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flacdsp_lpc_template: add comment to explain the CONFIG_SMALL code

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I found the optimisation of 2 samples per iteration obscured the
underlying algorithm.  I had to write it out on paper and translate into
a mathematical sum to see that the two samples are unconnected.  I hope
that if anyone else is struggling to understand the code that this will
be useful.

Reviewed-by: Christophe Gisquet <christophe.gisquet@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
This commit is contained in:
James Darnley 2014-02-14 12:40:10 +01:00 committed by Michael Niedermayer
parent 9ae8e23188
commit 91126dc481
1 changed files with 18 additions and 0 deletions
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18
libavcodec/flacdsp_lpc_template.c
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@ -139,3 +139,21 @@ static void FUNC(flac_lpc_encode_c)(int32_t *res, const int32_t *smp, int len,
}
#endif
}
/* Comment for clarity/de-obfuscation.
*
* for (int i = order; i < len; i++) {
* int32_t p = 0;
* for (int j = 0; j < order; j++) {
* int c = coefs[j];
* int s = smp[(i-1)-j];
* p += c*s;
* }
* res[i] = smp[i] - (p >> shift);
* }
*
* The CONFIG_SMALL code above simplifies to this, in the case of SAMPLE_SIZE
* not being equal to 32 (at the present time that means for 16-bit audio). The
* code above does 2 samples per iteration. Commit bfdd5bc ( made all the way
* back in 2007) says that way is faster.
*/