Update the gain widget to support updating direct monitor mix controls

The 4th Gen Solo and 2i2 have controls to set custom Mix A/B gains
when Direct Monitor is enabled. Update those controls when the Mix A/B
gains are updated so that they are remembered.

src/widget-gain.c

@@ -2,10 +2,13 @@
 // SPDX-License-Identifier: GPL-3.0-or-later
 
 #include "gtkdial.h"
+#include "stringhelper.h"
 #include "widget-gain.h"
 
 struct gain {
   struct alsa_elem *elem;
+  struct alsa_elem *direct_monitor_elem;
+  struct alsa_elem *monitor_mix_elem[2];
   GtkWidget        *vbox;
   GtkWidget        *dial;
   GtkWidget        *label;
@@ -15,8 +18,28 @@ struct gain {
 
 static void gain_changed(GtkWidget *widget, struct gain *data) {
   int value = gtk_dial_get_value(GTK_DIAL(data->dial));
-
   alsa_set_elem_value(data->elem, value);
+
+  // check if there is a corresponding Direct Monitor Mix control to
+  // update as well
+
+  // Direct Monitor control?
+  if (!data->direct_monitor_elem)
+    return;
+
+  // Direct Monitor enabled?
+  int direct_monitor = alsa_get_elem_value(data->direct_monitor_elem);
+
+  if (!direct_monitor)
+    return;
+
+  // Get the corresponding Mix control
+  struct alsa_elem *monitor_mix = data->monitor_mix_elem[direct_monitor - 1];
+  if (!monitor_mix)
+    return;
+
+  // Update it
+  alsa_set_elem_value(monitor_mix, value);
 }
 
 static void gain_updated(
@@ -56,6 +79,70 @@ static void gain_updated(
   gtk_label_set_text(GTK_LABEL(data->label), s);
 }
 
+// 4th Gen Solo and 2i2 have Mix & Direct Monitor controls which
+// interact. If direct monitor is enabled and the Mix A/B controls are
+// changed, then the Monitor Mix Playback Volume controls are changed
+// too so that the mix settings are restored when direct monitor is
+// later enabled again.
+static void find_direct_monitor_controls(struct gain *data) {
+  struct alsa_elem *elem = data->elem;
+  GArray *elems = elem->card->elems;
+
+  // Card has no direct monitor control?
+  struct alsa_elem *direct_monitor_elem = get_elem_by_prefix(
+    elems,
+    "Direct Monitor Playback"
+  );
+  if (!direct_monitor_elem)
+    return;
+
+  // Card has no mixer?
+  if (strncmp(elem->name, "Mix ", 4) != 0 ||
+      !strstr(elem->name, "Playback Volume"))
+    return;
+
+  char mix_letter = elem->name[4];
+  int input_num = get_num_from_string(elem->name);
+
+  // Find the Monitor Mix control for the 4th Gen Solo
+  if (strstr(direct_monitor_elem->name, "Switch")) {
+    char s[80];
+    sprintf(
+      s,
+      "Monitor Mix %c Input %02d Playback Volume",
+      mix_letter, input_num
+    );
+
+    struct alsa_elem *monitor_mix_elem = get_elem_by_name(elems, s);
+    if (!monitor_mix_elem)
+      return;
+
+    data->direct_monitor_elem = direct_monitor_elem;
+    data->monitor_mix_elem[0] = monitor_mix_elem;
+
+  // Find the Monitor Mix controls for the 4th Gen 2i2
+  } else if (strstr(direct_monitor_elem->name, "Enum")) {
+    for (int i = 0; i <= 1; i++) {
+      char s[80];
+      sprintf(
+        s,
+        "Monitor %d Mix %c Input %02d Playback Volume",
+        i + 1, mix_letter, input_num
+      );
+
+      struct alsa_elem *monitor_mix_elem = get_elem_by_name(elems, s);
+      if (!monitor_mix_elem)
+        return;
+
+      data->direct_monitor_elem = direct_monitor_elem;
+      data->monitor_mix_elem[i] = monitor_mix_elem;
+    }
+
+  } else {
+    fprintf(stderr, "Couldn't find direct monitor mix control\n");
+  }
+}
+
 //GList *make_gain_alsa_elem(struct alsa_elem *elem) {
 GtkWidget *make_gain_alsa_elem(
   struct alsa_elem *elem,
@@ -63,7 +150,7 @@ GtkWidget *make_gain_alsa_elem(
   int               widget_taper,
   int               can_control
 ) {
-  struct gain *data = g_malloc(sizeof(struct gain));
+  struct gain *data = calloc(1, sizeof(struct gain));
   data->elem = elem;
   data->vbox = gtk_box_new(GTK_ORIENTATION_VERTICAL, 0);
   gtk_widget_set_hexpand(data->vbox, TRUE);
@@ -110,6 +197,8 @@ GtkWidget *make_gain_alsa_elem(
 
   data->zero_is_off = zero_is_off;
 
+  find_direct_monitor_controls(data);
+
   g_signal_connect(
     data->dial, "value-changed", G_CALLBACK(gain_changed), data
   );