# A very simple little class for doing some basic fast statistics sampling.
# You feed it either samples of numeric data you want measured or you call
# Stats.tick to get it to add a time delta between the last time you called it.
# When you're done either call sum, sumsq, n, min, max, mean or sd to get 
# the information.  The other option is to just call dump and see everything.
#
# It does all of this very fast and doesn't take up any memory since the samples
# are not stored but instead all the values are calculated on the fly.
class Stats
  attr_reader :sum, :sumsq, :n, :min, :max

  def initialize(name)
    @name = name
    reset
  end

  # Resets the internal counters so you can start sampling again.
  def reset
    @sum = 0.0
    @sumsq = 0.0
    @last_time = Time.new
    @n = 0.0
    @min = 0.0
    @max = 0.0
  end

  # Adds a sampling to the calculations.
  def sample(s)
    @sum += s
    @sumsq += s * s
    if @n == 0
      @min = @max = s
    else
      @min = s if @min > s
      @max = s if @max < s
    end
    @n+=1
  end

  # Dump this Stats object with an optional additional message.
  def dump(msg = "", out=STDERR)
    out.puts "[#{@name}] #{msg} : SUM=#@sum, SUMSQ=#@sumsq, N=#@n, MEAN=#{mean}, SD=#{sd}, MIN=#@min, MAX=#@max"
  end

  # Calculates and returns the mean for the data passed so far.
  def mean
    @sum / @n
  end

  # Calculates the standard deviation of the data so far.
  def sd
    # (sqrt( ((s).sumsq - ( (s).sum * (s).sum / (s).n)) / ((s).n-1) ))
    Math.sqrt( (@sumsq - ( @sum * @sum / @n)) / (@n-1) )
  end


  # Adds a time delta between now and the last time you called this.  This
  # will give you the average time between two activities.
  # 
  # An example is:
  #
  #  t = Stats.new("do_stuff")
  #  10000.times { do_stuff(); t.tick }
  #  t.dump("time")
  #
  def tick
    now = Time.now
    sample(now - @last_time)
    @last_time = now
  end
end