ClickHouse/dbms/include/DB/DataStreams/SquashingTransform.h

#include <DB/Core/Block.h>


namespace DB
{


/** Merging consecutive passed blocks to specified minimum size.
  *
  * (But if one of input blocks has already at least specified size,
  *  then don't merge it with neighbours, even if neighbours are small.)
  *
  * Used to prepare blocks to adequate size for INSERT queries,
  *  because such storages as Memory, StripeLog, Log, TinyLog...
  *  store or compress data in blocks exactly as passed to it,
  *  and blocks of small size are not efficient.
  *
  * Order of data is kept.
  */
class SquashingTransform
{
public:
	/// Conditions on rows and bytes are OR-ed. If one of them is zero, then corresponding condition is ignored.
	SquashingTransform(size_t min_block_size_rows, size_t min_block_size_bytes);

	/// When not ready, you need to pass more blocks to add function.
	struct Result
	{
		bool ready = false;
		Block block;

		Result(bool ready_) : ready(ready_) {}
		Result(Block && block_) : ready(true), block(std::move(block_)) {}
	};

	/** Add next block and possibly returns squashed block.
	  * At end, you need to pass empty block. As the result for last (empty) block, you will get last Result with ready = true.
	  */
	Result add(Block && block);

private:
	size_t min_block_size_rows;
	size_t min_block_size_bytes;

	Block accumulated_block;

	void append(Block && block);

	bool isEnoughSize(size_t rows, size_t bytes) const;
};

}