mtortonesi/ruby-erv

require 'erv/distribution'


module ERV
  
  class WeibullDistribution < Distribution

    def initialize(opts={})
      super(opts)

      raise ArgumentError unless opts[:scale] and opts[:shape]
      @scale = opts[:scale].to_f
      @shape = opts[:shape].to_f

      raise ArgumentError, "scale parameter must be greater than zero!" unless @scale > 0.0
      raise ArgumentError, "shape parameter must be greater than zero!" unless @shape > 0.0

    end

    # For more details, see [KROESE11], section 4.2.18, algorithm 4.66 and 
    def sample
      u = @rng.rand
      x = @scale * (-Math::log(1 - u)) ** (1.0 / @shape)
    end

  end
end