def get_unit(key)
    @counters[key] ||= 0
    @counters[key] += @interval
    @counters[key] = @counters[key] - 1.0 if @counters[key] > 1
    @counters[key] = @counters[key] + 1.0 if @counters[key] < 0
    @counters[key]
  end
def get_unit(key)
    @counters[key] ||= 0
    @counters[key] += @interval
    @counters[key] = @counters[key] - 1.0 if @counters[key] > 1
    @counters[key] = @counters[key] + 1.0 if @counters[key] < 0
    @counters[key]
  end
Takes a hash with all keys supported by Factory, plus these keys and defaults:
  :interval => 0.01
Increment counter for key, looping it around to opposite side if it exits boundary.