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Clarify Section 7.1.

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* Provide exhaustive list of all the ways the last two bytes can be
sourced from.

* Also make a clear connection in this section that there are only 64
context IDs for literals. This is important for the indexing math
in context maps to make sense.
This commit is contained in:
Joe Tsai 2015-10-29 08:32:11 -07:00
parent 6db33debac
commit 0e4cb52a8b
1 changed files with 7 additions and 2 deletions
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docs/draft-alakuijala-brotli-07.nroff
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@ -1062,8 +1062,9 @@ code in the array of literal and distance prefix codes.
The context for encoding the next literal is defined by the last
two bytes in the stream (p1, p2, where p1 is the most recent
byte), regardless if these bytes are produced by backward
references or by literal insertions. At the start of the stream
byte), regardless of whether these bytes are produced by
uncompressed meta-blocks, backward references, static dictionary
references, or by literal insertions. At the start of the stream
p1 and p2 are initialized to zero.
There are four methods, called context modes, to compute the
@ -1159,6 +1160,10 @@ uncompressed byte the context IDs can be computed as follows:
For Signed: Context ID = (Lut2[p1] << 3) | Lut2[p2]
.fi
From the lookup tables defined above and the operations to compute the
context IDs, we can see that context IDs for literals are in the range
of 0..63.
The context modes LSB6, MSB6, UTF8, and Signed are denoted by
integers 0, 1, 2, 3.