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87 lines
2.7 KiB
C++
87 lines
2.7 KiB
C++
#pragma once
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#include <type_traits>
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#include <Common/HashTable/HashSet.h>
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/** A hash table that allows you to clear the table in O(1).
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* Even simpler than HashSet: Key and Mapped must be POD-types.
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*
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* Instead of this class, you could just use the pair (version, key) in the HashSet as the key
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* but then the table would accumulate all the keys that it ever stored, and it was unreasonably growing.
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* This class goes a step further and considers the keys with the old version empty in the hash table.
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*/
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struct ClearableHashSetState
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{
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UInt32 version = 1;
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/// Serialization, in binary and text form.
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void write(DB::WriteBuffer & wb) const { DB::writeBinary(version, wb); }
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void writeText(DB::WriteBuffer & wb) const { DB::writeText(version, wb); }
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/// Deserialization, in binary and text form.
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void read(DB::ReadBuffer & rb) { DB::readBinary(version, rb); }
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void readText(DB::ReadBuffer & rb) { DB::readText(version, rb); }
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};
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template <typename Key, typename BaseCell>
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struct ClearableHashTableCell : public BaseCell
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{
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using State = ClearableHashSetState;
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using value_type = typename BaseCell::value_type;
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UInt32 version;
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bool isZero(const State & state) const { return version != state.version; }
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static bool isZero(const Key & /*key*/, const State & /*state*/) { return false; }
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/// Set the key value to zero.
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void setZero() { version = 0; }
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/// Do I need to store the zero key separately (that is, can a zero key be inserted into the hash table).
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static constexpr bool need_zero_value_storage = false;
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ClearableHashTableCell() {}
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ClearableHashTableCell(const Key & key_, const State & state) : BaseCell(key_, state), version(state.version) {}
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};
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template
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<
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typename Key,
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typename Hash = DefaultHash<Key>,
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typename Grower = HashTableGrower<>,
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typename Allocator = HashTableAllocator
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>
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class ClearableHashSet : public HashTable<Key, ClearableHashTableCell<Key, HashTableCell<Key, Hash, ClearableHashSetState>>, Hash, Grower, Allocator>
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{
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public:
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using Base = HashTable<Key, ClearableHashTableCell<Key, HashTableCell<Key, Hash, ClearableHashSetState>>, Hash, Grower, Allocator>;
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using typename Base::LookupResult;
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void clear()
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{
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++this->version;
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this->m_size = 0;
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}
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};
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template
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<
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typename Key,
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typename Hash = DefaultHash<Key>,
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typename Grower = HashTableGrower<>,
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typename Allocator = HashTableAllocator
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>
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class ClearableHashSetWithSavedHash: public HashTable<Key, ClearableHashTableCell<Key, HashSetCellWithSavedHash<Key, Hash, ClearableHashSetState>>, Hash, Grower, Allocator>
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{
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public:
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void clear()
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{
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++this->version;
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this->m_size = 0;
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}
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};
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