Calculating VAT

vat_calc.rb

module VatCalc

  def VatCalc.vat_rate
    @vat_rate ||= BigDecimal.new('20')
  end

  def VatCalc.vat_rate=(value)
    @vat_rate = BigDecimal.new(value.to_s)
  end

  def VatCalc.vat_divisor
    BigDecimal.new((VatCalc.vat_rate / BigDecimal('100')) + BigDecimal('1'))
  end

  # http://www.hmrc.gov.uk/vat/forms-rates/rates/calculating.htm
  # Do we round up or down?
  # http://customs.hmrc.gov.uk/channelsPortalWebApp/channelsPortalWebApp.portal?_nfpb=true&_pageLabel=pageVAT_ShowContent&id=HMCE_CL_001596&propertyType=document#P1493_117367

  def incl_vat_figures
    figures = {}     

    gross = BigDecimal.new(self.to_s).round(2)
    net   = (gross / VatCalc.vat_divisor).round(2)

    figures[:gross] = gross
    figures[:net]   = net
    figures[:vat]   = gross - net
    figures[:rate]  = VatCalc.vat_rate
    figures
  end

  def plus_vat_figures
    figures = {}    

    net = BigDecimal.new(self.to_s).round(2)
    vat = ((net * VatCalc.vat_divisor) - net).round(2)

    figures[:net]   = net
    figures[:vat]   = vat 
    figures[:gross] = (net + vat).round(2)
    figures[:rate]  = VatCalc.vat_rate
    figures
  end

end

require 'bigdecimal'

include VatCalc

300.23.plus_vat_figures.each {|x,y| puts "#{x}: #{y.to_f}"}
300.23.incl_vat_figures.each {|x,y| puts "#{x}: #{y.to_f}"}