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/**
* Copyright (C) 2020-present MongoDB, Inc.
*
* This program is free software: you can redistribute it and/or modify
* it under the terms of the Server Side Public License, version 1,
* as published by MongoDB, Inc.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* Server Side Public License for more details.
*
* You should have received a copy of the Server Side Public License
* along with this program. If not, see
* <http://www.mongodb.com/licensing/server-side-public-license>.
*
* As a special exception, the copyright holders give permission to link the
* code of portions of this program with the OpenSSL library under certain
* conditions as described in each individual source file and distribute
* linked combinations including the program with the OpenSSL library. You
* must comply with the Server Side Public License in all respects for
* all of the code used other than as permitted herein. If you modify file(s)
* with this exception, you may extend this exception to your version of the
* file(s), but you are not obligated to do so. If you do not wish to do so,
* delete this exception statement from your version. If you delete this
* exception statement from all source files in the program, then also delete
* it in the license file.
*/
#pragma once
#include <queue>
#include "mongo/db/exec/sbe/stages/stages.h"
namespace mongo::sbe {
/**
* Merges the outputs of N branches, each of which is sorted. The output is also sorted.
*
* Each branch has a pointer to a "stream" which must implement a getNext() method like so:
*
* class MyStream {
* public:
* PlanState getNext() { ... }
* };
*
* A stream may be a PlanStage but it does not have to be.
*/
template <class SortedStream>
class SortedStreamMerger final {
public:
SortedStreamMerger(std::vector<std::vector<value::SlotAccessor*>> inputKeyAccessors,
std::vector<SortedStream*> streams,
std::vector<value::SortDirection> dirs,
std::vector<value::SwitchAccessor>& outAccessors)
: _dirs(convertDirs(dirs)), _outAccessors(outAccessors), _heap(&_dirs) {
invariant(inputKeyAccessors.size() == streams.size());
invariant(!streams.empty());
const auto keySize = inputKeyAccessors.front().size();
for (size_t i = 0; i < streams.size(); ++i) {
invariant(keySize == inputKeyAccessors[i].size());
_branches.push_back(Branch{streams[i], std::move(inputKeyAccessors[i]), i});
}
}
void clear() {
_heap = decltype(_heap)(&_dirs);
}
void init() {
clear();
for (auto&& branch : _branches) {
if (branch.stream->getNext() == PlanState::ADVANCED) {
_heap.push(&branch);
}
}
_lastBranchPopped = nullptr;
}
PlanState getNext() {
if (_lastBranchPopped && _lastBranchPopped->stream->getNext() == PlanState::ADVANCED) {
// This branch was removed in the last call to getNext() on the stage.
_heap.push(_lastBranchPopped);
_lastBranchPopped = nullptr;
} else if (_heap.empty()) {
return PlanState::IS_EOF;
}
auto top = _heap.top();
_heap.pop();
_lastBranchPopped = top;
for (size_t i = 0; i < _outAccessors.size(); ++i) {
_outAccessors[i].setIndex(top->branchSwitchId);
}
return PlanState::ADVANCED;
}
private:
struct Branch {
SortedStream* stream = nullptr;
std::vector<value::SlotAccessor*> inputKeyAccessors;
size_t branchSwitchId{0};
};
class BranchComparator {
public:
BranchComparator(const std::vector<int>* dirs) : _dirs(dirs) {}
bool operator()(const Branch*, const Branch*);
private:
// Guaranteed not to be not null.
const std::vector<int>* _dirs;
};
static std::vector<int> convertDirs(const std::vector<value::SortDirection>& dirs) {
std::vector<int> outDirs;
for (auto&& dir : dirs) {
outDirs.push_back(dir == value::SortDirection::Ascending ? 1 : -1);
}
return outDirs;
}
const std::vector<int> _dirs;
// Switched output.
std::vector<value::SwitchAccessor>& _outAccessors;
// Branches are owned here.
std::vector<Branch> _branches;
// Heap for maintaining order.
std::priority_queue<Branch*, std::vector<Branch*>, BranchComparator> _heap;
// Indicates the last branch which we popped from. At the beginning of a getNext() call, this
// branch will _not_ be in the heap and must be reinserted. This is done to avoid calling
// getNext() on the branch whose value is being returned, which would require an extra copy of
// the output value.
Branch* _lastBranchPopped = nullptr;
};
template <class SortedStream>
bool SortedStreamMerger<SortedStream>::BranchComparator::operator()(const Branch* left,
const Branch* right) {
// Because this comparator is used with std::priority_queue, which is a max heap,
// return _true_ when left > right.
for (size_t i = 0; i < left->inputKeyAccessors.size(); ++i) {
auto lhsTagVal = left->inputKeyAccessors[i]->getViewOfValue();
auto rhsTagVal = right->inputKeyAccessors[i]->getViewOfValue();
auto [tag, val] = value::compareValue(
lhsTagVal.first, lhsTagVal.second, rhsTagVal.first, rhsTagVal.second);
uassert(5073804,
str::stream() << "Could not compare values with type " << lhsTagVal.first << " and "
<< rhsTagVal.first,
tag == value::TypeTags::NumberInt32);
const auto result = value::bitcastTo<int32_t>(val) * ((*_dirs)[i]);
if (result < 0) {
return false;
}
if (result > 0) {
return true;
}
}
return false;
}
} // namespace mongo::sbe
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