I'm trying to implement proper rate-limiting and I'm finding it difficult to adequately rate-limit a server when dealing with untrusted []byte inputs since I don't know how large the decompressed output will end up. I could convert the []byte into a io.Reader and perform rate limiting based on the number of bytes read from zstd.Decoder, but that seems somewhat unfortunate.

I propose the addition of:

// DecodedLength reports the decompressed length of src in bytes.
func (*Decoder) DecodedLength(src []byte) (int64, error)

It may or may not make sense as a method on Decoder.

IIUC, this should only need to perform work proportional to O(len(src)). The amount of memory it will occupy is primarily what's needed to initialize the ANS tables and to read each symbol. We would then count each literal symbol and the length of each LZ77 back-reference symbol.

This API also assists in determining what sized buffers to grab for the output.