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LibCompress: Fix off-by-one error in generate_huffman_lengths

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Previously we would calculate the index of the first parent node as
heap.size() (which is initialized to non_zero_freqs), so in the edge
case in which all symbols had a non-zero frequency, we would use the
Size-index entry in the array for both the first symbol's leaf node,
and the first parent node.

The result would either be a non-optimal huffman code (bad), or an
illegal huffman code that would then go on to crash due to an error
check in CanonicalCode::from_bytes. (worse)

We now store parent nodes starting at heap.size() - 1, which eliminates
the potential overlap, and resolves the issue.
This commit is contained in:
Idan Horowitz 2023-12-01 20:42:54 +02:00 • committed by Tim Schumacher
parent ec081a2ef5
commit b749167506
4 changed files with 21 additions and 3 deletions
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1
Tests/LibCompress/CMakeLists.txt
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@ -12,3 +12,4 @@ foreach(source IN LISTS TEST_SOURCES)
endforeach()
install(DIRECTORY brotli-test-files DESTINATION usr/Tests/LibCompress)
install(DIRECTORY deflate-test-files DESTINATION usr/Tests/LibCompress)