1/*
2 * Copyright 2015 Google Inc.
3 *
4 * Use of this source code is governed by a BSD-style license that can be
5 * found in the LICENSE file.
6 */
7
8#ifndef SkTDPQueue_DEFINED
9#define SkTDPQueue_DEFINED
10
11#include "SkTDArray.h"
12
13/**
14 * This class implements a priority queue. T is the type of the elements in the queue. LESS is a
15 * function that compares two Ts and returns true if the first is higher priority than the second.
16 *
17 * Optionally objects may know their index into the priority queue. The queue will update the index
18 * as the objects move through the queue. This is enabled by using a non-NULL function for INDEX.
19 * When an INDEX function is provided random deletes from the queue are allowed using remove().
20 * Additionally, the * priority is allowed to change as long as priorityDidChange() is called
21 * afterwards. In debug builds the index will be set to -1 before an element is removed from the
22 * queue.
23 */
24template <typename T,
25          bool (*LESS)(const T&, const T&),
26          int* (*INDEX)(const T&) = (int* (*)(const T&))NULL>
27class SkTDPQueue : public SkNoncopyable {
28public:
29    SkTDPQueue() {}
30
31    /** Number of items in the queue. */
32    int count() const { return fArray.count(); }
33
34    /** Gets the next item in the queue without popping it. */
35    const T& peek() const { return fArray[0]; }
36    T& peek() { return fArray[0]; }
37
38    /** Removes the next item. */
39    void pop() {
40        this->validate();
41        SkDEBUGCODE(if (SkToBool(INDEX)) { *INDEX(fArray[0]) = -1; })
42        if (1 == fArray.count()) {
43            fArray.pop();
44            return;
45        }
46
47        fArray[0] = fArray[fArray.count() - 1];
48        this->setIndex(0);
49        fArray.pop();
50        this->percolateDownIfNecessary(0);
51
52        this->validate();
53    }
54
55    /** Inserts a new item in the queue based on its priority. */
56    void insert(T entry) {
57        this->validate();
58        int index = fArray.count();
59        *fArray.append() = entry;
60        this->setIndex(fArray.count() - 1);
61        this->percolateUpIfNecessary(index);
62        this->validate();
63    }
64
65    /** Random access removal. This requires that the INDEX function is non-NULL. */
66    void remove(T entry) {
67        SkASSERT(NULL != INDEX);
68        int index = *INDEX(entry);
69        SkASSERT(index >= 0 && index < fArray.count());
70        this->validate();
71        SkDEBUGCODE(*INDEX(fArray[index]) = -1;)
72        if (index == fArray.count() - 1) {
73            fArray.pop();
74            return;
75        }
76        fArray[index] = fArray[fArray.count() - 1];
77        fArray.pop();
78        this->setIndex(index);
79        this->percolateUpOrDown(index);
80        this->validate();
81    }
82
83    /** Notification that the priority of an entry has changed. This must be called after an
84        item's priority is changed to maintain correct ordering. Changing the priority is only
85        allowed if an INDEX function is provided. */
86    void priorityDidChange(T entry) {
87        SkASSERT(NULL != INDEX);
88        int index = *INDEX(entry);
89        SkASSERT(index >= 0 && index < fArray.count());
90        this->validate(index);
91        this->percolateUpOrDown(index);
92        this->validate();
93    }
94
95    /** Gets the item at index i in the priority queue (for i < this->count()). at(0) is equivalent
96        to peek(). Otherwise, there is no guarantee about ordering of elements in the queue. */
97    T at(int i) const { return fArray[i]; }
98
99private:
100    static int LeftOf(int x) { SkASSERT(x >= 0); return 2 * x + 1; }
101    static int ParentOf(int x) { SkASSERT(x > 0); return (x - 1) >> 1; }
102
103    void percolateUpOrDown(int index) {
104        SkASSERT(index >= 0);
105        if (!percolateUpIfNecessary(index)) {
106            this->validate(index);
107            this->percolateDownIfNecessary(index);
108        }
109    }
110
111    bool percolateUpIfNecessary(int index) {
112        SkASSERT(index >= 0);
113        bool percolated = false;
114        do {
115            if (0 == index) {
116                this->setIndex(index);
117                return percolated;
118            }
119            int p = ParentOf(index);
120            if (LESS(fArray[index], fArray[p])) {
121                SkTSwap(fArray[index], fArray[p]);
122                this->setIndex(index);
123                index = p;
124                percolated = true;
125            } else {
126                this->setIndex(index);
127                return percolated;
128            }
129            this->validate(index);
130        } while (true);
131    }
132
133    void percolateDownIfNecessary(int index) {
134        SkASSERT(index >= 0);
135        do {
136            int child = LeftOf(index);
137
138            if (child >= fArray.count()) {
139                // We're a leaf.
140                this->setIndex(index);
141                return;
142            }
143
144            if (child + 1 >= fArray.count()) {
145                // We only have a left child.
146                if (LESS(fArray[child], fArray[index])) {
147                    SkTSwap(fArray[child], fArray[index]);
148                    this->setIndex(child);
149                    this->setIndex(index);
150                    return;
151                }
152            } else if (LESS(fArray[child + 1], fArray[child])) {
153                // The right child is the one we should swap with, if we swap.
154                child++;
155            }
156
157            // Check if we need to swap.
158            if (LESS(fArray[child], fArray[index])) {
159                SkTSwap(fArray[child], fArray[index]);
160                this->setIndex(index);
161                index = child;
162            } else {
163                // We're less than both our children.
164                this->setIndex(index);
165                return;
166            }
167            this->validate(index);
168        } while (true);
169    }
170
171    void setIndex(int index) {
172        SkASSERT(index < fArray.count());
173        if (SkToBool(INDEX)) {
174            *INDEX(fArray[index]) = index;
175        }
176    }
177
178    void validate(int excludedIndex = -1) const {
179#ifdef SK_DEBUG
180        for (int i = 1; i < fArray.count(); ++i) {
181            int p = ParentOf(i);
182            if (excludedIndex != p && excludedIndex != i) {
183                SkASSERT(!(LESS(fArray[i], fArray[p])));
184                SkASSERT(!SkToBool(INDEX) || *INDEX(fArray[i]) == i);
185            }
186        }
187#endif
188    }
189
190    SkTDArray<T> fArray;
191
192    typedef SkNoncopyable INHERITED;
193};
194
195#endif
196