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// Copyright (c) 2011-present, Facebook, Inc. All rights reserved.
// This source code is licensed under both the GPLv2 (found in the
// COPYING file in the root directory) and Apache 2.0 License
// (found in the LICENSE.Apache file in the root directory).
#pragma once
#include <algorithm>
#include <cstdint>
#include <functional>
#include "port/port.h"
#include "util/autovector.h"
namespace rocksdb {
// Binary heap implementation optimized for use in multi-way merge sort.
// Comparison to std::priority_queue:
// - In libstdc++, std::priority_queue::pop() usually performs just over logN
// comparisons but never fewer.
// - std::priority_queue does not have a replace-top operation, requiring a
// pop+push. If the replacement element is the new top, this requires
// around 2logN comparisons.
// - This heap's pop() uses a "schoolbook" downheap which requires up to ~2logN
// comparisons.
// - This heap provides a replace_top() operation which requires [1, 2logN]
// comparisons. When the replacement element is also the new top, this
// takes just 1 or 2 comparisons.
//
// The last property can yield an order-of-magnitude performance improvement
// when merge-sorting real-world non-random data. If the merge operation is
// likely to take chunks of elements from the same input stream, only 1
// comparison per element is needed. In RocksDB-land, this happens when
// compacting a database where keys are not randomly distributed across L0
// files but nearby keys are likely to be in the same L0 file.
//
// The container uses the same counterintuitive ordering as
// std::priority_queue: the comparison operator is expected to provide the
// less-than relation, but top() will return the maximum.
template<typename T, typename Compare = std::less<T>>
class BinaryHeap {
public:
BinaryHeap() { }
explicit BinaryHeap(Compare cmp) : cmp_(std::move(cmp)) { }
void push(const T& value) {
data_.push_back(value);
upheap(data_.size() - 1);
}
void push(T&& value) {
data_.push_back(std::move(value));
upheap(data_.size() - 1);
}
const T& top() const {
assert(!empty());
return data_.front();
}
void replace_top(const T& value) {
assert(!empty());
data_.front() = value;
downheap(get_root());
}
void replace_top(T&& value) {
assert(!empty());
data_.front() = std::move(value);
downheap(get_root());
}
void pop() {
assert(!empty());
data_.front() = std::move(data_.back());
data_.pop_back();
if (!empty()) {
downheap(get_root());
} else {
reset_root_cmp_cache();
}
}
void swap(BinaryHeap &other) {
std::swap(cmp_, other.cmp_);
data_.swap(other.data_);
std::swap(root_cmp_cache_, other.root_cmp_cache_);
}
void clear() {
data_.clear();
reset_root_cmp_cache();
}
bool empty() const { return data_.empty(); }
size_t size() const { return data_.size(); }
void reset_root_cmp_cache() { root_cmp_cache_ = port::kMaxSizet; }
private:
static inline size_t get_root() { return 0; }
static inline size_t get_parent(size_t index) { return (index - 1) / 2; }
static inline size_t get_left(size_t index) { return 2 * index + 1; }
static inline size_t get_right(size_t index) { return 2 * index + 2; }
void upheap(size_t index) {
T v = std::move(data_[index]);
while (index > get_root()) {
const size_t parent = get_parent(index);
if (!cmp_(data_[parent], v)) {
break;
}
data_[index] = std::move(data_[parent]);
index = parent;
}
data_[index] = std::move(v);
reset_root_cmp_cache();
}
void downheap(size_t index) {
T v = std::move(data_[index]);
size_t picked_child = port::kMaxSizet;
while (1) {
const size_t left_child = get_left(index);
if (get_left(index) >= data_.size()) {
break;
}
const size_t right_child = left_child + 1;
assert(right_child == get_right(index));
picked_child = left_child;
if (index == 0 && root_cmp_cache_ < data_.size()) {
picked_child = root_cmp_cache_;
} else if (right_child < data_.size() &&
cmp_(data_[left_child], data_[right_child])) {
picked_child = right_child;
}
if (!cmp_(v, data_[picked_child])) {
break;
}
data_[index] = std::move(data_[picked_child]);
index = picked_child;
}
if (index == 0) {
// We did not change anything in the tree except for the value
// of the root node, left and right child did not change, we can
// cache that `picked_child` is the smallest child
// so next time we compare againist it directly
root_cmp_cache_ = picked_child;
} else {
// the tree changed, reset cache
reset_root_cmp_cache();
}
data_[index] = std::move(v);
}
Compare cmp_;
autovector<T> data_;
// Used to reduce number of cmp_ calls in downheap()
size_t root_cmp_cache_ = port::kMaxSizet;
};
} // namespace rocksdb
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