How to compress / decompress an array of bytes?

[a1880]

I've been trying to compress / decompress a vector of bytes by feeding it into a BitOutputArchive or a BitInputArchive respectively. This was rejected as unpermitted operation. Should I use BitMemoryCompressor? How?

Any help would be appreciated.

Greetings,

Axel

[rikyoz]

I've been trying to compress / decompress a vector of bytes by feeding it into a BitOutputArchive or a BitInputArchive respectively.

Usually, you don't want to directly use BitOutputArchive/BitInputArchive, and should use BitArchiveWriter/BitArchiveReader instead. This is because the former classes need a separate BitAbstractArchiveCreator/BitAbstractArchiveHandler instance, while the writer and reader classes already implement these abstractions.

This was rejected as unpermitted operation.

What is the exact error message and error code thrown by bit7z?

Should I use BitMemoryCompressor?

BitMemCompressor internally uses BitOutputArchive, so it will likely behave exactly as this latter. In general, it is useful when you want to create multiple archives from memory data using the same compression settings/callbacks. In all other cases, e.g., creating only one archive, BitArchiveWriter is the standard choice.

The same applies to the BitMemExtractor vs BitArchiveReader.

[a1880]

Thank you for your time and your gracious help.

I tried the following:

bit7z::Bit7zLibrary lib{ LR"(C:\util\7z.dll)" };

void compress(const std::vector<bit7z::byte_t>& data, std::vector<bit7z::byte_t>& compressed)
{
	try
	{
		bit7z::BitArchiveWriter writer(lib, bit7z::BitFormat::GZip);
		writer.compressTo(compressed);

		std::cout << "Compression succeeded!" << std::endl;
	}
	catch (bit7z::BitException ex)
	{
		std::cout << "Compression failed! " << ex.what() << std::endl;
	}
}

This throws an exception in compressTo().
Within writer.compressTo(), the exception is thrown in compressOut().
Variable outMemStream might be the culprit.
Result of outArc->UpdateItems() is E_INVALIDARG One or more arguments are invalid.

[rikyoz]

The problem is that, in your function, you're not adding any data to the archive (e.g., using writer.addFile(data, "filename")).
7-Zip doesn't support creating empty GZip archives, hence your error.
Also, the GZip format itself doesn't allow adding more than one file, so only one file/buffer of data can be added to the archive; otherwise, a similar error will be thrown by the compressTo method.

[a1880]

That makes sense. Thank you again!

I've now managed to make it work.
Please find my code attached as example usage of bit7z.

BZip7.zip

[rikyoz]

That makes sense. Thank you again!

No problem!

As a side note, I definitely need to improve the error message and code in such cases, as 7-Zip is really not good at reporting the causes of such errors, unfortunately.

Please find my code attached as example usage of bit7z.

I took a look at your code and it seems fine to me.
Just a note: BitArchiveReader can directly extract to a buffer, so the decompression function can be simplified as follows

void BZip7::decompress(const std::vector<bit7z::byte_t>& data, std::vector<bit7z::byte_t>& decompressed) const
{
	try
	{
		bit7z::BitArchiveReader reader(lib, data, bit7z::BitFormat::GZip);
		reader.extractTo(decompressed);
		std::cout << "Decompression succeeded!" << std::endl;
	}
	catch (bit7z::BitException ex)
	{
		std::cout << "Decompression failed! " << ex.what() << std::endl;
	}
}