--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/Resources/Orthanc/Core/Cache/LeastRecentlyUsedIndex.h	Fri Mar 23 11:04:03 2018 +0100
@@ -0,0 +1,347 @@
+/**
+ * Orthanc - A Lightweight, RESTful DICOM Store
+ * Copyright (C) 2012-2016 Sebastien Jodogne, Medical Physics
+ * Department, University Hospital of Liege, Belgium
+ * Copyright (C) 2017-2018 Osimis S.A., Belgium
+ *
+ * This program is free software: you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License as
+ * published by the Free Software Foundation, either version 3 of the
+ * License, or (at your option) any later version.
+ *
+ * In addition, as a special exception, the copyright holders of this
+ * program give permission to link the code of its release with the
+ * OpenSSL project's "OpenSSL" library (or with modified versions of it
+ * that use the same license as the "OpenSSL" library), and distribute
+ * the linked executables. You must obey the GNU General Public License
+ * in all respects for all of the code used other than "OpenSSL". If you
+ * modify file(s) with this exception, you may extend this exception to
+ * your version of the file(s), but you are not obligated to do so. If
+ * you do not wish to do so, delete this exception statement from your
+ * version. If you delete this exception statement from all source files
+ * in the program, then also delete it here.
+ * 
+ * This program is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+ * General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program. If not, see <http://www.gnu.org/licenses/>.
+ **/
+
+
+#pragma once
+
+#include <list>
+#include <map>
+#include <boost/noncopyable.hpp>
+#include <cassert>
+
+#include "../OrthancException.h"
+#include "../Toolbox.h"
+
+namespace Orthanc
+{
+  /**
+   * This class implements the index of a cache with least recently
+   * used (LRU) recycling policy. All the items of the cache index
+   * can be associated with a payload.
+   * Reference: http://stackoverflow.com/a/2504317
+   **/
+  template <typename T, typename Payload = NullType>
+  class LeastRecentlyUsedIndex : public boost::noncopyable
+  {
+  private:
+    typedef std::list< std::pair<T, Payload> >  Queue;
+    typedef std::map<T, typename Queue::iterator>  Index;
+
+    Index  index_;
+    Queue  queue_;
+
+    /**
+     * Internal method for debug builds to check whether the internal
+     * data structures are not corrupted.
+     **/
+    void CheckInvariants() const;
+
+  public:
+    /**
+     * Add a new element to the cache index, and make it the most
+     * recent element.
+     * \param id The ID of the element.
+     * \param payload The payload of the element.
+     **/
+    void Add(T id, Payload payload = Payload());
+
+    void AddOrMakeMostRecent(T id, Payload payload = Payload());
+
+    /**
+     * When accessing an element of the cache, this method tags the
+     * element as the most recently used.
+     * \param id The most recently accessed item.
+     **/
+    void MakeMostRecent(T id);
+
+    void MakeMostRecent(T id, Payload updatedPayload);
+
+    /**
+     * Remove an element from the cache index.
+     * \param id The item to remove.
+     **/
+    Payload Invalidate(T id);
+
+    /**
+     * Get the oldest element in the cache and remove it.
+     * \return The oldest item.
+     **/
+    T RemoveOldest();
+
+    /**
+     * Get the oldest element in the cache, remove it and return the
+     * associated payload.
+     * \param payload Where to store the associated payload.
+     * \return The oldest item.
+     **/
+    T RemoveOldest(Payload& payload);
+
+    /**
+     * Check whether an element is contained in the cache.
+     * \param id The item.
+     * \return \c true iff the item is indexed by the cache.
+     **/
+    bool Contains(T id) const
+    {
+      return index_.find(id) != index_.end();
+    }
+
+    bool Contains(T id, Payload& payload) const
+    {
+      typename Index::const_iterator it = index_.find(id);
+      if (it == index_.end())
+      {
+        return false;
+      }
+      else
+      {
+        payload = it->second->second;
+        return true;
+      }
+    }
+
+    /**
+     * Return the number of elements in the cache.
+     * \return The number of elements.
+     **/
+    size_t GetSize() const
+    {
+      assert(index_.size() == queue_.size());
+      return queue_.size();
+    }
+
+    /**
+     * Check whether the cache index is empty.
+     * \return \c true iff the cache is empty.
+     **/
+    bool IsEmpty() const
+    {
+      return index_.empty();
+    }
+
+    const T& GetOldest() const;
+    
+    const Payload& GetOldestPayload() const;
+  };
+
+
+
+
+  /******************************************************************
+   ** Implementation of the template
+   ******************************************************************/
+
+  template <typename T, typename Payload>
+  void LeastRecentlyUsedIndex<T, Payload>::CheckInvariants() const
+  {
+#ifndef NDEBUG
+    assert(index_.size() == queue_.size());
+
+    for (typename Index::const_iterator 
+           it = index_.begin(); it != index_.end(); it++)
+    {
+      assert(it->second != queue_.end());
+      assert(it->second->first == it->first);
+    }
+#endif
+  }
+
+
+  template <typename T, typename Payload>
+  void LeastRecentlyUsedIndex<T, Payload>::Add(T id, Payload payload)
+  {
+    if (Contains(id))
+    {
+      throw OrthancException(ErrorCode_BadSequenceOfCalls);
+    }
+
+    queue_.push_front(std::make_pair(id, payload));
+    index_[id] = queue_.begin();
+
+    CheckInvariants();
+  }
+
+
+  template <typename T, typename Payload>
+  void LeastRecentlyUsedIndex<T, Payload>::MakeMostRecent(T id)
+  {
+    if (!Contains(id))
+    {
+      throw OrthancException(ErrorCode_InexistentItem);
+    }
+
+    typename Index::iterator it = index_.find(id);
+    assert(it != index_.end());
+
+    std::pair<T, Payload> item = *(it->second);
+    
+    queue_.erase(it->second);
+    queue_.push_front(item);
+    index_[id] = queue_.begin();
+
+    CheckInvariants();
+  }
+
+
+  template <typename T, typename Payload>
+  void LeastRecentlyUsedIndex<T, Payload>::AddOrMakeMostRecent(T id, Payload payload)
+  {
+    typename Index::iterator it = index_.find(id);
+
+    if (it != index_.end())
+    {
+      // Already existing. Make it most recent.
+      std::pair<T, Payload> item = *(it->second);
+      item.second = payload;
+      queue_.erase(it->second);
+      queue_.push_front(item);
+    }
+    else
+    {
+      // New item
+      queue_.push_front(std::make_pair(id, payload));
+    }
+
+    index_[id] = queue_.begin();
+
+    CheckInvariants();
+  }
+
+
+  template <typename T, typename Payload>
+  void LeastRecentlyUsedIndex<T, Payload>::MakeMostRecent(T id, Payload updatedPayload)
+  {
+    if (!Contains(id))
+    {
+      throw OrthancException(ErrorCode_InexistentItem);
+    }
+
+    typename Index::iterator it = index_.find(id);
+    assert(it != index_.end());
+
+    std::pair<T, Payload> item = *(it->second);
+    item.second = updatedPayload;
+    
+    queue_.erase(it->second);
+    queue_.push_front(item);
+    index_[id] = queue_.begin();
+
+    CheckInvariants();
+  }
+
+
+  template <typename T, typename Payload>
+  Payload LeastRecentlyUsedIndex<T, Payload>::Invalidate(T id)
+  {
+    if (!Contains(id))
+    {
+      throw OrthancException(ErrorCode_InexistentItem);
+    }
+
+    typename Index::iterator it = index_.find(id);
+    assert(it != index_.end());
+
+    Payload payload = it->second->second;
+    queue_.erase(it->second);
+    index_.erase(it);
+
+    CheckInvariants();
+    return payload;
+  }
+
+
+  template <typename T, typename Payload>
+  T LeastRecentlyUsedIndex<T, Payload>::RemoveOldest(Payload& payload)
+  {
+    if (IsEmpty())
+    {
+      throw OrthancException(ErrorCode_BadSequenceOfCalls);
+    }
+
+    std::pair<T, Payload> item = queue_.back();
+    T oldest = item.first;
+    payload = item.second;
+
+    queue_.pop_back();
+    assert(index_.find(oldest) != index_.end());
+    index_.erase(oldest);
+
+    CheckInvariants();
+
+    return oldest;
+  }
+
+
+  template <typename T, typename Payload>
+  T LeastRecentlyUsedIndex<T, Payload>::RemoveOldest()
+  {
+    if (IsEmpty())
+    {
+      throw OrthancException(ErrorCode_BadSequenceOfCalls);
+    }
+
+    std::pair<T, Payload> item = queue_.back();
+    T oldest = item.first;
+
+    queue_.pop_back();
+    assert(index_.find(oldest) != index_.end());
+    index_.erase(oldest);
+
+    CheckInvariants();
+
+    return oldest;
+  }
+
+
+  template <typename T, typename Payload>
+  const T& LeastRecentlyUsedIndex<T, Payload>::GetOldest() const
+  {
+    if (IsEmpty())
+    {
+      throw OrthancException(ErrorCode_BadSequenceOfCalls);
+    }
+
+    return queue_.back().first;
+  }
+
+
+  template <typename T, typename Payload>
+  const Payload& LeastRecentlyUsedIndex<T, Payload>::GetOldestPayload() const
+  {
+    if (IsEmpty())
+    {
+      throw OrthancException(ErrorCode_BadSequenceOfCalls);
+    }
+
+    return queue_.back().second;
+  }
+}