private static class RunningAvg {
      private final long[] slots;
      private int          offset;
      private long     sum;

      public RunningAvg(int slotCount) {
         this.slots = new long[slotCount];
         this.offset = 0;
      }

      public void add(long value) {
          // add new value and remove old value (last in array) from sum
          sum = value - slots[(offset+1) % slots.length];
          slots[offset++ % slots.length] = value;
      }

      public long avg() {
         int sampleCount = Math.min(this.offset, this.slots.length);
         return sum / sampleCount;
      }
   }