Ruby Calc

calc.rb

class Calc
  MINUTE  = 60
  HOUR    = 3600
  DAY     = 86400
  WEEK    = 604800

  class << self
    def time_of_day_weighted(value, time = Time.now.utc)
      value.to_f * (time.seconds_since_midnight / DAY)
    end

    def percent_difference(x1, x2)
      x1, x2 = x1.to_f, x2.to_f
      ((x1 - x2) / ((x1 + x2)/2)).abs * 100.0
    end

    def percent_change(x1, x2)
      return 0 if x1 == 0
      x1, x2 = x1.to_f, x2.to_f
      ((x2 - x1) / x1) * 100.0
    end

    # Pass sample=true if you only pass a subset of population
    def variance(population, sample = false)
      n = 0
      mean = 0.0
      s = 0.0
      population.each { |x|
        n = n + 1
        delta = x - mean
        mean = mean + (delta / n)
        s = s + delta * (x - mean)
      }

      if sample
        s / (n - 1)
      else
        s / n
      end
    end

    def standard_deviation(population, sample = false)
      Math.sqrt(variance(population, sample))
    end
  end
end