From 05668eea967cc1a8636d71889f4ab1c680635205 Mon Sep 17 00:00:00 2001
From: James Rhew <james@rhew.org>
Date: Sat, 23 Aug 2014 20:43:35 -0400
Subject: [PATCH] Support relative volumes for groupSetVolume()

---
 lib/player.js | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/lib/player.js b/lib/player.js
index 6cdc5375..417b77f1 100644
--- a/lib/player.js
+++ b/lib/player.js
@@ -498,9 +498,18 @@ Player.prototype.groupSetVolume = function (volumeLevel, callback) {
   var actions = [];
   var count = 0;
   var groupVolume = this.groupState.volume;
-  var deltaVolume = volumeLevel - groupVolume;
-  var newVolume;
 
+  var deltaVolume;
+  // If prefixed with + or -
+  if (/^[+-]/.test(volumeLevel)) {
+    deltaVolume = parseInt(volumeLevel);
+    volumeLevel = groupVolume + parseInt(volumeLevel);
+  } else {
+    volumeLevel = parseInt(volumeLevel);
+    deltaVolume = volumeLevel - groupVolume;
+  }
+
+  var newVolume;
   for (var uuid in this.discovery.players) {
     var player = this.discovery.players[uuid];
     if (player.coordinator.uuid != this.uuid) continue;