From 14bd2a9f25fc0de4fb1a2d4afaef09162c51bb35 Mon Sep 17 00:00:00 2001
From: Loren Merritt <lorenm@u.washington.edu>
Date: Sat, 29 Sep 2007 05:41:27 +0000
Subject: [PATCH] replace brute force find_optimal_param() with a closed-form
solution. overall flac encoding: 4-15% faster. output is not identical to the
previous algorithm due to occasional rounding errors, but the differece is
less than .0005% bitrate.
Originally committed as revision 10612 to svn://svn.ffmpeg.org/ffmpeg/trunk
---
libavcodec/flacenc.c | 23 +++++++++++------------
1 file changed, 11 insertions(+), 12 deletions(-)
diff --git a/libavcodec/flacenc.c b/libavcodec/flacenc.c
index e997bca129e..3b01a3ba1a3 100644
--- a/libavcodec/flacenc.c
+++ b/libavcodec/flacenc.c
@@ -447,20 +447,19 @@ static void copy_samples(FlacEncodeContext *s, int16_t *samples)
#define rice_encode_count(sum, n, k) (((n)*((k)+1))+((sum-(n>>1))>>(k)))
+/**
+ * Solve for d/dk(rice_encode_count) = n-((sum-(n>>1))>>(k+1)) = 0
+ */
static int find_optimal_param(uint32_t sum, int n)
{
- int k, k_opt;
- uint32_t nbits[MAX_RICE_PARAM+1];
-
- k_opt = 0;
- nbits[0] = UINT32_MAX;
- for(k=0; k<=MAX_RICE_PARAM; k++) {
- nbits[k] = rice_encode_count(sum, n, k);
- if(nbits[k] < nbits[k_opt]) {
- k_opt = k;
- }
- }
- return k_opt;
+ int k;
+ uint32_t sum2;
+
+ if(sum <= n>>1)
+ return 0;
+ sum2 = sum-(n>>1);
+ k = av_log2(n<256 ? FASTDIV(sum2,n) : sum2/n);
+ return FFMIN(k, MAX_RICE_PARAM);
}
static uint32_t calc_optimal_rice_params(RiceContext *rc, int porder,
--
GitLab