[flac-dev] About a comment in stream_decoder.c
lvqcl
lvqcl.mail at gmail.com
Mon Apr 20 10:00:20 PDT 2015
I don't understand the comment in src/libFLAC/stream_decoder.c:
/*@@@@@@ technically not pessimistic enough, should be more like
if( (FLAC__uint64)order * ((((FLAC__uint64)1)<<bps)-1) * ((1<<subframe->qlp_coeff_precision)-1) < (((FLAC__uint64)-1) << 32) )
*/
if(bps + subframe->qlp_coeff_precision + FLAC__bitmath_ilog2(order) <= 32)
see http://git.xiph.org/?p=flac.git;a=commitdiff;h=8534bbe4e92300609fd6dc289d882130b69d48cd
First, I suspect that there's a typo and the intended code is
... < (((FLAC__uint64)1) << 32) )
(without an unary minus).
Second, this condition (in algebraic form)
order * (2^bps - 1) * (2^qlp_coeff_precision - 1) < 2^32
makes sense if qlp_coeff[] and data[] in FLAC__lpc_restore_signal() are
unsigned integers. But they are signed, and the condition should be
order * 2^(bps-1) * 2^(qlp_coeff_precision-1) < 2^31
which is equivalent to the current
bps + subframe->qlp_coeff_precision + FLAC__bitmath_ilog2(order) <= 32
So IMHO the comment is incorrect.
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