+    number_of_present_entries_ = 1;
+    first_packet_ = packet_number;
+    return true;
+  }
+
+  // Do not allow insertion out-of-order.
+  if (packet_number <= last_packet()) {
+    return false;
+  }
+
+  // Handle potentially missing elements.
+  size_t offset = packet_number - first_packet_;
+  if (offset > entries_.size()) {
+    entries_.resize(offset);
+  }
+
+  number_of_present_entries_++;
+  entries_.emplace_back(std::forward<Args>(args)...);
+  DCHECK_EQ(packet_number, last_packet());
+  return true;
+}
+
+template <typename T>
+bool PacketNumberIndexedQueue<T>::Remove(QuicPacketNumber packet_number) {
+  EntryWrapper* entry = GetEntryWrapper(packet_number);
+  if (entry == nullptr) {
+    return false;
+  }
+  entry->present = false;
+  number_of_present_entries_--;
+
+  if (packet_number == first_packet()) {
+    Cleanup();
+  }
+  return true;
+}
+
+template <typename T>
+void PacketNumberIndexedQueue<T>::Cleanup() {
+  while (!entries_.empty() && !entries_.front().present) {
+    entries_.pop_front();
+    first_packet_++;
+  }
+  if (entries_.empty()) {
+    first_packet_ = 0;
+  }
+}
+
+template <typename T>
+auto PacketNumberIndexedQueue<T>::GetEntryWrapper(QuicPacketNumber offset)
+    -> EntryWrapper* {
+  if (offset < first_packet_) {
+    return nullptr;
+  }
+
+  offset -= first_packet_;
+  if (offset >= entries_.size()) {
+    return nullptr;
+  }
+
+  EntryWrapper* entry = &entries_[offset];
+  if (!entry->present) {
+    return nullptr;
+  }
+
+  return entry;
+}
+
+}  // namespace net
+
+#endif  // NET_QUIC_CORE_PACKET_NUMBER_INDEXED_QUEUE_H_