src/base/circular_queue.hh - public/gem5 - Git at Google

 /*
  * Copyright (c) 2017-2018 ARM Limited
  * All rights reserved
  *
  * The license below extends only to copyright in the software and shall
  * not be construed as granting a license to any other intellectual
  * property including but not limited to intellectual property relating
  * to a hardware implementation of the functionality of the software
  * licensed hereunder.  You may use the software subject to the license
  * terms below provided that you ensure that this notice is replicated
  * unmodified and in its entirety in all distributions of the software,
  * modified or unmodified, in source code or in binary form.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met: redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer;
  * redistributions in binary form must reproduce the above copyright
  * notice, this list of conditions and the following disclaimer in the
  * documentation and/or other materials provided with the distribution;
  * neither the name of the copyright holders nor the names of its
  * contributors may be used to endorse or promote products derived from
  * this software without specific prior written permission.
  *
  * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
  * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
  * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
  * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
  * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
  * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
  * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
  * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
  * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
  * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  *
  * Authors: Rekai Gonzalez-Alberquilla
  */

 #ifndef __BASE_CIRCULAR_QUEUE_HH__
 #define __BASE_CIRCULAR_QUEUE_HH__

 #include <vector>

 /** Circular queue.
  * Circular queue implemented on top of a standard vector. Instead of using
  * a sentinel entry, we use a boolean to distinguish the case in which the
  * queue is full or empty.
  * Thus, a circular queue is represented by the 5-tuple
  *  (Capacity, IsEmpty?, Head, Tail, Round)
  * Where:
  *   - Capacity is the size of the underlying vector.
  *   - IsEmpty? can be T or F.
  *   - Head is the index in the vector of the first element of the queue.
  *   - Tail is the index in the vector of the last element of the queue.
  *   - Round is the counter of how many times the Tail has wrapped around.
  * A queue is empty when
  *     Head == (Tail + 1 mod Capacity) && IsEmpty?.
  * Conversely, a queue if full when
  *     Head == (Tail + 1 mod Capacity) && !IsEmpty?.
  * Comments may show depictions of the underlying vector in the following
  * format: '|' delimit the 'cells' of the underlying vector. '-' represents
  * an element of the vector that is out-of-bounds of the circular queue,
  * while 'o' represents and element that is inside the bounds. The
  * characters '[' and ']' are added to mark the entries that hold the head
  * and tail of the circular queue respectively.
  * E.g.:
  *   - Empty queues of capacity 4:
  *     (4,T,1,0,_): |-]|[-|-|-|        (4,T,3,2): |-|-|-]|[-|
  *   - Full queues of capacity 4:
  *     (4,F,1,0,_): |o]|[o|o|o|        (4,F,3,2): |o|o|o]|[o|
  *   - Queues of capacity 4 with 2 elements:
  *     (4,F,0,1,_): |[o|o]|-|-|        (4,F,3,0): |o]|-|-|[o|
  *
  * The Round number is only relevant for checking validity of indices,
  * therefore it will be omitted or shown as '_'
  */
 template <typename T>
 class CircularQueue : private std::vector<T>
 {
   protected:
     using Base = std::vector<T>;
     using typename Base::reference;
     using typename Base::const_reference;
     const uint32_t _capacity;
     uint32_t _head;
     uint32_t _tail;
     uint32_t _empty;

     /** Counter for how many times the tail wraps around.
      * Some parts of the code rely on getting the past the end iterator, and
      * expect to use it after inserting on the tail. To support this without
      * ambiguity, we need the round number to guarantee that it did not become
      * a before-the-beginning iterator.
      */
     uint32_t _round;

     /** General modular addition. */
     static uint32_t
     moduloAdd(uint32_t op1, uint32_t op2, uint32_t size)
     {
         return (op1 + op2) % size;
     }

     /** General modular subtraction. */
     static uint32_t
     moduloSub(uint32_t op1, uint32_t op2, uint32_t size)
     {
         int32_t ret = sub(op1, op2, size);
         return ret >= 0 ? ret : ret + size;
     }

     static int32_t
     sub(uint32_t op1, uint32_t op2, uint32_t size)
     {
         if (op1 > op2)
             return (op1 - op2) % size;
         else
             return -((op2 - op1) % size);
     }

     void increase(uint32_t& v, size_t delta = 1)
     {
         v = moduloAdd(v, delta, _capacity);
     }

     void decrease(uint32_t& v)
     {
         v = (v ? v : _capacity) - 1;
     }

     /** Iterator to the circular queue.
      * iterator implementation to provide the circular-ness that the
      * standard std::vector<T>::iterator does not implement.
      * Iterators to a queue are represented by a pair of a character and the
      * round counter. For the character, '*' denotes the element pointed to by
      * the iterator if it is valid. 'x' denotes the element pointed to by the
      * iterator when it is BTB or PTE.
      * E.g.:
      *   - Iterator to the head of a queue of capacity 4 with 2 elems.
      *     (4,F,0,1,R): |[(*,R)|o]|-|-|        (4,F,3,0): |o]|-|-|[(*,R)|
      *   - Iterator to the tail of a queue of capacity 4 with 2 elems.
      *     (4,F,0,1,R): |[o|(*,R)]|-|-|        (4,F,3,0): |(*,R)]|-|-|[o|
      *   - Iterator to the end of a queue of capacity 4 with 2 elems.
      *     (4,F,0,1,R): |[o|o]|(x,R)|-|        (4,F,3,0): |o]|(x,R)|-|[o|
      */
   public:
     struct iterator {
         CircularQueue* _cq;
         uint32_t _idx;
         uint32_t _round;

       public:
         iterator(CircularQueue* cq, uint32_t idx, uint32_t round)
             : _cq(cq), _idx(idx), _round(round) {}

         /** Iterator Traits */
         using value_type = T;
         using difference_type = std::ptrdiff_t;
         using reference = value_type&;
         using const_reference = const value_type&;
         using pointer = value_type*;
         using const_pointer = const value_type*;
         using iterator_category = std::random_access_iterator_tag;

         /** Trait reference type
          * iterator satisfies OutputIterator, therefore reference
          * must be T& */
         static_assert(std::is_same<reference, T&>::value,
                 "reference type is not assignable as required");

         iterator() : _cq(nullptr), _idx(0), _round(0) { }

         iterator(const iterator& it)
             : _cq(it._cq), _idx(it._idx), _round(it._round) {}

         iterator&
         operator=(const iterator& it)
         {
             _cq = it._cq;
             _idx = it._idx;
             _round = it._round;
             return *this;
         }

         ~iterator() { _cq = nullptr; _idx = 0; _round = 0; }

         /** Test dereferenceability.
          * An iterator is dereferenceable if it is pointing to a non-null
          * circular queue, it is not the past-the-end iterator  and the
          * index is a valid index to that queue. PTE test is required to
          * distinguish between:
          * - An iterator to the first element of a full queue
          *    (4,F,1,0): |o]|[*|o|o|
          * - The end() iterator of a full queue
          *    (4,F,1,0): |o]|x[o|o|o|
          * Sometimes, though, users will get the PTE iterator and expect it
          * to work after growing the buffer on the tail, so we have to
          * check if the iterator is still PTE.
          */
         bool
         dereferenceable() const
         {
             return _cq != nullptr && _cq->isValidIdx(_idx, _round);
         }

         /** InputIterator. */

         /** Equality operator.
          * Two iterators must point to the same, possibly null, circular
          * queue and the same element on it, including PTE, to be equal.
          * In case the clients the the PTE iterator and then grow on the back
          * and expect it to work, we have to check if the PTE is still PTE
          */
         bool operator==(const iterator& that) const
         {
             return _cq == that._cq && _idx == that._idx &&
                 _round == that._round;
         }

         /** Inequality operator.
          * Conversely, two iterators are different if they both point to
          * different circular queues or they point to different elements.
          */
         bool operator!=(const iterator& that)
         {
             return !(*this == that);
         }

         /** Dereference operator. */
         reference operator*()
         {
             /* this has to be dereferenceable. */
             return (*_cq)[_idx];
         }

         const_reference operator*() const
         {
             /* this has to be dereferenceable. */
             return (*_cq)[_idx];
         }

         /** Dereference operator.
          * Rely on operator* to check for dereferenceability.
          */
         pointer operator->()
         {
             return &((*_cq)[_idx]);
         }

         const_pointer operator->() const
         {
             return &((*_cq)[_idx]);
         }

         /** Pre-increment operator. */
         iterator& operator++()
         {
             /* this has to be dereferenceable. */
             _cq->increase(_idx);
             if (_idx == 0)
                 ++_round;
             return *this;
         }

         /** Post-increment operator. */
         iterator
         operator++(int)
         {
             iterator t = *this;
             ++*this;
             return t;
         }

         /** ForwardIterator
          * The multipass guarantee is provided by the reliance on _idx.
          */

         /** BidirectionalIterator requirements. */
       private:
         /** Test decrementability.
          * An iterator to a non-null circular queue is not-decrementable
          * if it is pointing to the head element, unless the queue is full
          * and we are talking about the past-the-end iterator. In that case,
          * the iterator round equals the cq round unless the head is at the
          * zero position and the round is one more than the cq round.
          */
         bool
         decrementable() const
         {
             return _cq && !(_idx == _cq->head() &&
                             (_cq->empty() ||
                              (_idx == 0 && _round != _cq->_round + 1) ||
                              (_idx !=0 && _round != _cq->_round)));
         }

       public:
         /** Pre-decrement operator. */
         iterator& operator--()
         {
             /* this has to be decrementable. */
             assert(decrementable());
             if (_idx == 0)
                 --_round;
             _cq->decrease(_idx);
             return *this;
         }

         /** Post-decrement operator. */
         iterator operator--(int ) { iterator t = *this; --*this; return t; }

         /** RandomAccessIterator requirements.*/
         iterator& operator+=(const difference_type& t)
         {
             assert(_cq);
             _round += (t + _idx) / _cq->capacity();
             _idx = _cq->moduloAdd(_idx, t);
             return *this;
         }

         iterator& operator-=(const difference_type& t)
         {
             assert(_cq);

             /* C does not do euclidean division, so we have to adjust */
             if (t >= 0) {
                 _round += (-t + _idx) / _cq->capacity();
                 _idx = _cq->moduloSub(_idx, t);
             } else {
                 *this += -t;
             }
             return *this;
         }

         /** Addition operator. */
         iterator operator+(const difference_type& t)
         {
             iterator ret(*this);
             return ret += t;
         }

         friend iterator operator+(const difference_type& t, iterator& it)
         {
             iterator ret = it;
             return ret += t;
         }

         /** Substraction operator. */
         iterator operator-(const difference_type& t)
         {
             iterator ret(*this);
             return ret -= t;
         }

         friend iterator operator-(const difference_type& t, iterator& it)
         {
             iterator ret = it;
             return ret -= t;
         }

         /** Difference operator.
          * that + ret == this
          */
         difference_type operator-(const iterator& that)
         {
             /* If a is already at the end, we can safely return 0. */
             auto ret = _cq->sub(this->_idx, that._idx, _cq->capacity());

             if (this->_round != that._round) {
                 ret += ((this->_round - that._round) * _cq->capacity());
             }
             return ret;
         }

         /** Index operator.
          * The use of * tests for dereferenceability.
          */
         template<typename Idx>
         typename std::enable_if<std::is_integral<Idx>::value,reference>::type
         operator[](const Idx& index) { return *(*this + index); }

         /** Comparisons. */
         bool
         operator<(const iterator& that) const
         {
             assert(_cq && that._cq == _cq);
             return (this->_round < that._round) ||
                 (this->_round == that._round && _idx < that._idx);
         }

         bool
         operator>(const iterator& that) const
         { return !(*this <= that); }

         bool operator>=(const iterator& that) const
         { return !(*this < that); }

         bool operator<=(const iterator& that) const
         { return !(that < *this); }

         /** OutputIterator has no extra requirements.*/
         size_t idx() const { return _idx; }
     };

   public:
     using Base::operator[];

     explicit CircularQueue(uint32_t size = 0)
         : _capacity(size), _head(1), _tail(0), _empty(true), _round(0)
     {
         Base::resize(size);
     }

     /**
      * Remove all the elements in the queue.
      *
      * Note: This does not actually remove elements from the backing
      * store.
      */
     void flush()
     {
         _head = 1;
         _round = 0;
         _tail = 0;
         _empty = true;
     }

     /** Test if the index is in the range of valid elements. */
     bool isValidIdx(size_t idx) const
     {
         /* An index is invalid if:
          *   - The queue is empty.
          *   (6,T,3,2): |-|-|-]|[-|-|x|
          *   - head is small than tail and:
          *       - It is greater than both head and tail.
          *       (6,F,1,3): |-|[o|o|o]|-|x|
          *       - It is less than both head and tail.
          *       (6,F,1,3): |x|[o|o|o]|-|-|
          *   - It is greater than the tail and not than the head.
          *   (6,F,4,1): |o|o]|-|x|[o|o|
          */
         return !(_empty || (
             (_head < _tail) && (
                 (_head < idx && _tail < idx) ||
                 (_head > idx && _tail > idx)
             )) || (_tail < idx && idx < _head));
     }

     /** Test if the index is in the range of valid elements.
      * The round counter is used to disambiguate aliasing.
      */
     bool isValidIdx(size_t idx, uint32_t round) const
     {
         /* An index is valid if:
          *   - The queue is not empty.
          *      - round == R and
          *          - index <= tail (if index > tail, that would be PTE)
          *          - Either:
          *             - head <= index
          *               (6,F,1,3,R): |-|[o|(*,r)|o]|-|-|
          *             - head > tail
          *               (6,F,5,3,R): |o|o|(*,r)|o]|-|[o|
          *            The remaining case means the the iterator is BTB:
          *               (6,F,3,4,R): |-|-|(x,r)|[o|o]|-|
          *      - round + 1 == R and:
          *          - index > tail. If index <= tail, that would be BTB:
          *               (6,F,2,3,r):   | -|- |[(*,r)|o]|-|-|
          *               (6,F,0,1,r+1): |[o|o]| (x,r)|- |-|-|
          *               (6,F,0,3,r+1): |[o|o | (*,r)|o]|-|-|
          *          - index >= head. If index < head, that would be BTB:
          *               (6,F,5,2,R): |o|o]|-|-|(x,r)|[o|
          *          - head > tail. If head <= tail, that would be BTB:
          *               (6,F,3,4,R): |[o|o]|(x,r)|-|-|-|
          *      Other values of the round meand that the index is PTE or BTB
          */
         return (!_empty && (
                     (round == _round && idx <= _tail && (
                         _head <= idx || _head > _tail)) ||
                     (round + 1 == _round &&
                      idx > _tail &&
                      idx >= _head &&
                      _head > _tail)
                     ));
     }

     reference front() { return (*this)[_head]; }
     reference back() { return (*this)[_tail]; }
     uint32_t head() const { return _head; }
     uint32_t tail() const { return _tail; }
     size_t capacity() const { return _capacity; }

     uint32_t size() const
     {
         if (_empty)
             return 0;
         else if (_head <= _tail)
             return _tail - _head + 1;
         else
             return _capacity - _head + _tail + 1;
     }

     uint32_t moduloAdd(uint32_t s1, uint32_t s2) const
     {
         return moduloAdd(s1, s2, _capacity);
     }

     uint32_t moduloSub(uint32_t s1, uint32_t s2) const
     {
         return moduloSub(s1, s2, _capacity);
     }

     /** Circularly increase the head pointer.
      * By increasing the head pointer we are removing elements from
      * the begin of the circular queue.
      * Check that the queue is not empty. And set it to empty if it
      * had only one value prior to insertion.
      *
      * @params num_elem number of elements to remove
      */
     void pop_front(size_t num_elem = 1)
     {
         if (num_elem == 0) return;
         auto hIt = begin();
         hIt += num_elem;
         assert(hIt <= end());
         _empty = hIt == end();
         _head = hIt._idx;
     }

     /** Circularly decrease the tail pointer. */
     void pop_back()
     {
         assert (!_empty);
         _empty = _head == _tail;
         if (_tail == 0)
             --_round;
         decrease(_tail);
     }

     /** Pushes an element at the end of the queue. */
     void push_back(typename Base::value_type val)
     {
         advance_tail();
         (*this)[_tail] = val;
     }

     /** Increases the tail by one.
      * Check for wrap-arounds to update the round counter.
      */
     void advance_tail()
     {
         increase(_tail);
         if (_tail == 0)
             ++_round;

         if (_tail == _head && !_empty)
             increase(_head);

         _empty = false;
     }

     /** Increases the tail by a specified number of steps
      *
      * @param len Number of steps
      */
     void advance_tail(uint32_t len)
     {
         for (auto idx = 0; idx < len; idx++)
             advance_tail();
     }

     /** Is the queue empty? */
     bool empty() const { return _empty; }

     /** Is the queue full?
      * A queue is full if the head is the 0^{th} element and the tail is
      * the (size-1)^{th} element, or if the head is the n^{th} element and
      * the tail the (n-1)^{th} element.
      */
     bool full() const
     {
         return !_empty &&
             (_tail + 1 == _head || (_tail + 1 == _capacity && _head == 0));
     }

     /** Iterators. */
     iterator begin()
     {
         if (_empty)
             return end();
         else if (_head > _tail)
             return iterator(this, _head, _round - 1);
         else
             return iterator(this, _head, _round);
     }

     /* TODO: This should return a const_iterator. */
     iterator begin() const
     {
         if (_empty)
             return end();
         else if (_head > _tail)
             return iterator(const_cast<CircularQueue*>(this), _head,
                     _round - 1);
         else
             return iterator(const_cast<CircularQueue*>(this), _head,
                     _round);
     }

     iterator end()
     {
         auto poi = moduloAdd(_tail, 1);
         auto round = _round;
         if (poi == 0)
             ++round;
         return iterator(this, poi, round);
     }

     iterator end() const
     {
         auto poi = moduloAdd(_tail, 1);
         auto round = _round;
         if (poi == 0)
             ++round;
         return iterator(const_cast<CircularQueue*>(this), poi, round);
     }

     /** Return an iterator to an index in the vector.
      * This poses the problem of round determination. By convention, the round
      * is picked so that isValidIndex(idx, round) is true. If that is not
      * possible, then the round value is _round, unless _tail is at the end of
      * the storage, in which case the PTE wraps up and becomes _round + 1
      */
     iterator getIterator(size_t idx)
     {
         assert(isValidIdx(idx) || moduloAdd(_tail, 1) == idx);
         if (_empty)
             return end();

         uint32_t round = _round;
         if (idx > _tail) {
             if (idx >= _head && _head > _tail) {
                 round -= 1;
             }
         } else if (idx < _head && _tail + 1 == _capacity) {
             round += 1;
         }
         return iterator(this, idx, round);
     }
 };

 #endif /* __BASE_CIRCULARQUEUE_HH__ */
	/*
	* Copyright (c) 2017-2018 ARM Limited
	* All rights reserved
	*
	* The license below extends only to copyright in the software and shall
	* not be construed as granting a license to any other intellectual
	* property including but not limited to intellectual property relating
	* to a hardware implementation of the functionality of the software
	* licensed hereunder. You may use the software subject to the license
	* terms below provided that you ensure that this notice is replicated
	* unmodified and in its entirety in all distributions of the software,
	* modified or unmodified, in source code or in binary form.
	*
	* Redistribution and use in source and binary forms, with or without
	* modification, are permitted provided that the following conditions are
	* met: redistributions of source code must retain the above copyright
	* notice, this list of conditions and the following disclaimer;
	* redistributions in binary form must reproduce the above copyright
	* notice, this list of conditions and the following disclaimer in the
	* documentation and/or other materials provided with the distribution;
	* neither the name of the copyright holders nor the names of its
	* contributors may be used to endorse or promote products derived from
	* this software without specific prior written permission.
	*
	* THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
	* "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
	* LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
	* A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
	* OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
	* SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
	* LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
	* DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
	* THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
	* (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
	* OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
	*
	* Authors: Rekai Gonzalez-Alberquilla
	*/

	#ifndef __BASE_CIRCULAR_QUEUE_HH__
	#define __BASE_CIRCULAR_QUEUE_HH__

	#include <vector>

	/** Circular queue.
	* Circular queue implemented on top of a standard vector. Instead of using
	* a sentinel entry, we use a boolean to distinguish the case in which the
	* queue is full or empty.
	* Thus, a circular queue is represented by the 5-tuple
	* (Capacity, IsEmpty?, Head, Tail, Round)
	* Where:
	* - Capacity is the size of the underlying vector.
	* - IsEmpty? can be T or F.
	* - Head is the index in the vector of the first element of the queue.
	* - Tail is the index in the vector of the last element of the queue.
	* - Round is the counter of how many times the Tail has wrapped around.
	* A queue is empty when
	* Head == (Tail + 1 mod Capacity) && IsEmpty?.
	* Conversely, a queue if full when
	* Head == (Tail + 1 mod Capacity) && !IsEmpty?.
	* Comments may show depictions of the underlying vector in the following
	* format: '\|' delimit the 'cells' of the underlying vector. '-' represents
	* an element of the vector that is out-of-bounds of the circular queue,
	* while 'o' represents and element that is inside the bounds. The
	* characters '[' and ']' are added to mark the entries that hold the head
	* and tail of the circular queue respectively.
	* E.g.:
	* - Empty queues of capacity 4:
	* (4,T,1,0,_): \|-]\|[-\|-\|-\| (4,T,3,2): \|-\|-\|-]\|[-\|
	* - Full queues of capacity 4:
	* (4,F,1,0,_): \|o]\|[o\|o\|o\| (4,F,3,2): \|o\|o\|o]\|[o\|
	* - Queues of capacity 4 with 2 elements:
	* (4,F,0,1,_): \|[o\|o]\|-\|-\| (4,F,3,0): \|o]\|-\|-\|[o\|
	*
	* The Round number is only relevant for checking validity of indices,
	* therefore it will be omitted or shown as '_'
	*/
	template <typename T>
	class CircularQueue : private std::vector<T>
	{
	protected:
	using Base = std::vector<T>;
	using typename Base::reference;
	using typename Base::const_reference;
	const uint32_t _capacity;
	uint32_t _head;
	uint32_t _tail;
	uint32_t _empty;

	/** Counter for how many times the tail wraps around.
	* Some parts of the code rely on getting the past the end iterator, and
	* expect to use it after inserting on the tail. To support this without
	* ambiguity, we need the round number to guarantee that it did not become
	* a before-the-beginning iterator.
	*/
	uint32_t _round;

	/** General modular addition. */
	static uint32_t
	moduloAdd(uint32_t op1, uint32_t op2, uint32_t size)
	{
	return (op1 + op2) % size;
	}

	/** General modular subtraction. */
	static uint32_t
	moduloSub(uint32_t op1, uint32_t op2, uint32_t size)
	{
	int32_t ret = sub(op1, op2, size);
	return ret >= 0 ? ret : ret + size;
	}

	static int32_t
	sub(uint32_t op1, uint32_t op2, uint32_t size)
	{
	if (op1 > op2)
	return (op1 - op2) % size;
	else
	return -((op2 - op1) % size);
	}

	void increase(uint32_t& v, size_t delta = 1)
	{
	v = moduloAdd(v, delta, _capacity);
	}

	void decrease(uint32_t& v)
	{
	v = (v ? v : _capacity) - 1;
	}

	/** Iterator to the circular queue.
	* iterator implementation to provide the circular-ness that the
	* standard std::vector<T>::iterator does not implement.
	* Iterators to a queue are represented by a pair of a character and the
	* round counter. For the character, '*' denotes the element pointed to by
	* the iterator if it is valid. 'x' denotes the element pointed to by the
	* iterator when it is BTB or PTE.
	* E.g.:
	* - Iterator to the head of a queue of capacity 4 with 2 elems.
	* (4,F,0,1,R): \|[(,R)\|o]\|-\|-\| (4,F,3,0): \|o]\|-\|-\|[(,R)\|
	* - Iterator to the tail of a queue of capacity 4 with 2 elems.
	* (4,F,0,1,R): \|[o\|(,R)]\|-\|-\| (4,F,3,0): \|(,R)]\|-\|-\|[o\|
	* - Iterator to the end of a queue of capacity 4 with 2 elems.
	* (4,F,0,1,R): \|[o\|o]\|(x,R)\|-\| (4,F,3,0): \|o]\|(x,R)\|-\|[o\|
	*/
	public:
	struct iterator {
	CircularQueue* _cq;
	uint32_t _idx;
	uint32_t _round;

	public:
	iterator(CircularQueue* cq, uint32_t idx, uint32_t round)
	: _cq(cq), _idx(idx), _round(round) {}

	/** Iterator Traits */
	using value_type = T;
	using difference_type = std::ptrdiff_t;
	using reference = value_type&;
	using const_reference = const value_type&;
	using pointer = value_type*;
	using const_pointer = const value_type*;
	using iterator_category = std::random_access_iterator_tag;

	/** Trait reference type
	* iterator satisfies OutputIterator, therefore reference
	* must be T& */
	static_assert(std::is_same<reference, T&>::value,
	"reference type is not assignable as required");

	iterator() : _cq(nullptr), _idx(0), _round(0) { }

	iterator(const iterator& it)
	: _cq(it._cq), _idx(it._idx), _round(it._round) {}

	iterator&
	operator=(const iterator& it)
	{
	_cq = it._cq;
	_idx = it._idx;
	_round = it._round;
	return *this;
	}

	~iterator() { _cq = nullptr; _idx = 0; _round = 0; }

	/** Test dereferenceability.
	* An iterator is dereferenceable if it is pointing to a non-null
	* circular queue, it is not the past-the-end iterator and the
	* index is a valid index to that queue. PTE test is required to
	* distinguish between:
	* - An iterator to the first element of a full queue
	* (4,F,1,0): \|o]\|[*\|o\|o\|
	* - The end() iterator of a full queue
	* (4,F,1,0): \|o]\|x[o\|o\|o\|
	* Sometimes, though, users will get the PTE iterator and expect it
	* to work after growing the buffer on the tail, so we have to
	* check if the iterator is still PTE.
	*/
	bool
	dereferenceable() const
	{
	return _cq != nullptr && _cq->isValidIdx(_idx, _round);
	}

	/** InputIterator. */

	/** Equality operator.
	* Two iterators must point to the same, possibly null, circular
	* queue and the same element on it, including PTE, to be equal.
	* In case the clients the the PTE iterator and then grow on the back
	* and expect it to work, we have to check if the PTE is still PTE
	*/
	bool operator==(const iterator& that) const
	{
	return _cq == that._cq && _idx == that._idx &&
	_round == that._round;
	}

	/** Inequality operator.
	* Conversely, two iterators are different if they both point to
	* different circular queues or they point to different elements.
	*/
	bool operator!=(const iterator& that)
	{
	return !(*this == that);
	}

	/** Dereference operator. */
	reference operator*()
	{
	/* this has to be dereferenceable. */
	return (*_cq)[_idx];
	}

	const_reference operator*() const
	{
	/* this has to be dereferenceable. */
	return (*_cq)[_idx];
	}

	/** Dereference operator.
	* Rely on operator* to check for dereferenceability.
	*/
	pointer operator->()
	{
	return &((*_cq)[_idx]);
	}

	const_pointer operator->() const
	{
	return &((*_cq)[_idx]);
	}

	/** Pre-increment operator. */
	iterator& operator++()
	{
	/* this has to be dereferenceable. */
	_cq->increase(_idx);
	if (_idx == 0)
	++_round;
	return *this;
	}

	/** Post-increment operator. */
	iterator
	operator++(int)
	{
	iterator t = *this;
	++*this;
	return t;
	}

	/** ForwardIterator
	* The multipass guarantee is provided by the reliance on _idx.
	*/

	/** BidirectionalIterator requirements. */
	private:
	/** Test decrementability.
	* An iterator to a non-null circular queue is not-decrementable
	* if it is pointing to the head element, unless the queue is full
	* and we are talking about the past-the-end iterator. In that case,
	* the iterator round equals the cq round unless the head is at the
	* zero position and the round is one more than the cq round.
	*/
	bool
	decrementable() const
	{
	return _cq && !(_idx == _cq->head() &&
	(_cq->empty() \|\|
	(_idx == 0 && _round != _cq->_round + 1) \|\|
	(_idx !=0 && _round != _cq->_round)));
	}

	public:
	/** Pre-decrement operator. */
	iterator& operator--()
	{
	/* this has to be decrementable. */
	assert(decrementable());
	if (_idx == 0)
	--_round;
	_cq->decrease(_idx);
	return *this;
	}

	/** Post-decrement operator. */
	iterator operator--(int ) { iterator t = this; --this; return t; }

	/** RandomAccessIterator requirements.*/
	iterator& operator+=(const difference_type& t)
	{
	assert(_cq);
	_round += (t + _idx) / _cq->capacity();
	_idx = _cq->moduloAdd(_idx, t);
	return *this;
	}

	iterator& operator-=(const difference_type& t)
	{
	assert(_cq);

	/* C does not do euclidean division, so we have to adjust */
	if (t >= 0) {
	_round += (-t + _idx) / _cq->capacity();
	_idx = _cq->moduloSub(_idx, t);
	} else {
	*this += -t;
	}
	return *this;
	}

	/** Addition operator. */
	iterator operator+(const difference_type& t)
	{
	iterator ret(*this);
	return ret += t;
	}

	friend iterator operator+(const difference_type& t, iterator& it)
	{
	iterator ret = it;
	return ret += t;
	}

	/** Substraction operator. */
	iterator operator-(const difference_type& t)
	{
	iterator ret(*this);
	return ret -= t;
	}

	friend iterator operator-(const difference_type& t, iterator& it)
	{
	iterator ret = it;
	return ret -= t;
	}

	/** Difference operator.
	* that + ret == this
	*/
	difference_type operator-(const iterator& that)
	{
	/* If a is already at the end, we can safely return 0. */
	auto ret = _cq->sub(this->_idx, that._idx, _cq->capacity());

	if (this->_round != that._round) {
	ret += ((this->_round - that._round) * _cq->capacity());
	}
	return ret;
	}

	/** Index operator.
	* The use of * tests for dereferenceability.
	*/
	template<typename Idx>
	typename std::enable_if<std::is_integral<Idx>::value,reference>::type
	operator[](const Idx& index) { return (this + index); }

	/** Comparisons. */
	bool
	operator<(const iterator& that) const
	{
	assert(_cq && that._cq == _cq);
	return (this->_round < that._round) \|\|
	(this->_round == that._round && _idx < that._idx);
	}

	bool
	operator>(const iterator& that) const
	{ return !(*this <= that); }

	bool operator>=(const iterator& that) const
	{ return !(*this < that); }

	bool operator<=(const iterator& that) const
	{ return !(that < *this); }

	/** OutputIterator has no extra requirements.*/
	size_t idx() const { return _idx; }
	};

	public:
	using Base::operator[];

	explicit CircularQueue(uint32_t size = 0)
	: _capacity(size), _head(1), _tail(0), _empty(true), _round(0)
	{
	Base::resize(size);
	}

	/**
	* Remove all the elements in the queue.
	*
	* Note: This does not actually remove elements from the backing
	* store.
	*/
	void flush()
	{
	_head = 1;
	_round = 0;
	_tail = 0;
	_empty = true;
	}

	/** Test if the index is in the range of valid elements. */
	bool isValidIdx(size_t idx) const
	{
	/* An index is invalid if:
	* - The queue is empty.
	* (6,T,3,2): \|-\|-\|-]\|[-\|-\|x\|
	* - head is small than tail and:
	* - It is greater than both head and tail.
	* (6,F,1,3): \|-\|[o\|o\|o]\|-\|x\|
	* - It is less than both head and tail.
	* (6,F,1,3): \|x\|[o\|o\|o]\|-\|-\|
	* - It is greater than the tail and not than the head.
	* (6,F,4,1): \|o\|o]\|-\|x\|[o\|o\|
	*/
	return !(_empty \|\| (
	(_head < _tail) && (
	(_head < idx && _tail < idx) \|\|
	(_head > idx && _tail > idx)
	)) \|\| (_tail < idx && idx < _head));
	}

	/** Test if the index is in the range of valid elements.
	* The round counter is used to disambiguate aliasing.
	*/
	bool isValidIdx(size_t idx, uint32_t round) const
	{
	/* An index is valid if:
	* - The queue is not empty.
	* - round == R and
	* - index <= tail (if index > tail, that would be PTE)
	* - Either:
	* - head <= index
	* (6,F,1,3,R): \|-\|[o\|(*,r)\|o]\|-\|-\|
	* - head > tail
	* (6,F,5,3,R): \|o\|o\|(*,r)\|o]\|-\|[o\|
	* The remaining case means the the iterator is BTB:
	* (6,F,3,4,R): \|-\|-\|(x,r)\|[o\|o]\|-\|
	* - round + 1 == R and:
	* - index > tail. If index <= tail, that would be BTB:
	* (6,F,2,3,r): \| -\|- \|[(*,r)\|o]\|-\|-\|
	* (6,F,0,1,r+1): \|[o\|o]\| (x,r)\|- \|-\|-\|
	* (6,F,0,3,r+1): \|[o\|o \| (*,r)\|o]\|-\|-\|
	* - index >= head. If index < head, that would be BTB:
	* (6,F,5,2,R): \|o\|o]\|-\|-\|(x,r)\|[o\|
	* - head > tail. If head <= tail, that would be BTB:
	* (6,F,3,4,R): \|[o\|o]\|(x,r)\|-\|-\|-\|
	* Other values of the round meand that the index is PTE or BTB
	*/
	return (!_empty && (
	(round == _round && idx <= _tail && (
	_head <= idx \|\| _head > _tail)) \|\|
	(round + 1 == _round &&
	idx > _tail &&
	idx >= _head &&
	_head > _tail)
	));
	}

	reference front() { return (*this)[_head]; }
	reference back() { return (*this)[_tail]; }
	uint32_t head() const { return _head; }
	uint32_t tail() const { return _tail; }
	size_t capacity() const { return _capacity; }

	uint32_t size() const
	{
	if (_empty)
	return 0;
	else if (_head <= _tail)
	return _tail - _head + 1;
	else
	return _capacity - _head + _tail + 1;
	}

	uint32_t moduloAdd(uint32_t s1, uint32_t s2) const
	{
	return moduloAdd(s1, s2, _capacity);
	}

	uint32_t moduloSub(uint32_t s1, uint32_t s2) const
	{
	return moduloSub(s1, s2, _capacity);
	}

	/** Circularly increase the head pointer.
	* By increasing the head pointer we are removing elements from
	* the begin of the circular queue.
	* Check that the queue is not empty. And set it to empty if it
	* had only one value prior to insertion.
	*
	* @params num_elem number of elements to remove
	*/
	void pop_front(size_t num_elem = 1)
	{
	if (num_elem == 0) return;
	auto hIt = begin();
	hIt += num_elem;
	assert(hIt <= end());
	_empty = hIt == end();
	_head = hIt._idx;
	}

	/** Circularly decrease the tail pointer. */
	void pop_back()
	{
	assert (!_empty);
	_empty = _head == _tail;
	if (_tail == 0)
	--_round;
	decrease(_tail);
	}

	/** Pushes an element at the end of the queue. */
	void push_back(typename Base::value_type val)
	{
	advance_tail();
	(*this)[_tail] = val;
	}

	/** Increases the tail by one.
	* Check for wrap-arounds to update the round counter.
	*/
	void advance_tail()
	{
	increase(_tail);
	if (_tail == 0)
	++_round;

	if (_tail == _head && !_empty)
	increase(_head);

	_empty = false;
	}

	/** Increases the tail by a specified number of steps
	*
	* @param len Number of steps
	*/
	void advance_tail(uint32_t len)
	{
	for (auto idx = 0; idx < len; idx++)
	advance_tail();
	}

	/** Is the queue empty? */
	bool empty() const { return _empty; }

	/** Is the queue full?
	* A queue is full if the head is the 0^{th} element and the tail is
	* the (size-1)^{th} element, or if the head is the n^{th} element and
	* the tail the (n-1)^{th} element.
	*/
	bool full() const
	{
	return !_empty &&
	(_tail + 1 == _head \|\| (_tail + 1 == _capacity && _head == 0));
	}

	/** Iterators. */
	iterator begin()
	{
	if (_empty)
	return end();
	else if (_head > _tail)
	return iterator(this, _head, _round - 1);
	else
	return iterator(this, _head, _round);
	}

	/* TODO: This should return a const_iterator. */
	iterator begin() const
	{
	if (_empty)
	return end();
	else if (_head > _tail)
	return iterator(const_cast<CircularQueue*>(this), _head,
	_round - 1);
	else
	return iterator(const_cast<CircularQueue*>(this), _head,
	_round);
	}

	iterator end()
	{
	auto poi = moduloAdd(_tail, 1);
	auto round = _round;
	if (poi == 0)
	++round;
	return iterator(this, poi, round);
	}

	iterator end() const
	{
	auto poi = moduloAdd(_tail, 1);
	auto round = _round;
	if (poi == 0)
	++round;
	return iterator(const_cast<CircularQueue*>(this), poi, round);
	}

	/** Return an iterator to an index in the vector.
	* This poses the problem of round determination. By convention, the round
	* is picked so that isValidIndex(idx, round) is true. If that is not
	* possible, then the round value is _round, unless _tail is at the end of
	* the storage, in which case the PTE wraps up and becomes _round + 1
	*/
	iterator getIterator(size_t idx)
	{
	assert(isValidIdx(idx) \|\| moduloAdd(_tail, 1) == idx);
	if (_empty)
	return end();

	uint32_t round = _round;
	if (idx > _tail) {
	if (idx >= _head && _head > _tail) {
	round -= 1;
	}
	} else if (idx < _head && _tail + 1 == _capacity) {
	round += 1;
	}
	return iterator(this, idx, round);
	}
	};

	#endif /* __BASE_CIRCULARQUEUE_HH__ */