af_rubberband: add af-command to multiply current pitch

This commit introduces the multiply-pitch af-command. Users may bind
keys to this command in order to incrementally adjust the pitch of a
track. This will probably mostly be useful for musicians trying to
transpose up and down by semi tones without having to calculate
the correct ratio beforehand.

As an example, here is an input.conf to test this feature:

    { af-command all multiply-pitch 0.9438743126816935
    } af-command all multiply-pitch 1.059463094352953

2 files changed, 22 insertions, 6 deletions

diff --git a/DOCS/man/af.rst b/DOCS/man/af.rst
index 5a13bb4495..eb139bc833 100644
--- a/DOCS/man/af.rst
+++ b/DOCS/man/af.rst
@@ -225,6 +225,12 @@ Available filters are:
         change the playback pitch at runtime. Note that speed is controlled
         using the standard ``speed`` property, not ``af-command``.
 
+    ``multiply-pitch <factor>``
+        Multiply the current value of ``<pitch-scale>`` dynamically.  For
+        example: 0.5 to go down by an octave, 1.5 to go up by a perfect fifth.
+        If you want to go up or down by semi-tones, use 1.059463094352953 and
+        0.9438743126816935
+
 ``lavfi=graph``
     Filter audio using FFmpeg's libavfilter.
 
diff --git a/audio/filter/af_rubberband.c b/audio/filter/af_rubberband.c
index a4deb3d48c..58cf077d8b 100644
--- a/audio/filter/af_rubberband.c
+++ b/audio/filter/af_rubberband.c
@@ -45,12 +45,16 @@ static void update_speed(struct af_instance *af, double new_speed)
     rubberband_set_time_ratio(p->rubber, 1.0 / p->speed);
 }
 
-static void update_pitch(struct af_instance *af, double new_pitch)
+static bool update_pitch(struct af_instance *af, double new_pitch)
 {
+    if (new_pitch < 0.01 || new_pitch > 100.0)
+        return false;
+
     struct priv *p = af->priv;
 
     p->pitch = new_pitch;
     rubberband_set_pitch_scale(p->rubber, p->pitch);
+    return true;
 }
 
 static int control(struct af_instance *af, int cmd, void *arg)
@@ -99,13 +103,19 @@ static int control(struct af_instance *af, int cmd, void *arg)
         return AF_OK;
     case AF_CONTROL_COMMAND: {
         char **args = arg;
+        char *endptr;
+        double pitch = p->pitch;
         if (!strcmp(args[0], "set-pitch")) {
-            char *endptr;
-            double pitch = strtod(args[1], &endptr);
-            if (*endptr || pitch < 0.01 || pitch > 100.0)
+            pitch = strtod(args[1], &endptr);
+            if (*endptr)
+                return CONTROL_ERROR;
+            return update_pitch(af, pitch) ? CONTROL_OK : CONTROL_ERROR;
+        } else if (!strcmp(args[0], "multiply-pitch")) {
+            double mult = strtod(args[1], &endptr);
+            if (*endptr || mult <= 0)
                 return CONTROL_ERROR;
-            update_pitch(af, pitch);
-            return CONTROL_OK;
+            pitch *= mult;
+            return update_pitch(af, pitch) ? CONTROL_OK : CONTROL_ERROR;
         } else {
             return CONTROL_ERROR;
         }