AK: Combine SinglyLinkedList and SinglyLinkedListWithCount

Using policy based design `SinglyLinkedList` and
`SinglyLinkedListWithCount` can be combined into one class which takes
a policy to determine how to keep track of the size of the list. The
default policy is to use list iteration to count the items in the list
each time. The `WithCount` form is a different policy which tracks the
size, but comes with the overhead of storing the count and
incrementing/decrementing on each modification.

This model is extensible to have other forms of counting by
implementing only a new policy instead of implementing a totally new
type.

AK/Forward.h

@@ -7,6 +7,7 @@
 #pragma once
 
 #include <AK/DefaultDelete.h>
+#include <AK/SinglyLinkedListSizePolicy.h>
 #include <AK/Types.h>
 
 namespace AK {
@@ -64,7 +65,7 @@ using Bytes = Span<u8>;
 template<typename T, AK::MemoryOrder DefaultMemoryOrder>
 class Atomic;
 
-template<typename T>
+template<typename T, typename TSizeCalculationPolicy = DefaultSizeCalculationPolicy>
 class SinglyLinkedList;
 
 template<typename T>

AK/SinglyLinkedList.h

@@ -63,7 +63,7 @@ class SinglyLinkedListIterator {
  bool m_removed { false };
 };
 
-template<typename T>
+template<typename T, typename TSizeCalculationPolicy>
 class SinglyLinkedList {
 private:
  struct Node {
@@ -96,12 +96,9 @@ class SinglyLinkedList {
 
  bool is_empty() const { return !head(); }
 
- inline size_t size_slow() const
+ inline size_t size() const
  {
- size_t size = 0;
- for (auto* node = m_head; node; node = node->next)
- ++size;
- return size;
+ return m_size_policy.size(m_head);
  }
 
  void clear()
@@ -113,6 +110,7 @@ class SinglyLinkedList {
  }
  m_head = nullptr;
  m_tail = nullptr;
+ m_size_policy.reset();
  }
 
  T& first()
@@ -144,6 +142,7 @@ class SinglyLinkedList {
  if (m_tail == m_head)
  m_tail = nullptr;
  m_head = m_head->next;
+ m_size_policy.decrease_size(value);
  delete prev_head;
  return value;
  }
@@ -154,6 +153,7 @@ class SinglyLinkedList {
  auto* node = new (nothrow) Node(forward<U>(value));
  if (!node)
  return Error::from_errno(ENOMEM);
+ m_size_policy.increase_size(value);
  if (!m_head) {
  m_head = node;
  m_tail = node;
@@ -170,6 +170,7 @@ class SinglyLinkedList {
  auto* node = new (nothrow) Node(forward<U>(value));
  if (!node)
  return Error::from_errno(ENOMEM);
+ m_size_policy.increase_size(value);
  if (!m_head) {
  m_head = node;
  m_tail = node;
@@ -237,6 +238,7 @@ class SinglyLinkedList {
  auto* node = new (nothrow) Node(forward<U>(value));
  if (!node)
  return Error::from_errno(ENOMEM);
+ m_size_policy.increase_size(value);
  node->next = iterator.m_node;
  if (m_head == iterator.m_node)
  m_head = node;
@@ -254,6 +256,7 @@ class SinglyLinkedList {
  auto* node = new (nothrow) Node(forward<U>(value));
  if (!node)
  return Error::from_errno(ENOMEM);
+ m_size_policy.increase_size(value);
  node->next = iterator.m_node->next;
 
  iterator.m_node->next = node;
@@ -286,6 +289,7 @@ class SinglyLinkedList {
  m_tail = iterator.m_prev;
  if (iterator.m_prev)
  iterator.m_prev->next = iterator.m_node->next;
+ m_size_policy.decrease_size(iterator.m_node->value);
  delete iterator.m_node;
  }
 
@@ -298,8 +302,8 @@ class SinglyLinkedList {
 
  Node* m_head { nullptr };
  Node* m_tail { nullptr };
+ TSizeCalculationPolicy m_size_policy {};
 };
-
 }
 
 #if USING_AK_GLOBALLY

AK/SinglyLinkedListSizePolicy.h

@@ -0,0 +1,47 @@
+/*
+ * Copyright (c) 2022, the SerenityOS developers.
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Types.h>
+
+namespace AK {
+
+struct DefaultSizeCalculationPolicy {
+ constexpr void increase_size(auto const&) { }
+
+ constexpr void decrease_size(auto const&) { }
+
+ constexpr void reset() { }
+
+ constexpr size_t size(auto const* head) const
+ {
+ size_t size = 0;
+ for (auto* node = head; node; node = node->next)
+ ++size;
+ return size;
+ }
+};
+
+struct CountingSizeCalculationPolicy {
+ constexpr void increase_size(auto const&) { ++m_size; }
+
+ constexpr void decrease_size(auto const&) { --m_size; }
+
+ constexpr void reset() { m_size = 0; }
+
+ constexpr size_t size(auto const*) const { return m_size; }
+
+private:
+ size_t m_size { 0 };
+};
+
+}
+
+#ifdef USING_AK_GLOBALLY
+using AK::CountingSizeCalculationPolicy;
+using AK::DefaultSizeCalculationPolicy;
+#endif

Kernel/Net/IPv4Socket.h

@@ -7,7 +7,7 @@
 #pragma once
 
 #include <AK/HashMap.h>
-#include <AK/SinglyLinkedListWithCount.h>
+#include <AK/SinglyLinkedList.h>
 #include <Kernel/DoubleBuffer.h>
 #include <Kernel/KBuffer.h>
 #include <Kernel/Locking/MutexProtected.h>
@@ -116,7 +116,7 @@ class IPv4Socket : public Socket {
  OwnPtr<KBuffer> data;
  };
 
- SinglyLinkedListWithCount<ReceivedPacket> m_receive_queue;
+ SinglyLinkedList<ReceivedPacket, CountingSizeCalculationPolicy> m_receive_queue;
 
  OwnPtr<DoubleBuffer> m_receive_buffer;