module Train::Tax::Calculator
  module WithholdingTax

    TRAIN_TAX_TABLE = [
      { lowest:       0.00, highest:  20_833.00, base:       0.00, excess: 0.00 },
      { lowest:  20_833.00, highest:  33_333.00, base:       0.00, excess: 0.20 },
      { lowest:  33_333.00, highest:  66_667.00, base:   2_500.00, excess: 0.25 },
      { lowest: 66_667.000, highest: 166_667.00, base:  10_833.33, excess: 0.30 },
      { lowest: 166_667.00, highest: 666_667.00, base:  40_833.33, excess: 0.32 },
      { lowest: 666_667.00, highest: +1.00/0.00, base: 200_833.33, excess: 0.35 },
    ]

    def self.compute(deductions, basic_salary)
      taxable_income = basic_salary - deductions

      tax_bracket = TRAIN_TAX_TABLE.select do |bracket|
        taxable_income >= bracket[:lowest] &&
        taxable_income < bracket[:highest]
      end.first

      withholding_tax = tax_bracket[:base] + ((taxable_income - tax_bracket[:lowest]) * tax_bracket[:excess])
      withholding_tax.round(2)
    end

  end
end