less a few checks.

Income calculator

STUDENT_LOAN_RATE = 0.09 # 9%
STUDENT_LOAN_THRESHOLD = 15_000

BASIC_TAX_THRESHOLD = 9_440
BASIC_TAX_RATE = 0.20

HIGHER_TAX_RATE = 0.40
HIGHER_TAX_THRESHOLD = 32_010

ADDITIONAL_TAX_RATE = 0.50
ADDITIONA_TAX_THRESHOLD = 150_001

NATIONAL_INSURANCE_RATE = 0.12
NATIONAL_INSURANNCE_THRESHOLD = 7_748

class Income
  @@student_loan_rate = STUDENT_LOAN_RATE
  @@student_loan_threshold = STUDENT_LOAN_THRESHOLD

  @@basic_tax_threshold = BASIC_TAX_THRESHOLD
  @@basic_tax_rate = BASIC_TAX_RATE

  @@higher_tax_rate = HIGHER_TAX_RATE
  @@higher_tax_threshold = HIGHER_TAX_THRESHOLD

  @@additional_tax_rate = ADDITIONAL_TAX_RATE
  @@additiona_tax_threshold = ADDITIONA_TAX_THRESHOLD

  @@national_insurance_rate = NATIONAL_INSURANCE_RATE
  @@national_insurannce_threshold = NATIONAL_INSURANNCE_THRESHOLD

  def self.student_loan_rate=(n)
    @@student_loan_rate = n
  end
  def self.student_loan_threshold=(n)
    @@student_loan_threshold = n
  end
  def self.basic_tax_threshold=(n)
    @@basic_tax_threshold = n
  end
  def self.basic_tax_rate=(n)
    @@basic_tax_rate = n
  end
  def self.higher_tax_rate=(n)
    @@higher_tax_rate = n
  end
  def self.higher_tax_threshold=(n)
    @@higher_tax_threshold = n
  end
  def self.additional_tax_rate=(n)
    @@additional_tax_rate = n
  end
  def self.additiona_tax_threshold=(n)
    @@additiona_tax_threshold = n
  end
  def self.national_insurance_rate=(n)
    @@national_insurance_rate = n
  end
  def self.national_insurannce_threshold=(n)
    @@national_insurannce_threshold = n
  end
  def self.student_loan_rate
    @@student_loan_rate
  end
  def self.student_loan_threshold
    @@student_loan_threshold
  end
  def self.basic_tax_threshold
    @@basic_tax_threshold
  end
  def self.basic_tax_rate
    @@basic_tax_rate
  end
  def self.higher_tax_rate
    @@higher_tax_rate
  end
  def self.higher_tax_threshold
    @@higher_tax_threshold
  end
  def self.additional_tax_rate
    @@additional_tax_rate
  end
  def self.additiona_tax_threshold
    @@additiona_tax_threshold
  end
  def self.national_insurance_rate
    @@national_insurance_rate
  end
  def self.national_insurannce_threshold
    @@national_insurannce_threshold
  end

  def initialize(opt = {})
    @student = opt[:student] ? true : false
    @income = opt[:income] || 0
  end

  def income=(income)
    @income = income
  end
  def income
    @income
  end
  def student=(s=false)
    @student = s ? true : false
  end
  def student?
    @student
  end

  def self.calculate_tax(amount)
    tax = 0

    if amount > @@basic_tax_threshold
      tax += (amount - @@basic_tax_threshold) * @@basic_tax_rate
    end

    if amount > @@higher_tax_threshold
      tax += (amount - @@higher_tax_threshold) * @@higher_tax_rate
    end

    if amount > @@additiona_tax_threshold
      tax += (amount - @@additiona_tax_threshold) * @@additional_tax_rate
    end

    tax.round(2)
  end

  def calculate_tax(amount=false)
    amount ||= @income
    self.class.calculate_tax(amount)
  end

  def calculate_nat_ins(amount=false)
    amount ||= @income

    nat_ins = 0
    if amount > @@national_insurannce_threshold
      nat_ins += (amount - @@national_insurannce_threshold) * @@national_insurance_rate
    end

    nat_ins.round(2)
  end

  def calculate_student_deduction(amount=false)
    return 0 if !student?

    amount ||= @income

    payment = 0
    if amount > @@student_loan_threshold
      payment += (amount - @@student_loan_threshold) * @@student_loan_rate
    end

    # student? ? payment.round(2) : 0
    payment.round(2)
  end

  def total_deductions(amount=false)
    amount ||= @income

    if student?
      calculate_tax(amount) + calculate_nat_ins(amount) + calculate_student_deduction(amount)
    else
      calculate_tax(amount) + calculate_nat_ins(amount)
    end
  end

  def net_salary(amount=false)
    amount ||= @income

    amount - total_deductions(amount).round(2)
  end

  def show_earnings
    format="%20s\t%12s\t%12s\t%12s\n"
    printf(format, "", "Yearly", "Monthly", "Weekly")
    printf(format, "", "------", "-------", "------")

    format="%20s\t%12.2f\t%12.2f\t%12.2f\n"
    salary = net_salary

    taxable = @income - BASIC_TAX_THRESHOLD
    printf(format, "=Taxable", taxable, (taxable)/12, (taxable)/52 )

    printf(format, "-Tax due", calculate_tax, (calculate_tax)/12, (calculate_tax)/52 )
    printf(format, "-Nat ins due", calculate_nat_ins, (calculate_nat_ins)/12, (calculate_nat_ins)/52 )

    printf(format, "-Student repayments", calculate_student_deduction, (calculate_student_deduction)/12, (calculate_student_deduction)/52 ) if student?


    printf(format, "=Total deductions", total_deductions, (total_deductions/12), (total_deductions/52) )
    printf(format, "=Net", salary, (salary/12), (salary/52) )
  end
end