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authorAndreas Rheinhardt <andreas.rheinhardt@gmail.com>2020-09-23 15:17:33 +0200
committerAndreas Rheinhardt <andreas.rheinhardt@gmail.com>2020-09-26 20:38:30 +0200
commit1bf30a1beb7527887c9b466d07765d20fa3b5412 (patch)
tree9bc6f30a6fe72c385d2ede87622ed347fdcc386c /tests/ref/lavf/mp4
parent59f7d685141d724d6ef845a9c0f81106eb87771c (diff)
downloadffmpeg-1bf30a1beb7527887c9b466d07765d20fa3b5412.tar.gz
avcodec/magicyuv: Avoid AV_QSORT when creating Huffman table
The MagicYUV format stores Huffman tables in its bitstream by coding the length of a given symbol; it does not code the actual code directly, instead this is to be inferred by the rule that a symbol is to the left of every shorter symbol in the Huffman tree and that for symbols of the same length the symbol is ascending from left to right. Our decoder implemented this by first sorting the array containing length and symbol of each element according to descending length and for equal length, according to ascending symbol. Afterwards, the current state in the tree got encoded in a variable code; if the next array entry had length len, then the len most significant bits of code contained the code of this entry. Whenever an entry of the array of length len was processed, code was incremented by 1U << (32 - len). So two entries of length len have the same effect as incrementing code by 1U << (32 - (len - 1)), which corresponds to the parent node of length len - 1 of the two nodes of length len etc. This commit modifies this to avoid sorting the entries before calculating the codes. This is done by calculating how many non-leaf nodes there are on each level of the tree before calculating the codes. Afterwards every leaf node on this level gets assigned the number of nodes already on this level as code. This of course works only because the entries are already sorted by their symbol initially, so that this algorithm indeed gives ascending symbols from left to right on every level. This offers both speed- as well as (obvious) codesize advantages. With Clang 10 the number of decicycles for build_huffman decreased from 1561987 to 1228405; for GCC 9 it went from 1825096 decicyles to 1429921. These tests were carried out with a sample with 150 frames that was looped 13 times; and this was iterated 10 times. The earlier reference point here is from the point when the loop generating the codes was traversed in reverse order (as the patch reversing the order led to performance penalties). Reviewed-by: Paul B Mahol <onemda@gmail.com> Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
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