# File lib/tour/facets/array/percentile.rb, line 12
  def percentile(p)
    sorted_array = self.sort
    rank = (p.to_f / 100) * (self.length + 1)

    if self.length == 0
      return nil
    elsif rank.to_i == rank #fractional_part?
      sample_0 = sorted_array[rank.truncate - 1]
      sample_1 = sorted_array[rank.truncate]

      fractional_part = rank.abs.modulo(1)
      return (fractional_part * (sample_1 - sample_0)) + sample_0
    else
      return sorted_array[rank.to_i - 1]
    end    
  end