/*

 * Copyright © 2020  Google, Inc.

 *

 *  This is part of HarfBuzz, a text shaping library.

 *

 * Permission is hereby granted, without written agreement and without

 * license or royalty fees, to use, copy, modify, and distribute this

 * software and its documentation for any purpose, provided that the

 * above copyright notice and the following two paragraphs appear in

 * all copies of this software.

 *

 * IN NO EVENT SHALL THE COPYRIGHT HOLDER BE LIABLE TO ANY PARTY FOR

 * DIRECT, INDIRECT, SPECIAL, INCIDENTAL, OR CONSEQUENTIAL DAMAGES

 * ARISING OUT OF THE USE OF THIS SOFTWARE AND ITS DOCUMENTATION, EVEN

 * IF THE COPYRIGHT HOLDER HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH

 * DAMAGE.

 *

 * THE COPYRIGHT HOLDER SPECIFICALLY DISCLAIMS ANY WARRANTIES, INCLUDING,

 * BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND

 * FITNESS FOR A PARTICULAR PURPOSE.  THE SOFTWARE PROVIDED HEREUNDER IS

 * ON AN "AS IS" BASIS, AND THE COPYRIGHT HOLDER HAS NO OBLIGATION TO

 * PROVIDE MAINTENANCE, SUPPORT, UPDATES, ENHANCEMENTS, OR MODIFICATIONS.

 *

 * Google Author(s): Garret Rieger

 */

#ifndef HB_PRIORITY_QUEUE_HH

#define HB_PRIORITY_QUEUE_HH

#include "hb.hh"

#include "hb-vector.hh"

/*

 * hb_priority_queue_t

 *

 * Priority queue implemented as a binary heap. Supports extract minimum

 * and insert operations.

 *

 * The priority queue is implemented as a binary heap, which is a complete

 * binary tree. The root of the tree is the minimum element. The heap

 * property is that the priority of a node is less than or equal to the

 * priority of its children. The heap is stored in an array, with the

 * children of node i stored at indices 2i + 1 and 2i + 2.

 */

template <typename K>

struct hb_priority_queue_t

{

 private:

  typedef hb_pair_t<K, unsigned> item_t;

  hb_vector_t<item_t> heap;

 public:

  void reset () { heap.resize (0); }

  bool in_error () const { return heap.in_error (); }

  bool alloc (unsigned size)

  { return heap.alloc (size); }

#ifndef HB_OPTIMIZE_SIZE

  HB_ALWAYS_INLINE

#endif

  void insert (K priority, unsigned value)

  {

    heap.push (item_t (priority, value));

    if (unlikely (heap.in_error ())) return;

    bubble_up (heap.length - 1);

  }

#ifndef HB_OPTIMIZE_SIZE

  HB_ALWAYS_INLINE

#endif

  item_t pop_minimum ()

  {

    assert (!is_empty ());

    item_t result = heap.arrayZ[0];

    heap.arrayZ[0] = heap.arrayZ[heap.length - 1];

    heap.resize (heap.length - 1);

    if (!is_empty ())

      bubble_down (0);

    return result;

  }

  const item_t& minimum ()

  {

    return heap[0];

  }

  bool is_empty () const { return heap.length == 0; }

  explicit operator bool () const { return !is_empty (); }

  unsigned int get_population () const { return heap.length; }

  /* Sink interface. */

  hb_priority_queue_t& operator << (item_t item)

  { insert (item.first, item.second); return *this; }

 private:

  static constexpr unsigned parent (unsigned index)

  {

    return (index - 1) / 2;

  }

  static constexpr unsigned left_child (unsigned index)

  {

    return 2 * index + 1;

  }

  static constexpr unsigned right_child (unsigned index)

  {

    return 2 * index + 2;

  }

  HB_ALWAYS_INLINE

  void bubble_down (unsigned index)

  {

    repeat:

    assert (index < heap.length);

    unsigned left = left_child (index);

    unsigned right = right_child (index);

    bool has_left = left < heap.length;

    if (!has_left)

      // If there's no left, then there's also no right.

      return;

    bool has_right = right < heap.length;

    if (heap.arrayZ[index].first <= heap.arrayZ[left].first

        && (!has_right || heap.arrayZ[index].first <= heap.arrayZ[right].first))

      return;

    unsigned child;

    if (!has_right || heap.arrayZ[left].first < heap.arrayZ[right].first)

      child = left;

    else

      child = right;

    swap (index, child);

    index = child;

    goto repeat;

  }

  HB_ALWAYS_INLINE

  void bubble_up (unsigned index)

  {

    repeat:

    assert (index < heap.length);

    if (index == 0) return;

    unsigned parent_index = parent (index);

    if (heap.arrayZ[parent_index].first <= heap.arrayZ[index].first)

      return;

    swap (index, parent_index);

    index = parent_index;

    goto repeat;

  }

  void swap (unsigned a, unsigned b) noexcept

  {

    assert (a < heap.length);

    assert (b < heap.length);

    hb_swap (heap.arrayZ[a], heap.arrayZ[b]);

  }

};

#endif /* HB_PRIORITY_QUEUE_HH */