Tax Calculation Challenge in CoffeeScript

gistfile1.coffee

#---BEGIN---
class TaxSlice
	#---BEGIN---
	constructor: (min, max, rate) ->
		if min is null
			throw new Error "null value is not acceptable for min"

		if min >= max and max is not null
			throw new Error "min cannot be greater than or equal max"

		if rate < 0
			throw new Error "positive tax rate is required."

		@min = min
		@max = max
		@rate = rate
	#---END---

	#---BEGIN---
	calculate: (amount) ->
		if (amount == 0 or amount < @min)
			return 0

		if @max is null
			taxedAmount = amount - @min
		else
			taxedAmount = if amount > @max then @max - @min else amount - @min

		return Number (taxedAmount * @rate).toFixed(2)
    #---END---
#---END---


#---BEGIN---
class TaxCalculator
	constructor: ->
		@taxSlices = new Array

	#---BEGIN---
	addTaxSlice: (min, max, rate) ->
		slice = new TaxSlice min, max, rate
		@taxSlices.push slice
		console.info "Added tax slice #{min} #{max} #{rate}"
	#---END---

	#---BEGIN---
	calculateTax: (amount) ->
		console.info "Number of tax slices is #{@taxSlices.length}"
		taxes  = 0; count = 0
		for slice in @taxSlices
			tax = slice.calculate amount
			taxes += tax; count++
			console.info "Tax #{count} is #{tax}"
		return taxes
	#---END---
#---END---


#---BEGIN---
class App
	main: ->
		calc = new TaxCalculator
		calc.addTaxSlice 0, 5070, 0.10
		calc.addTaxSlice 5071, 8660, 0.14
		calc.addTaxSlice 8661, 14070, 0.23
		calc.addTaxSlice 14071, 212410, 0.30
		calc.addTaxSlice 212411, 40230, 0.33
		calc.addTaxSlice 40231, null, 0.45
		taxes = calc.calculateTax 5800
		alert "Taxes: " + taxes
#---END---

# Run application
app = new App
app.main()