Implement ff_decode_pitch_lag() that is used by both AMR and SIPR.

Based on code written by Colin McQuillan during his SoC project.

Originally committed as revision 20540 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/acelp_pitch_delay.c

@@ -140,3 +140,47 @@ float ff_amr_set_fixed_gain(float fixed_gain_factor, float fixed_mean_energy,
 
     return val;
 }
+
+void ff_decode_pitch_lag(int *lag_int, int *lag_frac, int pitch_index,
+                         const int prev_lag_int, const int subframe,
+                         int third_as_first, int resolution)
+{
+    /* Note n * 10923 >> 15 is floor(x/3) for 0 <= n <= 32767 */
+    if (subframe == 0 || (subframe == 2 && third_as_first)) {
+
+        if (pitch_index < 197)
+            pitch_index += 59;
+        else
+            pitch_index = 3 * pitch_index - 335;
+
+    } else {
+        if (resolution == 4) {
+            int search_range_min = av_clip(prev_lag_int - 5, PITCH_DELAY_MIN,
+                                           PITCH_DELAY_MAX - 9);
+
+            // decoding with 4-bit resolution
+            if (pitch_index < 4) {
+                // integer only precision for [search_range_min, search_range_min+3]
+                pitch_index = 3 * (pitch_index + search_range_min) + 1;
+            } else if (pitch_index < 12) {
+                // 1/3 fractional precision for [search_range_min+3 1/3, search_range_min+5 2/3]
+                pitch_index += 3 * search_range_min + 7;
+            } else {
+                // integer only precision for [search_range_min+6, search_range_min+9]
+                pitch_index = 3 * (pitch_index + search_range_min - 6) + 1;
+            }
+        } else {
+            // decoding with 5 or 6 bit resolution, 1/3 fractional precision
+            pitch_index--;
+
+            if (resolution == 5) {
+                pitch_index += 3 * av_clip(prev_lag_int - 10, PITCH_DELAY_MIN,
+                                           PITCH_DELAY_MAX - 19);
+            } else
+                pitch_index += 3 * av_clip(prev_lag_int - 5, PITCH_DELAY_MIN,
+                                           PITCH_DELAY_MAX - 9);
+        }
+    }
+    *lag_int  = pitch_index * 10923 >> 15;
+    *lag_frac = pitch_index - 3 * *lag_int - 1;
+}

libavcodec/acelp_pitch_delay.h

@@ -234,4 +234,22 @@ float ff_amr_set_fixed_gain(float fixed_gain_factor, float fixed_mean_energy,
                             float *prediction_error, float energy_mean,
                             const float *pred_table);
 
+
+/**
+ * Decode the adaptive codebook index to the integer and fractional parts
+ * of the pitch lag for one subframe at 1/3 fractional precision.
+ *
+ * The choice of pitch lag is described in 3GPP TS 26.090 section 5.6.1.
+ *
+ * @param lag_int             integer part of pitch lag of the current subframe
+ * @param lag_frac            fractional part of pitch lag of the current subframe
+ * @param pitch_index         parsed adaptive codebook (pitch) index
+ * @param prev_lag_int        integer part of pitch lag for the previous subframe
+ * @param subframe            current subframe number
+ * @param third_as_first      treat the third frame the same way as the first
+ */
+void ff_decode_pitch_lag(int *lag_int, int *lag_frac, int pitch_index,
+                         const int prev_lag_int, const int subframe,
+                         int third_as_first, int resolution);
+
 #endif /* AVCODEC_ACELP_PITCH_DELAY_H */