- Reject brotli streams where the number of
nibbles is too large for the size of the
meta-block
- Reject brotli streams where the padding
bits after a meta-block are not all zero
- Reject brotli streams where the symbol
in the simple prefix code is not in
the symbol alphabet
This is a partially backward incompatible format change,
that makes previously valid brotli streams that contain
larger than 16MB meta-blocks invalid.
The impact of this should be minimal, since the 'bro'
command-line tool does not create larger than 2MB
meta-blocks, so the only streams this change could
break are those created by a custom brotli encoder.
This commit contains only the specification update,
implementation in the decoder and encoder will
follow in later commits.
In the following three cases we allow more choices
for the compressor, which can potentially lead to
less compressed bits.
(1) Allow brotli streams where the block counts
do not count down to exactly zero at the end
of the meta-block. This makes it possible
for compressors to sometimes choose a block
count which can be represented with less bits
than the exact block count.
(2) Remove the restriction that prefix code
descriptions with exactly one non-zero
length symbol in the code length alphabet
must have 1 bit depth. This is because
bit depth 1 requires the most bits to encode.
(3) Allow any copy length value in the last
command where the copy part is ignored.
This makes it possible for a compressor
to choose a copy length which can be
represented with the least amount of bits.
In addition to the changes above, this commit also
has a wording clarification in the overview section
where the use of the 'context ID' expression is
changed to be consistent with the rest of the
specification, i.e. that it is a function of the
last two literals or the copy length.
Add a version of the brotli encoder that compresses
each meta-block independently, only using the
original input data from previous meta-blocks
and nothing from the compressor state.
This is a proof-of-concept to show that the
current format is flexible enough to support
parallel multi-threaded compression.
This will enable processing the input in smaller
chunks than the currently default 2MB for the
slow brotli, while still benefiting from the
larger sliding window.
Remove the hard-coded constants for window size
and meta-block size.
Initialize internal storage for each metablock
separately and reserve only as much as needed
for the actual input.
The new mode can be used by setting the greedy_block_split
field of BrotliParams to true.
This commit moves all the meta-block processing code
into its own library and moves the meta-block encoding
code to brotli_bit_stream.cc from encode.cc
