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/**
* Copyright (C) 2022-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 <algorithm>
#include <atomic>
#include <functional>
#include <utility>
#include <vector>
#include "mongo/bson/bsonobjbuilder.h"
#include "mongo/util/assert_util.h"
namespace mongo {
/**
* Generic histogram that supports data collection into intervals based on user-specified partitions
* over any continuous type. A binary predicate that establishes a strict weak ordering over the
* template parameter type `T` may be specified, otherwise `std::less<T>` is used. (read
* more here: https://en.cppreference.com/w/cpp/named_req/Compare).
*
* For some provided lowermost partition x and uppermost partition y, a value z will be counted
* in the (-inf, x) interval if z < x, and in the [y, inf) interval if z >= y. If no partitions are
* provided, z will be counted in the sole (-inf, inf) interval.
*/
template <typename T,
typename Cmp = std::less<T>,
typename Counter = std::atomic_int64_t> // NOLINT
class Histogram {
struct AtEnd {};
public:
explicit Histogram(std::vector<T> partitions, Cmp comparator = {})
: _partitions{std::move(partitions)},
_counts(_partitions.size() + 1),
_comparator{std::move(comparator)} {
auto ordered =
std::adjacent_find(_partitions.begin(), _partitions.end(), [&](const T& a, const T& b) {
return !_comparator(a, b);
}) == _partitions.end();
if (!ordered) {
iasserted(6101800, "Partitions must be strictly monotonically increasing");
}
}
void increment(const T& data) {
auto i = std::upper_bound(_partitions.begin(), _partitions.end(), data, _comparator) -
_partitions.begin();
++_counts[i];
}
const std::vector<T>& getPartitions() const {
return _partitions;
}
std::vector<int64_t> getCounts() const {
std::vector<int64_t> r(_counts.size());
std::transform(_counts.begin(), _counts.end(), r.begin(), [](auto&& x) { return x; });
return r;
}
/**
* An input iterator over the Histogram class that provides access to histogram buckets, each
* containing the count, lower and upper bound values. The `lower` data member set to nullptr
* signifies the lowermost extremity of the distribution. nullptr similarly represents the
* uppermost extremity when assigned to the `upper` data member.
*/
class iterator {
public:
struct Bucket {
int64_t count;
const T* lower;
const T* upper;
};
using difference_type = void;
using value_type = Bucket;
using pointer = const Bucket*;
using reference = const Bucket&;
using iterator_category = std::input_iterator_tag;
explicit iterator(const Histogram* hist) : _h{hist}, _pos{0} {}
iterator(const Histogram* hist, AtEnd) : _h{hist}, _pos{_h->_counts.size()} {}
reference operator*() const {
_b.count = _h->_counts[_pos];
_b.lower = (_pos == 0) ? nullptr : &_h->_partitions[_pos - 1];
_b.upper = (_pos == _h->_counts.size() - 1) ? nullptr : &_h->_partitions[_pos];
return _b;
}
pointer operator->() const {
return &**this;
}
iterator& operator++() {
++_pos;
return *this;
}
iterator operator++(int) {
iterator orig = *this;
++*this;
return orig;
}
friend bool operator==(const iterator& a, const iterator& b) {
return a._pos == b._pos;
}
friend bool operator!=(const iterator& a, const iterator& b) {
return !(a == b);
}
private:
const Histogram* _h;
size_t _pos; // position into _h->_counts
mutable Bucket _b;
};
iterator begin() const {
return iterator(this);
}
iterator end() const {
return iterator(this, AtEnd{});
}
protected:
std::vector<T> _partitions;
std::vector<Counter> _counts;
Cmp _comparator;
};
/**
* Appends data (i.e. count and lower/upper bounds of all buckets) of a histogram to the provided
* BSON object builder. `histKey` is used as the field name for the appended BSON object containing
* the data.
*/
template <typename... Ts>
void appendHistogram(BSONObjBuilder& bob, const Histogram<Ts...>& hist, const StringData histKey) {
BSONObjBuilder histBob(bob.subobjStart(histKey));
long long totalCount = 0;
using namespace fmt::literals;
for (auto&& [count, lower, upper] : hist) {
std::string bucketKey = "{}{}, {})"_format(lower ? "[" : "(",
lower ? "{}"_format(*lower) : "-inf",
upper ? "{}"_format(*upper) : "inf");
BSONObjBuilder(histBob.subobjStart(bucketKey))
.append("count", static_cast<long long>(count));
totalCount += count;
}
histBob.append("totalCount", totalCount);
}
} // namespace mongo
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