gistfile1.rb

# - A `Thing` can be 1% - 100% green
# - This is represented on the front end with a doughnut chart
# - The doughnut chart can't be generated since it's in an email
# - To get around this, we have 25 static doughnut images in increments of 4%

# This code takes the percentage and returns the relevant image.

# I'm sure there must be a more sensible way of doing this than using the
# world's longest switch statement. Any ideas?

class Thing
  key :percent_green, Integer

  def static_doughnut
    case percent_green
    when 1,2,3,4
      "1-4.gif"
    when 5,6,7,8
      "5-8.gif"
    when 9,10,11,12
      "9-12.gif"
    when 13,14,15,16
      "13-16.gif"
    when 17,18,19,20
      "17-20.gif"
    when 21,22,23,24
      "21-24.gif"
    when 25,26,27,28
      "25-28.gif"
    when 29,30,31,32
      "29-32.gif"
    when 33,34,35,36
      "33-36.gif"
    when 37,38,39,40
      "37-40.gif"
    when 41,42,43,44
      "41-44.gif"
    when 45,46,47,48
      "45-48.gif"
    when 49,50,51,52
      "49-52.gif"
    when 53,54,55,56
      "53-56.gif"
    when 57,58,59,60
      "57-60.gif"
    when 62,62,63,64
      "61-64.gif"
    when 65,66,67,68
      "65-68.gif"
    when 69,70,71,72
      "69-72.gif"
    when 73,74,75,76
      "73-76.gif"
    when 77,78,79,80
      "77-80.gif"
    when 81,82,83,84
      "81-84.gif"
    when 85,86,87,88
      "85-88.gif"
    when 89,90,91,92
      "89-92.gif"
    when 93,94,95,96
      "93-96.gif"
    when 97,98,99,100
      "97-100.gif"
    end
  end
end

The ranges follow an arithmetic progression so you just need to get everything together to plug into the arithmetic series formula.

def static_doughnut
  min = 1
  max = 100
  return if percent_green < min or percent_green > max # maintain original interface by doing nothing if outside range

  d = 4 # must be a factor of 'max' if 'min' is 1 for evenly spaced ranges
  n = (percent_green - 1) / d + 1 
  lower_bound = min + (n - 1) * d # arithmetic series formula
  upper_bound = lower_bound + d - 1

  "#{lower_bound}-#{upper_bound}.gif"
end