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ReadHuffmanCode: rm redundant num code lengths check

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4 bits are read ([0, 15]) with 4 added to the result. The check that the
result is not > NUM_CODE_LENGTH_CODES (19) is redundant. This makes the
check an assert for documentation purposes.

webp-lossless-bitstream-spec.txt is updated accordingly.

Bug: webp:611
Change-Id: Ie461017dc9de3da4cfe6856543d3dcf58f53a180
This commit is contained in:
James Zern 2023-08-14 17:59:42 -07:00
parent a2de25f6b6
commit aac5c5d0df
2 changed files with 2 additions and 5 deletions
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2
doc/webp-lossless-bitstream-spec.txt
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@ -904,8 +904,6 @@ First, `num_code_lengths` specifies the number of code lengths.
int num_code_lengths = 4 + ReadBits(4);
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
If `num_code_lengths` is greater than 19, the bitstream is invalid.
The code lengths are themselves encoded using prefix codes; lower-level code
lengths, `code_length_code_lengths`, first have to be read. The rest of those
`code_length_code_lengths` (according to the order in `kCodeLengthCodeOrder`)

5
src/dec/vp8l_dec.c
View File

@ -12,6 +12,7 @@
// Authors: Vikas Arora (vikaas.arora@gmail.com)
// Jyrki Alakuijala (jyrki@google.com)
#include <assert.h>
#include <stdlib.h>
#include "src/dec/alphai_dec.h"
@ -339,9 +340,7 @@ static int ReadHuffmanCode(int alphabet_size, VP8LDecoder* const dec,
int i;
int code_length_code_lengths[NUM_CODE_LENGTH_CODES] = { 0 };
const int num_codes = VP8LReadBits(br, 4) + 4;
if (num_codes > NUM_CODE_LENGTH_CODES) {
return VP8LSetError(dec, VP8_STATUS_BITSTREAM_ERROR);
}
assert(num_codes <= NUM_CODE_LENGTH_CODES);
for (i = 0; i < num_codes; ++i) {
code_length_code_lengths[kCodeLengthCodeOrder[i]] = VP8LReadBits(br, 3);