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

#pragma once

#include <DB/Core/FieldVisitors.h>
#include <DB/AggregateFunctions/IUnaryAggregateFunction.h>
#include <DB/AggregateFunctions/UniqVariadicHash.h>
#include <DB/DataTypes/DataTypesNumberFixed.h>
#include <DB/DataTypes/DataTypeTuple.h>


namespace DB
{


/** Counts the number of unique values up to no more than specified in the parameter.
  *
  * Example: uniqUpTo(3)(UserID)
  * - will count the number of unique visitors, return 1, 2, 3 or 4 if visitors > = 4.
  *
  * For strings, a non-cryptographic hash function is used, due to which the calculation may be a bit inaccurate.
  */

template <typename T>
struct __attribute__((__packed__)) AggregateFunctionUniqUpToData
{
/** If count == threshold + 1 - this means that it is "overflowed" (values greater than threshold).
  * In this case (for example, after calling the merge function), the `data` array does not necessarily contain the initialized values
  * - example: combine a state in which there are few values, with another state that has overflowed;
  *   then set count to `threshold + 1`, and values from another state are not copied.
  */
	UInt8 count = 0;

	T data[0];


	size_t size() const
	{
		return count;
	}

	/// threshold - for how many elements there is room in a `data`.
	void insert(T x, UInt8 threshold)
	{
		/// The state is already full - nothing needs to be done.
		if (count > threshold)
			return;

		/// Linear search for the matching element.
		for (size_t i = 0; i < count; ++i)
			if (data[i] == x)
				return;

		/// Did not find the matching element. If there is room for one more element, insert it.
		if (count < threshold)
			data[count] = x;

		/// After increasing count, the state may be overflowed.
		++count;
	}

	void merge(const AggregateFunctionUniqUpToData<T> & rhs, UInt8 threshold)
	{
		if (count > threshold)
			return;

		if (rhs.count > threshold)
		{
		/// If `rhs` is overflowed, then set `count` too also overflowed for the current state.
			count = rhs.count;
			return;
		}

		for (size_t i = 0; i < rhs.count; ++i)
			insert(rhs.data[i], threshold);
	}

	void write(WriteBuffer & wb, UInt8 threshold) const
	{
		writeBinary(count, wb);

		/// Write values only if the state is not overflowed. Otherwise, they are not needed, and only the fact that the state is overflowed is important.
		if (count <= threshold)
			wb.write(reinterpret_cast<const char *>(&data[0]), count * sizeof(data[0]));
	}

	void read(ReadBuffer & rb, UInt8 threshold)
	{
		readBinary(count, rb);

		if (count <= threshold)
			rb.read(reinterpret_cast<char *>(&data[0]), count * sizeof(data[0]));
	}


	void addImpl(const IColumn & column, size_t row_num, UInt8 threshold)
	{
		insert(static_cast<const ColumnVector<T> &>(column).getData()[row_num], threshold);
	}
};


/// For strings, their hashes are remembered.
template <>
struct AggregateFunctionUniqUpToData<String> : AggregateFunctionUniqUpToData<UInt64>
{
	void addImpl(const IColumn & column, size_t row_num, UInt8 threshold)
	{
		/// Keep in mind that calculations are approximate.
		StringRef value = column.getDataAt(row_num);
		insert(CityHash64(value.data, value.size), threshold);
	}
};


constexpr UInt8 uniq_upto_max_threshold = 100;

template <typename T>
class AggregateFunctionUniqUpTo final : public IUnaryAggregateFunction<AggregateFunctionUniqUpToData<T>, AggregateFunctionUniqUpTo<T> >
{
private:
	UInt8 threshold = 5;    /// Default value if the parameter is not specified.

public:
	size_t sizeOfData() const override
	{
		return sizeof(AggregateFunctionUniqUpToData<T>) + sizeof(T) * threshold;
	}

	String getName() const override { return "uniqUpTo"; }

	DataTypePtr getReturnType() const override
	{
		return std::make_shared<DataTypeUInt64>();
	}

	void setArgument(const DataTypePtr & argument)
	{
	}

	void setParameters(const Array & params) override
	{
		if (params.size() != 1)
			throw Exception("Aggregate function " + getName() + " requires exactly one parameter.", ErrorCodes::NUMBER_OF_ARGUMENTS_DOESNT_MATCH);

		UInt64 threshold_param = applyVisitor(FieldVisitorConvertToNumber<UInt64>(), params[0]);

		if (threshold_param > uniq_upto_max_threshold)
			throw Exception("Too large parameter for aggregate function " + getName() + ". Maximum: " + toString(uniq_upto_max_threshold),
				ErrorCodes::ARGUMENT_OUT_OF_BOUND);

		threshold = threshold_param;
	}

	void addImpl(AggregateDataPtr place, const IColumn & column, size_t row_num, Arena *) const
	{
		this->data(place).addImpl(column, row_num, threshold);
	}

	void merge(AggregateDataPtr place, ConstAggregateDataPtr rhs, Arena * arena) const override
	{
		this->data(place).merge(this->data(rhs), threshold);
	}

	void serialize(ConstAggregateDataPtr place, WriteBuffer & buf) const override
	{
		this->data(place).write(buf, threshold);
	}

	void deserialize(AggregateDataPtr place, ReadBuffer & buf, Arena *) const override
	{
		this->data(place).read(buf, threshold);
	}

	void insertResultInto(ConstAggregateDataPtr place, IColumn & to) const override
	{
		static_cast<ColumnUInt64 &>(to).getData().push_back(this->data(place).size());
	}
};


/** For multiple arguments. To compute, hashes them.
  * You can pass multiple arguments as is; You can also pass one argument - a tuple.
  * But (for the possibility of effective implementation), you can not pass several arguments, among which there are tuples.
  */
template <bool argument_is_tuple>
class AggregateFunctionUniqUpToVariadic final : public IAggregateFunctionHelper<AggregateFunctionUniqUpToData<UInt64>>
{
private:
	size_t num_args = 0;
	UInt8 threshold = 5;    /// Default value if the parameter is not specified.

public:
	size_t sizeOfData() const override
	{
		return sizeof(AggregateFunctionUniqUpToData<UInt64>) + sizeof(UInt64) * threshold;
	}

	String getName() const override { return "uniqUpTo"; }

	DataTypePtr getReturnType() const override
	{
		return std::make_shared<DataTypeUInt64>();
	}

	void setArguments(const DataTypes & arguments) override
	{
		if (argument_is_tuple)
			num_args = typeid_cast<const DataTypeTuple &>(*arguments[0]).getElements().size();
		else
			num_args = arguments.size();
	}

	void setParameters(const Array & params) override
	{
		if (params.size() != 1)
			throw Exception("Aggregate function " + getName() + " requires exactly one parameter.", ErrorCodes::NUMBER_OF_ARGUMENTS_DOESNT_MATCH);

		UInt64 threshold_param = applyVisitor(FieldVisitorConvertToNumber<UInt64>(), params[0]);

		if (threshold_param > uniq_upto_max_threshold)
			throw Exception("Too large parameter for aggregate function " + getName() + ". Maximum: " + toString(uniq_upto_max_threshold),
				ErrorCodes::ARGUMENT_OUT_OF_BOUND);

		threshold = threshold_param;
	}

	void add(AggregateDataPtr place, const IColumn ** columns, size_t row_num, Arena *) const override
	{
		this->data(place).insert(UniqVariadicHash<false, argument_is_tuple>::apply(num_args, columns, row_num), threshold);
	}

	void merge(AggregateDataPtr place, ConstAggregateDataPtr rhs, Arena * arena) const override
	{
		this->data(place).merge(this->data(rhs), threshold);
	}

	void serialize(ConstAggregateDataPtr place, WriteBuffer & buf) const override
	{
		this->data(place).write(buf, threshold);
	}

	void deserialize(AggregateDataPtr place, ReadBuffer & buf, Arena *) const override
	{
		this->data(place).read(buf, threshold);
	}

	void insertResultInto(ConstAggregateDataPtr place, IColumn & to) const override
	{
		static_cast<ColumnUInt64 &>(to).getData().push_back(this->data(place).size());
	}

	static void addFree(const IAggregateFunction * that, AggregateDataPtr place, const IColumn ** columns, size_t row_num, Arena *arena)
	{
		static_cast<const AggregateFunctionUniqUpToVariadic &>(*that).add(place, columns, row_num, arena);
	}

	IAggregateFunction::AddFunc getAddressOfAddFunction() const override final { return &addFree; }
};


}