Better formula to compute total consumption

People might not start with full tank and last one might not be full
either. So counting consumption from the first full tank till the last
one.

car.cpp

@@ -359,24 +359,24 @@ double Car::consumption() const
     unsigned long int maxDistance = 0;
     unsigned long int minDistance = 999999999;
     double totalConsumption = 0;
+    double partConsumption = 0;
 
     foreach(Tank *tank, _tanklist)
     {
-        if(tank->distance() > maxDistance)
-            maxDistance = tank->distance();
-        if(tank->distance() < minDistance)
-            minDistance = tank->distance();
-        totalConsumption += tank->quantity();
-    }
-    if(maxDistance == 0) return 0;
-    foreach(Tank *tank, _tanklist)
-    {
-        if(tank->distance() == minDistance)
+        if(tank->full())
         {
-            totalConsumption -= tank->quantity();
-            break;
+            totalConsumption += partConsumption;
+            partConsumption = 0;
+            if(tank->distance() > maxDistance)
+                maxDistance = tank->distance();
+            if(tank->distance() < minDistance)
+                minDistance = tank->distance();
         }
+        if(maxDistance > 0)
+            partConsumption += tank->quantity();
+
     }
+    if((maxDistance == 0) || (maxDistance == minDistance)) return 0;
     return totalConsumption / ((maxDistance - minDistance)/ 100.0);
 }