ClickHouse/dbms/include/DB/IO/CompressedReadBuffer.h

#pragma once

#include <DB/IO/CompressedReadBufferBase.h>


namespace DB
{

class CompressedReadBuffer : public CompressedReadBufferBase, public BufferWithOwnMemory<ReadBuffer>
{
private:
	size_t size_compressed = 0;

	bool nextImpl() override
	{
		size_t size_decompressed;
		size_t size_compressed_without_checksum;
		size_compressed = readCompressedData(size_decompressed, size_compressed_without_checksum);
		if (!size_compressed)
			return false;

		memory.resize(size_decompressed);
		working_buffer = Buffer(&memory[0], &memory[size_decompressed]);

		decompress(working_buffer.begin(), size_decompressed, size_compressed_without_checksum);

		return true;
	}

public:
	CompressedReadBuffer(ReadBuffer & in_)
		: CompressedReadBufferBase(&in_), BufferWithOwnMemory<ReadBuffer>(0)
	{
	}

	size_t readBig(char * to, size_t n)
	{
		size_t bytes_read = 0;

		/// Если в буфере есть непрочитанные байты, то скопируем сколько надо в to.
		if (pos < working_buffer.end())
			bytes_read += read(to, std::min(static_cast<size_t>(working_buffer.end() - pos), n));

		/// Если надо ещё прочитать - будем, по возможности, разжимать сразу в to.
		while (bytes_read < n)
		{
			size_t size_decompressed;
			size_t size_compressed_without_checksum;

			if (!readCompressedData(size_decompressed, size_compressed_without_checksum))
				return bytes_read;

			/// Если разжатый блок помещается целиком туда, куда его надо скопировать.
			if (size_decompressed <= n - bytes_read)
			{
				decompress(to + bytes_read, size_decompressed, size_compressed_without_checksum);
				bytes_read += size_decompressed;
				bytes += size_decompressed;
			}
			else
			{
				bytes += offset();
				memory.resize(size_decompressed);
				working_buffer = Buffer(&memory[0], &memory[size_decompressed]);
				pos = working_buffer.begin();

				decompress(working_buffer.begin(), size_decompressed, size_compressed_without_checksum);

				bytes_read += read(to + bytes_read, n - bytes_read);
				break;
			}
		}

		return bytes_read;
	}

	/// Сжатый размер текущего блока.
	size_t getSizeCompressed() const
	{
		return size_compressed;
	}
};

}