Tip Calculator

tip.rb

#!/usr/bin/env ruby
# Tip Calculator

# Usage: ./tip <cost> <minutes_waited> [percent]

exit if ARGV.length < 2

cost    = ARGV[0].to_f.round(2)
minutes = ARGV[1].to_i
percent = !ARGV[2].nil? ? ARGV[2].to_f / 100.0 : 0.2
max     = cost / 40 * 60
scale   = ( Math.cos( minutes * Math::PI / (120 * ( cost / 41 ))) * cost * percent )
tip     = ( minutes >= max ? 0 : minutes < 5 ? cost - (cost / 5 * minutes) : scale ).round(2)

def pad(n)
  return n < 10 ? "0#{n}" : n
end

printf(
  "Cost: $%.02f, Wait: %d:%s, Tip: $%0.2f\n",
  cost, minutes / 60.floor, pad(minutes % 60), tip
)