Add basic deque implementation.

This implementation allows one to add the deque_item property to any class and then manage pointers to these items in a deque. We currently use a spinlock for concurrent access.

lib/pls/CMakeLists.txt

 # List all required files here (cmake best practice to NOT automate this step!)
 add_library(pls STATIC
-            src/library.cpp include/pls/library.h
+            src/pls.cpp include/pls/pls.h
             src/internal/base/spin_lock.cpp include/pls/internal/base/spin_lock.h
             src/internal/base/thread.cpp include/pls/internal/base/thread.h
             include/pls/internal/base/prohibit_new.h
@@ -13,7 +13,7 @@ add_library(pls STATIC
             include/pls/internal/base/system_details.h
             src/internal/scheduling/run_on_n_threads_task.cpp include/pls/internal/scheduling/run_on_n_threads_task.h
             src/internal/scheduling/tbb_task.cpp include/pls/internal/scheduling/tbb_task.h
-        )
+            src/internal/base/deque.cpp include/pls/internal/base/deque.h)
 
 # Add everything in `./include` to be in the include path of this project
 target_include_directories(pls

lib/pls/include/pls/internal/base/deque.h

+
+#ifndef PLS_DEQUE_H
+#define PLS_DEQUE_H
+
+#include "spin_lock.h"
+
+namespace pls {
+    namespace internal {
+        namespace base {
+            class deque_item {
+                friend class deque_internal;
+
+                deque_item* prev_;
+                deque_item* next_;
+
+            };
+
+            class deque_internal {
+            protected:
+                deque_item* head_;
+                deque_item* tail_;
+
+                spin_lock lock_;
+
+                deque_item* pop_head_internal();
+                deque_item* pop_tail_internal();
+                void push_tail_internal(deque_item *new_item);
+            };
+
+            template<typename Item>
+            class deque: deque_internal {
+            public:
+                explicit deque(): deque_internal{} {}
+
+                inline Item* pop_head() {
+                    return static_cast<Item*>(pop_head_internal());
+                }
+
+                inline Item* pop_tail() {
+                    return static_cast<Item*>(pop_tail_internal());
+                }
+
+                inline void push_tail(Item* new_item) {
+                    push_tail_internal(new_item);
+                }
+            };
+        }
+    }
+}
+
+#endif //PLS_DEQUE_H

lib/pls/src/internal/base/deque.cpp

+#include <mutex>
+
+#include "pls/internal/base/deque.h"
+
+namespace pls {
+    namespace internal {
+        namespace base {
+            deque_item* deque_internal::pop_head_internal() {
+                std::lock_guard<spin_lock> lock{lock_};
+
+                if (head_ == nullptr) {
+                    return nullptr;
+                }
+
+                deque_item* result = head_;
+                head_ = head_->prev_;
+                if (head_ == nullptr) {
+                    tail_ = nullptr;
+                } else {
+                    head_->next_ = nullptr;
+                }
+
+                return result;
+            }
+
+            deque_item* deque_internal::pop_tail_internal() {
+                std::lock_guard<spin_lock> lock{lock_};
+
+                if (tail_ == nullptr) {
+                    return nullptr;
+                }
+
+                deque_item* result = tail_;
+                tail_ = tail_->next_;
+                if (tail_ == nullptr) {
+                    head_ = nullptr;
+                } else {
+                    tail_->prev_ = nullptr;
+                }
+
+                return result;
+            }
+
+            void deque_internal::push_tail_internal(deque_item *new_item) {
+                std::lock_guard<spin_lock> lock{lock_};
+
+                if (tail_ != nullptr) {
+                    tail_->prev_ = new_item;
+                } else {
+                    head_ = new_item;
+                }
+                new_item->next_ = tail_;
+                new_item->prev_ = nullptr;
+                tail_ = new_item;
+            }
+        }
+    }
+}