taxcalc.coffee

tax_payable = (band_info, gross) ->
  tax_in_band = (band) ->
    band.rate * (Math.min(band.max, gross) - Math.min(band.min, gross))

  total = 0
  bands = for band in band_info
    prev_total = total
    total += band.width + 0
    { width: band.width, rate: band.rate, min: prev_total, max: total }

  (bands.map tax_in_band).reduce (a, b) -> a + b

uk-2009-taxbands-approx.json

var bands = [ 
  { 
        width: 10000,
        rate: 0.0
  },
  { 
        width: 10000,
        rate: 0.1
  },
  { 
        width: 20000,
        rate: 0.2
  },
  { 
        width: 110000,
        rate: 0.4
  }
]