Provide decompressed file size

[djugei]

to my best understanding zstd has an optional header field telling us how big the uncompressed size is,
alternatively you could sum up all frames?

it would be nice to have
Decoder::size_hint(&self) -> Option <u64>

[gyscos]

Hi, and thanks for the report!

With the experimental flag enabled, there are two functions you can use:

• zstd_safe::get_frame_content_size: given a prefix of the frame (at least ~5 bytes long), returns the decompressed size of this frame.
• zstd_safe::find_decompressed_size: given the entire compressed content (could be a series of several frames), returns the total decompressed size (the sum of the decompressed size of each frame).

These are not available on Decoder itself - a Decoder could be in the middle of streaming, and have no memory of the header, and no knowledge of future frames in the stream.

We could at least re-export these methods in zstd itself.

[djugei]

progress: zstd::bulk::Decompressor::upper_bound does mostly what i need. issues with it:

• interface is wrong, returns an option, but its only none if a compile time feature is missing, imo it should return a straight value
• heavily overestimates on files created without a size header (for example by piping), probably unfixable
• needs access to a &[u8], basically forcing the use of mmap, this does not fit zstds usual file/writer/reader-based api