xref: /gem5/src/base/circular_queue.hh

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 * 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
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 * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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 * 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)
    {
        int ret = (uint32_t)(op1 - op2) % size;
        return ret >= 0 ? ret : ret + 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();
            else
                _round += (-t + _idx - _cq->capacity() + 1) / _cq->capacity();

            _idx = _cq->moduloSub(_idx, 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->moduloSub(this->_idx, that._idx);

            if (ret == 0 && this->_round != that._round) {
                ret += this->_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__ */