This changes the size limit on compressed blocks to match those of the other
block types: they may not be larger than the `Block_Maximum_Decompressed_Size`,
which is the smaller of the `Window_Size` and 128 KB, removing the additional
restriction that had been placed on `Compressed_Block`s, that they be smaller
than the decompressed content they represent.
Several things motivate removing this restriction. On the one hand, this
restriction is not useful for decoders: the decoder must nonetheless be
prepared to accept compressed blocks that are the full
`Block_Maximum_Decompressed_Size`. And on the other, this bound is actually
artificially limiting. If block representations were entirely independent,
a compressed representation of a block that is larger than the contents of the
block would be ipso facto useless, and it would be strictly better to send it
as an `Raw_Block`. However, blocks are not entirely independent, and it can
make sense to pay the cost of encoding custom entropy tables in a block, even
if that pushes that block size over the size of the data it represents,
because those tables can be re-used by subsequent blocks.
Finally, as far as I can tell, this restriction in the spec is not currently
enforced in any Zstandard implementation, nor has it ever been. This change
should therefore be safe to make.
If the current sequence has literal length of zero then an offset value
of three is handled in a special manner. While I implemented a golang
decoder I had to consult the educational decoder for clarification on
the update of the offset history in that case. This commit provides the
clarification that the offset value Repeated_Offset1-1 is handled as a
new offset is added to the offset history accordingly.
W. Felix Handte
Yann Collet
Ulrich Kunitz
Nick Terrell
Shawn Landden
Sean Purcell
Przemyslaw Skibinski