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Clarify Section 7.3

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* Acknowledge the fact that the context map is conceptually really a
two-dimensional matrix with 2 different keys, but in reality stored
as a one-dimensional array.

* Mention that InverseMoveToFrontTransform will not cause the
context map to have invalid indexes. This gives someone implementing
a decoder sanity that they do not have to go through the context
map again and check that all values are less than NTREES.
This commit is contained in:
Joe Tsai 2015-10-29 09:50:19 -07:00
parent ff3897df2d
commit 542a8b776e
1 changed files with 19 additions and 10 deletions
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docs/draft-alakuijala-brotli-07.nroff
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@ -1053,10 +1053,10 @@ implicit zero.
7. Context modeling 7. Context modeling
As described in Section 2, the prefix tree used to encode a literal As described in Section 2, the prefix tree used to encode a literal
byte or a distance code depends on the context ID and the block type. byte or a distance code depends on the block type and the context ID.
This section specifies how to compute the context ID for a particular This section specifies how to compute the context ID for a particular
literal and distance code, and how to encode the context map that literal and distance code, and how to encode the context map that
maps a <context ID, block type> pair to the index of a prefix maps a <block type, context ID> pair to the index of a prefix
code in the array of literal and distance prefix codes. code in the array of literal and distance prefix codes.
.ti 0 .ti 0
@ -1190,15 +1190,21 @@ context map is an integer between 0 and 255, indicating the index
of the prefix code to be used when encoding the next literal or of the prefix code to be used when encoding the next literal or
distance. distance.
The context map is encoded as a one-dimensional array, The context maps are two-dimensional matrices, encoded as
CMAPL[0..(64 * NBLTYPESL - 1)] and CMAPD[0..(4 * NBLTYPESD - 1)]. one-dimensional arrays:
.nf
CMAPL[0..(64 * NBLTYPESL - 1)]
CMAPD[0..(4 * NBLTYPESD - 1)]
.fi
The index of the prefix code for encoding a literal or distance The index of the prefix code for encoding a literal or distance
code with context ID, CIDx, and block type, BTYPE_x, is: code with block type, BTYPE_x, and context ID, CIDx, is:
.nf
index of literal prefix code = CMAPL[64 * BTYPE_L + CIDL] index of literal prefix code = CMAPL[64 * BTYPE_L + CIDL]
index of distance prefix code = CMAPD[4 * BTYPE_D + CIDD] index of distance prefix code = CMAPD[4 * BTYPE_D + CIDD]
.fi
The values of the context map are encoded with the combination The values of the context map are encoded with the combination
of run length encoding for zero values and prefix coding. Let of run length encoding for zero values and prefix coding. Let
@ -1245,11 +1251,11 @@ for literal and distance context maps):
.fi .fi
Note that RLEMAX may be larger than the value necessary to represent Note that RLEMAX may be larger than the value necessary to represent
the longest sequence of zero values. the longest sequence of zero values. Also, the NTREES value is encoded
right before the context map as described in Section 9.2.
For the encoding of NTREES see Section 9.2. We define the We define the inverse move-to-front transform used in this specification
inverse move-to-front transform used in this specification by the by the following C language function:
following C language function:
.nf .nf
void InverseMoveToFrontTransform(uint8_t* v, int v_len) { void InverseMoveToFrontTransform(uint8_t* v, int v_len) {
@ -1270,6 +1276,9 @@ following C language function:
} }
.fi .fi
Note that the inverse move-to-front transform will not produce values
outside the [0..NTREES-1] interval.
.ti 0 .ti 0
8. Static dictionary 8. Static dictionary