[mod_dptools:playback] fix audio speed change to preserve pitch

When  a  file  is  being played  with  the  playback
application  and  the  speed  is  changed  with  eg.
uuid_fileman  speed:+1  the audio  pitch  increases.
This commit tries to preserve the pitch even when the
speed is changed.

How?
+ Use longer sample chunks (where original freq is preserved)
+ Search for the best cut position using cross-correlation
+ Crossfade adjacent chunks to each other

This is  basically a copy  of my unreviewed  PR from
the old system
https://freeswitch.org/stash/projects/FS/repos/freeswitch/pull-requests/1707/commits/b8b98bbf310c6d8f138f68af7815d0938c440a13

src/switch_ivr_play_say.c

@@ -1280,6 +1280,14 @@ SWITCH_DECLARE(switch_status_t) switch_ivr_play_file(switch_core_session_t *sess
 
 
 	for (cur = 0; switch_channel_ready(channel) && !done && cur < argc; cur++) {
+		const int sp_fadeLen = 32;
+		const int sp_cut_src_rng = 64;
+		short sp_ovrlap[32];
+		short sp_has_overlap = 0;
+		int sp_prev_idx = 0;
+		int sp_prev_cap = 0;
+		short* sp_prev = NULL;
+
 		file = argv[cur];
 		eof = 0;
 
@@ -1781,43 +1789,92 @@ SWITCH_DECLARE(switch_status_t) switch_ivr_play_file(switch_core_session_t *sess
 
 			if (!switch_test_flag(fh, SWITCH_FILE_NATIVE) && fh->speed && do_speed) {
 				float factor = 0.25f * abs(fh->speed);
-				switch_size_t newlen, supplement, step;
-				short *bp = write_frame.data;
-				switch_size_t wrote = 0;
-
-				supplement = (int) (factor * olen);
-				if (!supplement) {
-					supplement = 1;
+				short* bp = write_frame.data;
+				switch_size_t supplement = (int) (factor * olen);
+				switch_size_t newlen = (fh->speed > 0) ? olen - supplement : olen + supplement;
+				int src_rng = (fh->speed > 0 ? supplement : sp_cut_src_rng)* sp_has_overlap;
+				int datalen = newlen + src_rng + sp_fadeLen;
+				int extra = datalen - olen;
+				short* data = NULL;
+				short* currp = NULL;
+				switch_zmalloc(data, datalen * 2);
+				if (sp_prev == NULL) {
+					sp_prev_cap = olen * 16;
+					switch_zmalloc(sp_prev, sp_prev_cap * 2);
 				}
-				newlen = (fh->speed > 0) ? olen - supplement : olen + supplement;
-
-				step = (fh->speed > 0) ? (newlen / supplement) : (olen / supplement);
-
 				if (!fh->sp_audio_buffer) {
 					switch_buffer_create_dynamic(&fh->sp_audio_buffer, 1024, 1024, 0);
 				}
 
-				while ((wrote + step) < newlen) {
-					switch_buffer_write(fh->sp_audio_buffer, bp, step * 2);
-					wrote += step;
-					bp += step;
-					if (fh->speed > 0) {
-						bp++;
+				if (extra > 0) {
+					int cont = 0;
+					currp = data;
+
+					if (extra > sp_prev_idx) {
+						// not enough data
+						switch_buffer_write(fh->sp_audio_buffer, bp, newlen * 2);
+						cont = 1;
 					} else {
-						float f;
-						short s;
-						f = (float) (*bp + *(bp + 1) + *(bp - 1));
-						f /= 3;
-						s = (short) f;
-						switch_buffer_write(fh->sp_audio_buffer, &s, 2);
-						wrote++;
+						memcpy(currp, sp_prev + sp_prev_idx - extra, extra * 2);
+						memcpy(currp + extra, bp, olen * 2);
 					}
+
+					if (sp_prev_cap < 2 * olen) {
+						switch_safe_free(sp_prev);
+						sp_prev_cap = olen * 16;
+						switch_zmalloc(sp_prev, sp_prev_cap * 2);
+						sp_prev_idx = 0;
+					}
+
+					if (sp_prev_idx > olen) {
+						memmove(sp_prev, sp_prev + sp_prev_idx - olen, olen * 2);
+						sp_prev_idx = olen;
+					}
+					memcpy(sp_prev + sp_prev_idx, bp, olen * 2);
+					sp_prev_idx += olen;
+
+					if (cont) {
+						continue;
+					}
+				} else {
+					sp_prev_idx = 0;
+					extra = 0;
+					currp = bp;
 				}
-				if (wrote < newlen) {
-					switch_size_t r = newlen - wrote;
-					switch_buffer_write(fh->sp_audio_buffer, bp, r * 2);
+
+				if (fh->speed != 0) {
+					int best_cut_idx = 0;
+					if (sp_has_overlap) {
+						double best = INT_MIN;
+						for (int idx_src = 0; idx_src < src_rng; idx_src++) {
+							double cc = 0;
+							for (int i = 0; i < sp_fadeLen; i++) {
+								cc += sp_ovrlap[i] * (*(currp + idx_src + i));
+							}
+							if (cc > best) {
+								best = cc;
+								best_cut_idx = idx_src;
+							}
+						}
+					}
+					currp += best_cut_idx;
+					switch_buffer_write(fh->sp_audio_buffer, currp, newlen * 2);
+					currp += newlen;
+
+					if (sp_has_overlap) {
+						short* ret = (short*)((unsigned char*)switch_buffer_get_head_pointer(fh->sp_audio_buffer) + switch_buffer_inuse(fh->sp_audio_buffer) - 2 * newlen);
+						// crossfade
+						for (int i = 0; i < sp_fadeLen; i++) {
+							double factor = ((double)i) / sp_fadeLen;
+							*(ret + i) = (short)(sp_ovrlap[i] * (1 - factor) + *(ret + i) * factor);
+						}
+					}
+					memcpy(sp_ovrlap, currp, sp_fadeLen * 2);
+					sp_has_overlap = 1;
 				}
+
 				last_speed = fh->speed;
+				switch_safe_free(data);
 				continue;
 			}