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.
