aacenc_ltp: adjust and speed up autocorrelation calculations

There were some errors in the calculation as well as an entire
unnecessary loop to find the gain coefficient. Merge the
two loops.
Thanks to @ubitux for the suggestions and testing.

1 files changed, 29 insertions, 31 deletions

diff --git a/libavcodec/aacenc_ltp.c b/libavcodec/aacenc_ltp.c
index 4a5a8ce5e1..d24046075a 100644
--- a/libavcodec/aacenc_ltp.c
+++ b/libavcodec/aacenc_ltp.c
@@ -66,6 +66,7 @@ void ff_aac_ltp_insert_new_frame(AACEncContext *s)
             memcpy(&sce->ltp_state[0],    &sce->ltp_state[1024], 1024*sizeof(sce->ltp_state[0]));
             memcpy(&sce->ltp_state[1024], &s->planar_samples[cur_channel][2048], 1024*sizeof(sce->ltp_state[0]));
             memcpy(&sce->ltp_state[2048], &sce->ret_buf[0], 1024*sizeof(sce->ltp_state[0]));
+            sce->ics.ltp.lag = 0;
         }
         start_ch += chans;
     }
@@ -77,54 +78,44 @@ void ff_aac_ltp_insert_new_frame(AACEncContext *s)
  */
 void ff_aac_update_ltp(AACEncContext *s, SingleChannelElement *sce)
 {
-    int i, j, lag;
-    float corr, s0, s1, max_corr = 0.0f;
-    float *samples = &s->planar_samples[s->cur_channel][1024];
+    int i, j, lag, samples_num;
+    float corr, max_ratio, max_corr;
     float *pred_signal = &sce->ltp_state[0];
-    int samples_num = 2048;
+    const float *samples = &s->planar_samples[s->cur_channel][1024];
 
     if (s->profile != FF_PROFILE_AAC_LTP)
         return;
 
     /* Calculate lag */
-    for (i = 0; i < samples_num; i++) {
-        s0 = s1 = 0.0f;
-        for (j = 0; j < samples_num; j++) {
-            if (j + 1024 < i)
-                continue;
-            s0 += samples[j]*pred_signal[j-i+1024];
-            s1 += pred_signal[j-i+1024]*pred_signal[j-i+1024];
+    max_corr = 0.0f;
+    for (i = 0; i < 2048; i++) {
+        float s0 = 0.0f, s1 = 0.0f;
+        const int start = FFMAX(0, i - 1024);
+        for (j = start; j < 2048; j++) {
+            const int idx = j - i + 1024;
+            s0 += samples[j]*pred_signal[idx];
+            s1 += pred_signal[idx]*pred_signal[idx];
         }
         corr = s1 > 0.0f ? s0/sqrt(s1) : 0.0f;
         if (corr > max_corr) {
             max_corr = corr;
             lag = i;
+            max_ratio = corr/(2048-start);
         }
     }
-    lag = av_clip_uintp2(lag, 11); /* 11 bits => 2^11 = 0->2047 */
 
-    if (!lag) {
-        sce->ics.ltp.lag = lag;
+    if (lag < 1)
         return;
-    }
 
-    s0 = s1 = 0.0f;
-    for (i = 0; i < lag; i++) {
-        s0 += samples[i];
-        s1 += pred_signal[i-lag+1024];
-    }
-
-    sce->ics.ltp.coef_idx = quant_array_idx(s0/s1, ltp_coef, 8);
-    sce->ics.ltp.coef     = ltp_coef[sce->ics.ltp.coef_idx];
+    sce->ics.ltp.lag = lag = av_clip_uintp2(lag, 11);
+    sce->ics.ltp.coef_idx  = quant_array_idx(max_ratio, ltp_coef, 8);
+    sce->ics.ltp.coef      = ltp_coef[sce->ics.ltp.coef_idx];
 
     /* Predict the new samples */
-    if (lag < 1024)
-        samples_num = lag + 1024;
-    for (i = 0; i < samples_num; i++)
-        pred_signal[i+1024] = sce->ics.ltp.coef*pred_signal[i-lag+1024];
+    samples_num = 1024 + (lag < 1024 ? lag : 1024);
+    for (i = 1024; i < samples_num + 1024; i++)
+        pred_signal[i] = sce->ics.ltp.coef*pred_signal[i-lag];
     memset(&pred_signal[samples_num], 0, (2048 - samples_num)*sizeof(float));
-
-    sce->ics.ltp.lag = lag;
 }
 
 void ff_aac_adjust_common_ltp(AACEncContext *s, ChannelElement *cpe)
@@ -163,8 +154,15 @@ void ff_aac_search_for_ltp(AACEncContext *s, SingleChannelElement *sce,
     float *PCD34 = &s->scoefs[128*2];
     const int max_ltp = FFMIN(sce->ics.max_sfb, MAX_LTP_LONG_SFB);
 
-    if (sce->ics.window_sequence[0] == EIGHT_SHORT_SEQUENCE ||
-        !sce->ics.ltp.lag)
+    if (sce->ics.window_sequence[0] == EIGHT_SHORT_SEQUENCE) {
+        if (sce->ics.ltp.lag) {
+            memset(&sce->lcoeffs[0], 0.0f, 3072*sizeof(sce->lcoeffs[0]));
+            memset(&sce->ics.ltp, 0, sizeof(LongTermPrediction));
+        }
+        return;
+    }
+
+    if (!sce->ics.ltp.lag)
         return;
 
     for (w = 0; w < sce->ics.num_windows; w += sce->ics.group_len[w]) {