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+/*
+ * 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.
+ */
+struct hb_priority_queue_t
+{
+ private:
+  typedef hb_pair_t<int64_t, unsigned> item_t;
+  hb_vector_t<item_t> heap;
+
+ public:
+
+  void reset () { heap.resize (0); }
+
+  bool in_error () const { return heap.in_error (); }
+
+  void insert (int64_t priority, unsigned value)
+  {
+    heap.push (item_t (priority, value));
+    if (unlikely (heap.in_error ())) return;
+    bubble_up (heap.length - 1);
+  }
+
+  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;
+  }
+
+  void bubble_down (unsigned index)
+  {
+    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;
+
+    if (!has_right || heap.arrayZ[left].first < heap.arrayZ[right].first)
+    {
+      swap (index, left);
+      bubble_down (left);
+      return;
+    }
+
+    swap (index, right);
+    bubble_down (right);
+  }
+
+  void bubble_up (unsigned index)
+  {
+    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);
+    bubble_up (parent_index);
+  }
+
+  void swap (unsigned a, unsigned b)
+  {
+    assert (a < heap.length);
+    assert (b < heap.length);
+    hb_swap (heap.arrayZ[a], heap.arrayZ[b]);
+  }
+};
+
+#endif /* HB_PRIORITY_QUEUE_HH */