AuroraMiddleware/brotli
1
0
Fork 0
mirror of https://github.com/google/brotli.git synced 2024-11-21 11:10:04 +00:00
Code Issues Packages Projects Releases Wiki Activity

Clarify simple and complex prefix codes

Browse Source 
* At the beginning of the simple prefix code section, telling us that "a value
of 1 indicates the number of leading zeros" is not very helpful. Instead, it
should indicate that it means a complex prefix code and point the reader to
the relevant section (which repeats this information in more detail)

* Clearly indicate that reusing a value is an error! This seems to be the
behavior of the of the reference implementation.

* Clarify what the termination conditions are while reading the prefix codes.
Also, indicate that it is an error if the prefix tree is over-subscribed or
under-subscribed.

* Clearly state what is the maximum number of individual symbols that may be
read. This ensures that it is forbidden to an stream that continually says that
the symbols have zero length.
This commit is contained in:
Joe Tsai 2015-11-01 17:01:38 -08:00
parent c5b6b5c7c1
commit 5c869c9de2
1 changed files with 10 additions and 8 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

18
docs/draft-alakuijala-brotli-07.nroff
View File

@ -617,8 +617,9 @@ for the various prefix codes and the sections where they are defined.
The first two bits of the compressed representation of each
prefix code distinguishes between simple and complex prefix
codes. If this value is 1, then a simple prefix code follows.
Otherwise the value indicates the number of leading zeros.
codes. If this value is 1, then a simple prefix code follows
as described in this section. Otherwise, a complex prefix code
follows as described in Section 3.5.
A simple prefix code can have only up to four symbols with
non-zero code length. The format of the simple prefix code is as
@ -637,9 +638,10 @@ The value of ALPHABET_BITS depends on the alphabet of the prefix
code: it is the smallest number of bits that can represent all
symbols in the alphabet. E.g. for the alphabet of literal bytes,
ALPHABET_BITS is 8. The value of each of the NSYM symbols above is
the value of the ALPHABETS_BITS width integer value. (If the integer
value is greater than or equal to the alphabet size, then the stream
should be rejected as invalid.)
the value of the ALPHABETS_BITS width integer value. If the integer
value is greater than or equal to the alphabet size, or the value
is identical to a previous value, then the stream should be rejected
as invalid.
Note that the NSYM symbols may not be presented in sorted order. Prefix codes
of the same bit length must be assigned to the symbols in sorted order.
@ -757,7 +759,7 @@ We can now define the format of the complex prefix code as follows:
If there are at least two non-zero code lengths, any
trailing zero code lengths are omitted, i.e. the last
code length in the sequence must be non-zero. In this
case the sum of (32 >> code length) over all the non-zero
case, the sum of (32 >> code length) over all the non-zero
code lengths must equal to 32.
If the lengths have been read for the entire code length
@ -775,8 +777,8 @@ We can now define the format of the complex prefix code as follows:
length is taken to be 8 before any code length code
lengths are read.
Sequence of code lengths symbols, encoded using the code
length prefix code. Any trailing 0 or 17 must be
Sequence of at most alphabet size code lengths symbols, encoded
using the code length prefix code. Any trailing 0 or 17 must be
omitted, i.e. the last encoded code length symbol must be
between 1 and 16. The sum of (32768 >> code length) over
all the non-zero code lengths in the alphabet, including