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/*
* Copyright (C) 2017 The Android Open Source Project
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
#pragma once
#include <utils/RefBase.h>
#include <unordered_map>
#include <vector>
using namespace android;
namespace android {
namespace os {
namespace statsd {
/** Defines a hash function for sp<const AA>, returning the hash of the underlying pointer. */
template <class AA>
struct SpHash {
size_t operator()(const sp<const AA>& k) const {
return std::hash<const AA*>()(k.get());
}
};
/**
* Min priority queue for generic type AA.
* Unlike a regular priority queue, this class is also capable of removing interior elements.
* @tparam Comparator must implement [bool operator()(sp<const AA> a, sp<const AA> b)], returning
* whether a should be closer to the top of the queue than b.
*/
template <class AA, class Comparator>
class indexed_priority_queue {
public:
indexed_priority_queue();
/** Adds a into the priority queue. If already present or a==nullptr, does nothing. */
void push(sp<const AA> a);
/*
* Removes a from the priority queue. If not present or a==nullptr, does nothing.
* Returns true if a had been present (and is now removed), else false.
*/
bool remove(sp<const AA> a);
/** Removes the top element, if there is one. */
void pop();
/** Removes all elements. */
void clear();
/** Returns whether priority queue contains a (not just a copy of a, but a itself). */
bool contains(sp<const AA> a) const;
/** Returns min element. Returns nullptr iff empty(). */
sp<const AA> top() const;
/** Returns number of elements in priority queue. */
size_t size() const {
return pq.size() - 1;
} // pq is 1-indexed
/** Returns true iff priority queue is empty. */
bool empty() const {
return size() < 1;
}
private:
/** Vector representing a min-heap (1-indexed, with nullptr at 0). */
std::vector<sp<const AA>> pq;
/** Mapping of each element in pq to its index in pq (i.e. the inverse of a=pq[i]). */
std::unordered_map<sp<const AA>, size_t, SpHash<AA>> indices;
void init();
void sift_up(size_t idx);
void sift_down(size_t idx);
/** Returns whether pq[idx1] is considered higher than pq[idx2], according to Comparator. */
bool higher(size_t idx1, size_t idx2) const;
void swap_indices(size_t i, size_t j);
};
// Implementation must be done in this file due to use of template.
template <class AA, class Comparator>
indexed_priority_queue<AA, Comparator>::indexed_priority_queue() {
init();
}
template <class AA, class Comparator>
void indexed_priority_queue<AA, Comparator>::push(sp<const AA> a) {
if (a == nullptr) return;
if (contains(a)) return;
pq.push_back(a);
size_t idx = size(); // index of last element since 1-indexed
indices.insert({a, idx});
sift_up(idx); // get the pq back in order
}
template <class AA, class Comparator>
bool indexed_priority_queue<AA, Comparator>::remove(sp<const AA> a) {
if (a == nullptr) return false;
if (!contains(a)) return false;
size_t idx = indices[a];
if (idx >= pq.size()) {
return false;
}
if (idx == size()) { // if a is the last element, i.e. at index idx == size() == (pq.size()-1)
pq.pop_back();
indices.erase(a);
return true;
}
// move last element (guaranteed not to be at idx) to idx, then delete a
sp<const AA> last_a = pq.back();
pq[idx] = last_a;
pq.pop_back();
indices[last_a] = idx;
indices.erase(a);
// get the heap back in order (since the element at idx is not in order)
sift_up(idx);
sift_down(idx);
return true;
}
// The same as, but slightly more efficient than, remove(top()).
template <class AA, class Comparator>
void indexed_priority_queue<AA, Comparator>::pop() {
sp<const AA> a = top();
if (a == nullptr) return;
const size_t idx = 1;
if (idx == size()) { // if a is the last element
pq.pop_back();
indices.erase(a);
return;
}
// move last element (guaranteed not to be at idx) to idx, then delete a
sp<const AA> last_a = pq.back();
pq[idx] = last_a;
pq.pop_back();
indices[last_a] = idx;
indices.erase(a);
// get the heap back in order (since the element at idx is not in order)
sift_down(idx);
}
template <class AA, class Comparator>
void indexed_priority_queue<AA, Comparator>::clear() {
pq.clear();
indices.clear();
init();
}
template <class AA, class Comparator>
sp<const AA> indexed_priority_queue<AA, Comparator>::top() const {
if (empty()) return nullptr;
return pq[1];
}
template <class AA, class Comparator>
void indexed_priority_queue<AA, Comparator>::init() {
pq.push_back(nullptr); // so that pq is 1-indexed.
indices.insert({nullptr, 0}); // just to be consistent with pq.
}
template <class AA, class Comparator>
void indexed_priority_queue<AA, Comparator>::sift_up(size_t idx) {
while (idx > 1) {
size_t parent = idx / 2;
if (higher(idx, parent))
swap_indices(idx, parent);
else
break;
idx = parent;
}
}
template <class AA, class Comparator>
void indexed_priority_queue<AA, Comparator>::sift_down(size_t idx) {
while (2 * idx <= size()) {
size_t child = 2 * idx;
if (child < size() && higher(child + 1, child)) child++;
if (higher(child, idx))
swap_indices(child, idx);
else
break;
idx = child;
}
}
template <class AA, class Comparator>
bool indexed_priority_queue<AA, Comparator>::higher(size_t idx1, size_t idx2) const {
if (!(0u < idx1 && idx1 < pq.size() && 0u < idx2 && idx2 < pq.size())) {
return false; // got to do something.
}
return Comparator()(pq[idx1], pq[idx2]);
}
template <class AA, class Comparator>
bool indexed_priority_queue<AA, Comparator>::contains(sp<const AA> a) const {
if (a == nullptr) return false; // publicly, we pretend that nullptr is not actually in pq.
return indices.count(a) > 0;
}
template <class AA, class Comparator>
void indexed_priority_queue<AA, Comparator>::swap_indices(size_t i, size_t j) {
if (!(0u < i && i < pq.size() && 0u < j && j < pq.size())) {
return;
}
sp<const AA> val_i = pq[i];
sp<const AA> val_j = pq[j];
pq[i] = val_j;
pq[j] = val_i;
indices[val_i] = j;
indices[val_j] = i;
}
} // namespace statsd
} // namespace os
} // namespace android
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