Calculate irish net income

calculate_income.rb

=begin

Tax Income
20% up to 32,800€
41% above 32,800€

Universal  Social Charge (USC)
2% first 10,036€
4% next 5,980€
7% on balance

Pay Related Social Insurance (PRSI)
4% Class A1

=end

require "csv"
require "awesome_print"

def calculate_income_tax(income)
  tax_break_point = 32800

  lower_rate  = tax_break_point*0.2
  high_rate   = (income - tax_break_point) * 0.41
  tax_credits = 3300
  net_tax     = lower_rate + high_rate - tax_credits
end

def calculate_prsi(income)
  income * 0.04
end

def calculate_usc(income)
  breaking_point_1 = 10036
  breaking_point_2 = 5980

  first  = breaking_point_1 * 0.02
  second = breaking_point_2 * 0.04
  third  = first + second + (income - breaking_point_1 - breaking_point_2) * 0.07
end

def calculate_net_income(income)
  income_tax = calculate_income_tax(income)
  prsi       = calculate_prsi(income)
  usc        = calculate_usc(income)
  net_income = income - (income_tax + prsi + usc)

  tax = {
      :income_tax         => income_tax,
      :prsi               => prsi,
      :usc                => usc,
      :annual_net_income  => net_income,
      :monthly_net_income => (net_income / 12).round(2)
  }

end


increase_vs_last        = 0
total_increase_vs_first = 0
start_income            = 40000
end_income              = 55000
start_monthly_income    = calculate_net_income(start_income)[:monthly_net_income]
last_monthly_income     = start_monthly_income
results                 = []

(start_income..end_income).step(500).to_a.each do |value|
  net     = calculate_net_income(value)
  pension = (0.05 * value / 12).round(2)

  increase_vs_last    = net[:monthly_net_income] - last_monthly_income
  increase_vs_first   = net[:monthly_net_income] - start_monthly_income
  last_monthly_income = net[:monthly_net_income]

  results.push(
      {
          :gross_income                     => value,
          :income_tax                       => net[:income_tax],
          :prsi                             => net[:prsi],
          :usc                              => net[:usc],
          :monthly_net_income               => net[:monthly_net_income],
          :monthly_pension                  => pension,
          :monthly_net_income_after_pension => net[:monthly_net_income] - pension,
          :increase_vs_last                 => increase_vs_last,
          :increase_vs_first                => increase_vs_first
      }
  )
end


CSV.open("data.csv", "wb") do |csv|

  csv << results[0].keys
  results.each do |hash|
    csv << hash.values
  end
end