# File lib/standard/facets/math/percentile.rb, line 18
  def self.percentile(array, pcnt)
    sorted_array = array.sort

    return nil if array.length == 0

    rank  = (pcnt.to_f / 100) * (array.length + 1)
    whole = rank.truncate
 
    # if has fractional part
    if whole != rank
      s0 = sorted_array[whole - 1]
      s1 = sorted_array[whole]

      f = (rank - rank.truncate).abs

      return (f * (s1 - s0)) + s0
    else
      return sorted_array[whole - 1]
    end
  end