track reordering!!!

src/playbin.vala

@@ -276,4 +276,53 @@ public class Playbin : GLib.Object {
         if (this.state == STOPPED) this.play_queue_position += 1;
         this._play_queue.append (song);
     }
+
+    public void move_track (uint from, uint to)
+            requires (from < this._play_queue.get_n_items ())
+            requires (to < this._play_queue.get_n_items ())
+    {
+        debug (@"moving track $from to $to");
+
+        if (from < to) {
+            // why offset to? because if the playlist is 01234,
+            // mpv takes "move 1 to 3" to mean 02134, not 02314
+            // that is, the target is a "gap", not a playlist entry
+            // from ->  0 1 2 3 4 5
+            //   to -> 0 1 2 3 4 5 6
+            assert(this.mpv.command({
+                "playlist-move",
+                from.to_string (),
+                (to+1).to_string (),
+            }) >= 0);
+
+            // F0123T -> 0123TF
+            var additions = new Object[to-from+1];
+            for (uint i = from+1; i < to; i += 1) {
+                additions[i-from-1] = this._play_queue.get_item (i);
+            }
+            additions[to-from-1] = this._play_queue.get_item (to);
+            additions[to-from] = this._play_queue.get_item (from);
+            this._play_queue.splice(from, to-from+1, additions);
+
+            if (this.play_queue_position == from) this.play_queue_position = to;
+            else if (this.play_queue_position > from && this.play_queue_position <= to) this.play_queue_position -= 1;
+        } else if (from > to) {
+            assert(this.mpv.command({
+                "playlist-move",
+                from.to_string (),
+                to.to_string (),
+            }) >= 0);
+
+            // T0123F -> FT0123
+            var additions = new Object[from-to+1];
+            additions[0] = this._play_queue.get_item (from);
+            for (uint i = to; i < from; i += 1) {
+                additions[i-to+1] = this._play_queue.get_item (i);
+            }
+            this._play_queue.splice (to, from-to+1, additions);
+
+            if (this.play_queue_position == from) this.play_queue_position = to;
+            else if (this.play_queue_position >= to && this.play_queue_position < from) this.play_queue_position += 1;
+        }
+    }
 }

src/ui/play_queue.vala

@@ -65,6 +65,7 @@ class Ui.PlayQueueSong : Gtk.ListBoxRow {
     [GtkCallback] private bool on_drop (Value value, double x, double y) {
         var source = value as PlayQueueSong;
         debug ("dropped %u on %u", source.displayed_position, this.displayed_position);
+        this.playbin.move_track (source.displayed_position-1, this.displayed_position-1);
         return false;
     }
 }
@@ -100,7 +101,7 @@ public class Ui.PlayQueue : Adw.NavigationPage {
         var item = object as Gtk.ListItem;
         var child = new PlayQueueSong (this.playbin);
 
-        child.draggable = false; // TODO
+        child.draggable = true;
         child.show_artist = true;
         child.show_album = true;