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webp-lossless-bitstream-spec: fix 'simple code' snippet

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based on https://crbug.com/webp/551#c3 & https://crbug.com/webp/551#c4

Bug: webp:551
Change-Id: I4bcc19643b0bdda999247e9c3457d6954a49e35f
This commit is contained in:
James Zern
2022-03-11 19:21:26 -08:00
parent 44dd765def
commit 232f22da5a
1 changed files with 14 additions and 10 deletions
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24
doc/webp-lossless-bitstream-spec.txt
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@@ -858,25 +858,29 @@ stream. This may be inefficient, but it is allowed by the format.
**(i) Simple Code Length Code:**
This variant is used in the special case when only 1 or 2 Huffman code lengths
are non-zero, and are in the range of \[0..255\]. All other Huffman code lengths
This variant is used in the special case when only 1 or 2 Huffman symbols are
in the range \[0, 255\] with code length `1`. All other Huffman code lengths
are implicitly zeros.
The first bit indicates the number of non-zero code lengths:
The first bit indicates the number of symbols:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
int num_code_lengths = ReadBits(1) + 1;
int num_symbols = ReadBits(1) + 1;
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The first code length is stored either using a 1-bit code for values of 0 and 1,
or using an 8-bit code for values in range \[0..255\]. The second code length,
when present, is coded as an 8-bit code.
Following are the symbol values.
This first symbol is coded using 1 or 8 bits depending on the value of
`is_first_8bits`. The range is \[0, 1\] or \[0, 255\], respectively.
The second symbol, if present, is always assumed to be in the range \[0, 255\]
and coded using 8 bits.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
int is_first_8bits = ReadBits(1);
code_lengths[0] = ReadBits(1 + 7 * is_first_8bits);
if (num_code_lengths == 2) {
code_lengths[1] = ReadBits(8);
symbol0 = ReadBits(1 + 7 * is_first_8bits);
code_lengths[symbol0] = 1;
if (num_symbols == 2) {
symbol1 = ReadBits(8);
code_lengths[symbol1] = 1;
}
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~