ClickHouse/dbms/include/DB/AggregateFunctions/UniqVariadicHash.h

#pragma once

#include <city.h>
#include <DB/Core/Defines.h>
#include <DB/Common/SipHash.h>
#include <DB/Common/UInt128.h>
#include <DB/Columns/ColumnTuple.h>


namespace DB
{

/** Hashes a set of arguments to the aggregate function
  *  to calculate the number of unique values
  *  and adds them to the set.
  *
  * Four options (2 x 2)
  *
  * - for approximate calculation, uses a non-cryptographic 64-bit hash function;
  * - for an accurate calculation, uses a cryptographic 128-bit hash function;
  *
  * - for several arguments passed in the usual way;
  * - for one argument-tuple.
  */

template <bool exact, bool for_tuple>
struct UniqVariadicHash;


template <>
struct UniqVariadicHash<false, false>
{
	static inline UInt64 apply(size_t num_args, const IColumn ** columns, size_t row_num)
	{
		UInt64 hash;

		const IColumn ** column = columns;
		const IColumn ** columns_end = column + num_args;

		{
			StringRef value = (*column)->getDataAt(row_num);
			hash = CityHash64(value.data, value.size);
			++column;
		}

		while (column < columns_end)
		{
			StringRef value = (*column)->getDataAt(row_num);
			hash = Hash128to64(uint128(CityHash64(value.data, value.size), hash));
			++column;
		}

		return hash;
	}
};

template <>
struct UniqVariadicHash<false, true>
{
	static inline UInt64 apply(size_t num_args, const IColumn ** columns, size_t row_num)
	{
		UInt64 hash;

		const Columns & tuple_columns = static_cast<const ColumnTuple *>(columns[0])->getColumns();

		const ColumnPtr * column = tuple_columns.data();
		const ColumnPtr * columns_end = column + num_args;

		{
			StringRef value = column->get()->getDataAt(row_num);
			hash = CityHash64(value.data, value.size);
			++column;
		}

		while (column < columns_end)
		{
			StringRef value = column->get()->getDataAt(row_num);
			hash = Hash128to64(uint128(CityHash64(value.data, value.size), hash));
			++column;
		}

		return hash;
	}
};

template <>
struct UniqVariadicHash<true, false>
{
	static inline UInt128 apply(size_t num_args, const IColumn ** columns, size_t row_num)
	{
		const IColumn ** column = columns;
		const IColumn ** columns_end = column + num_args;

		SipHash hash;

		while (column < columns_end)
		{
			(*column)->updateHashWithValue(row_num, hash);
			++column;
		}

		UInt128 key;
		hash.get128(key.first, key.second);
		return key;
	}
};

template <>
struct UniqVariadicHash<true, true>
{
	static inline UInt128 apply(size_t num_args, const IColumn ** columns, size_t row_num)
	{
		const Columns & tuple_columns = static_cast<const ColumnTuple *>(columns[0])->getColumns();

		const ColumnPtr * column = tuple_columns.data();
		const ColumnPtr * columns_end = column + num_args;

		SipHash hash;

		while (column < columns_end)
		{
			(*column)->updateHashWithValue(row_num, hash);
			++column;
		}

		UInt128 key;
		hash.get128(key.first, key.second);
		return key;
	}
};

}