ClickHouse/dbms/Common/SpaceSaving.h
Ivan 97f2a2213e
Move all folders inside /dbms one level up (#9974)
* Move some code outside dbms/src folder
* Fix paths
2020-04-02 02:51:21 +03:00

394 lines
10 KiB
C++

#pragma once
#include <iostream>
#include <vector>
#include <boost/range/adaptor/reversed.hpp>
#include <Common/ArenaWithFreeLists.h>
#include <Common/UInt128.h>
#include <Common/HashTable/Hash.h>
#include <Common/HashTable/HashMap.h>
#include <IO/WriteBuffer.h>
#include <IO/WriteHelpers.h>
#include <IO/ReadBuffer.h>
#include <IO/ReadHelpers.h>
#include <IO/VarInt.h>
/*
* Implementation of the Filtered Space-Saving for TopK streaming analysis.
* http://www.l2f.inesc-id.pt/~fmmb/wiki/uploads/Work/misnis.ref0a.pdf
* It implements suggested reduce-and-combine algorithm from Parallel Space Saving:
* https://arxiv.org/pdf/1401.0702.pdf
*/
namespace DB
{
/*
* Arena interface to allow specialized storage of keys.
* POD keys do not require additional storage, so this interface is empty.
*/
template <typename TKey>
struct SpaceSavingArena
{
SpaceSavingArena() {}
const TKey emplace(const TKey & key) { return key; }
void free(const TKey & /*key*/) {}
};
/*
* Specialized storage for StringRef with a freelist arena.
* Keys of this type that are retained on insertion must be serialized into local storage,
* otherwise the reference would be invalid after the processed block is released.
*/
template <>
struct SpaceSavingArena<StringRef>
{
const StringRef emplace(const StringRef & key)
{
auto ptr = arena.alloc(key.size);
std::copy(key.data, key.data + key.size, ptr);
return StringRef{ptr, key.size};
}
void free(const StringRef & key)
{
if (key.data)
arena.free(const_cast<char *>(key.data), key.size);
}
private:
ArenaWithFreeLists arena;
};
template
<
typename TKey,
typename Hash = DefaultHash<TKey>,
typename Grower = HashTableGrower<>,
typename Allocator = HashTableAllocator
>
class SpaceSaving
{
private:
// Suggested constants in the paper "Finding top-k elements in data streams", chap 6. equation (24)
// Round to nearest power of 2 for cheaper binning without modulo
constexpr uint64_t nextAlphaSize(uint64_t x)
{
constexpr uint64_t ALPHA_MAP_ELEMENTS_PER_COUNTER = 6;
return 1ULL << (sizeof(uint64_t) * 8 - __builtin_clzll(x * ALPHA_MAP_ELEMENTS_PER_COUNTER));
}
public:
using Self = SpaceSaving;
struct Counter
{
Counter() {}
Counter(const TKey & k, UInt64 c = 0, UInt64 e = 0, size_t h = 0)
: key(k), slot(0), hash(h), count(c), error(e) {}
void write(WriteBuffer & wb) const
{
writeBinary(key, wb);
writeVarUInt(count, wb);
writeVarUInt(error, wb);
}
void read(ReadBuffer & rb)
{
readBinary(key, rb);
readVarUInt(count, rb);
readVarUInt(error, rb);
}
// greater() taking slot error into account
bool operator> (const Counter & b) const
{
return (count > b.count) || (count == b.count && error < b.error);
}
TKey key;
size_t slot;
size_t hash;
UInt64 count;
UInt64 error;
};
SpaceSaving(size_t c = 10) : alpha_map(nextAlphaSize(c)), m_capacity(c) {}
~SpaceSaving() { destroyElements(); }
inline size_t size() const
{
return counter_list.size();
}
inline size_t capacity() const
{
return m_capacity;
}
void clear()
{
return destroyElements();
}
void resize(size_t new_capacity)
{
counter_list.reserve(new_capacity);
alpha_map.resize(nextAlphaSize(new_capacity));
m_capacity = new_capacity;
}
void insert(const TKey & key, UInt64 increment = 1, UInt64 error = 0)
{
// Increase weight of a key that already exists
auto hash = counter_map.hash(key);
auto counter = findCounter(key, hash);
if (counter)
{
counter->count += increment;
counter->error += error;
percolate(counter);
return;
}
// Key doesn't exist, but can fit in the top K
else if (unlikely(size() < capacity()))
{
auto c = new Counter(arena.emplace(key), increment, error, hash);
push(c);
return;
}
auto min = counter_list.back();
// The key doesn't exist and cannot fit in the current top K, but
// the new key has a bigger weight and is virtually more present
// compared to the element who is less present on the set. This part
// of the code is useful for the function topKWeighted
if (increment > min->count)
{
destroyLastElement();
push(new Counter(arena.emplace(key), increment, error, hash));
return;
}
const size_t alpha_mask = alpha_map.size() - 1;
auto & alpha = alpha_map[hash & alpha_mask];
if (alpha + increment < min->count)
{
alpha += increment;
return;
}
// Erase the current minimum element
alpha_map[min->hash & alpha_mask] = min->count;
destroyLastElement();
push(new Counter(arena.emplace(key), alpha + increment, alpha + error, hash));
}
/*
* Parallel Space Saving reduction and combine step from:
* https://arxiv.org/pdf/1401.0702.pdf
*/
void merge(const Self & rhs)
{
UInt64 m1 = 0;
UInt64 m2 = 0;
if (size() == capacity())
{
m1 = counter_list.back()->count;
}
if (rhs.size() == rhs.capacity())
{
m2 = rhs.counter_list.back()->count;
}
/*
* Updated algorithm to mutate current table in place
* without mutating rhs table or creating new one
* in the first step we expect that no elements overlap
* and in the second sweep we correct the error if they do.
*/
if (m2 > 0)
{
for (auto counter : counter_list)
{
counter->count += m2;
counter->error += m2;
}
}
// The list is sorted in descending order, we have to scan in reverse
for (auto counter : boost::adaptors::reverse(rhs.counter_list))
{
size_t hash = counter_map.hash(counter->key);
if (auto current = findCounter(counter->key, hash))
{
// Subtract m2 previously added, guaranteed not negative
current->count += (counter->count - m2);
current->error += (counter->error - m2);
}
else
{
// Counters not monitored in S1
counter_list.push_back(new Counter(arena.emplace(counter->key), counter->count + m1, counter->error + m1, hash));
}
}
std::sort(counter_list.begin(), counter_list.end(), [](Counter * l, Counter * r) { return *l > *r; });
if (counter_list.size() > m_capacity)
{
for (size_t i = m_capacity; i < counter_list.size(); ++i)
{
arena.free(counter_list[i]->key);
delete counter_list[i];
}
counter_list.resize(m_capacity);
}
for (size_t i = 0; i < counter_list.size(); ++i)
counter_list[i]->slot = i;
rebuildCounterMap();
}
std::vector<Counter> topK(size_t k) const
{
std::vector<Counter> res;
for (auto counter : counter_list)
{
res.push_back(*counter);
if (res.size() == k)
break;
}
return res;
}
void write(WriteBuffer & wb) const
{
writeVarUInt(size(), wb);
for (auto counter : counter_list)
counter->write(wb);
writeVarUInt(alpha_map.size(), wb);
for (auto alpha : alpha_map)
writeVarUInt(alpha, wb);
}
void read(ReadBuffer & rb)
{
destroyElements();
size_t count = 0;
readVarUInt(count, rb);
for (size_t i = 0; i < count; ++i)
{
auto counter = new Counter();
counter->read(rb);
counter->hash = counter_map.hash(counter->key);
push(counter);
}
readAlphaMap(rb);
}
void readAlphaMap(ReadBuffer & rb)
{
size_t alpha_size = 0;
readVarUInt(alpha_size, rb);
for (size_t i = 0; i < alpha_size; ++i)
{
UInt64 alpha = 0;
readVarUInt(alpha, rb);
alpha_map.push_back(alpha);
}
}
protected:
void push(Counter * counter)
{
counter->slot = counter_list.size();
counter_list.push_back(counter);
counter_map[counter->key] = counter;
percolate(counter);
}
// This is equivallent to one step of bubble sort
void percolate(Counter * counter)
{
while (counter->slot > 0)
{
auto next = counter_list[counter->slot - 1];
if (*counter > *next)
{
std::swap(next->slot, counter->slot);
std::swap(counter_list[next->slot], counter_list[counter->slot]);
}
else
break;
}
}
private:
void destroyElements()
{
for (auto counter : counter_list)
{
arena.free(counter->key);
delete counter;
}
counter_map.clear();
counter_list.clear();
alpha_map.clear();
}
void destroyLastElement()
{
auto last_element = counter_list.back();
arena.free(last_element->key);
delete last_element;
counter_list.pop_back();
++removed_keys;
if (removed_keys * 2 > counter_map.size())
rebuildCounterMap();
}
Counter * findCounter(const TKey & key, size_t hash)
{
auto it = counter_map.find(key, hash);
if (!it)
return nullptr;
return it->getMapped();
}
void rebuildCounterMap()
{
removed_keys = 0;
counter_map.clear();
for (auto counter : counter_list)
counter_map[counter->key] = counter;
}
using CounterMap = HashMap<TKey, Counter *, Hash, Grower, Allocator>;
CounterMap counter_map;
std::vector<Counter *> counter_list;
std::vector<UInt64> alpha_map;
SpaceSavingArena<TKey> arena;
size_t m_capacity;
size_t removed_keys = 0;
};
}