Tiny command line utility that lets you work out values for tax or before- or after-tax earnings. `tax earnings_from x` lets you know your after-tax earnings from your before-tax earnings. `tax tax_on x` lets you know how much you'll be taxed from your before-tax earnings. `tax to_earn x` lets you know how much you'll have to earn to end up with…

tax

#!/usr/bin/env ruby
# encoding: utf-8

class TaxCalculator
  INFINITY = 1.0/0.0
  DEFAULT_RATES = {
    0.0...9440.0       => 0.0 ,
    9440.0...32010.0   => 0.2 ,
    32010.0...150000.0 => 0.4 ,
    150000.0..INFINITY => 0.45
  }

  def initialize rates=nil
    @rates = rates || DEFAULT_RATES
  end

  def earnings_from amount
    amount - tax_on(amount)
  end

  def tax_on amount
    @rates.inject(0.0) do |sum, (range, rate)|
      amount > range.first ? ([amount, range.last].min - range.first)*rate + sum : sum
    end
  end

  def to_earn amount
    @rates.sort{|(a,_),(b,_)|a.first<=>b.first}.reverse.each do |range, rate|
      e = earnings_from range.first
      return range.first + (amount-e)/(1-rate) if e < amount
    end
  end

  def to_be_taxed amount
    @rates.sort{|(a,_),(b,_)|a.first<=>b.first}.reverse.each do |range, rate|
      e = tax_on range.first
      return range.first + (amount-e)/rate if e < amount
    end
  end
end

tax_calculator = TaxCalculator.new

command = ARGV[1] ? ARGV[0] : ARGV[0] ? 'tax_on' : nil
amount  = ARGV[1] ? ARGV[1] : ARGV[0] ? ARGV[0]  : nil

if ['earnings_from', 'tax_on', 'to_earn', 'to_be_taxed'].include? command
  puts "£%.2f" % tax_calculator.send(command, amount.to_f)
else
  puts "USAGE tax [earnings_from|tax_on|to_earn|to_be_taxed] amount"
end