loop through the lines
  parse out the key and the frequency
  save the key and frequency as a two-element array inside an array
  keep a running total of the frequencies
set an accumulator to 0
loop through the array
  replace each frequency value with a value representing the frequency
         plus the accumulator, divided by the total of the frequencies
  add the original frequency to the accumulator
get a random positive number less than 1
loop through the array one more time
  if the random number is less than the relative frequency value 
    we've found our key, print it and exit the loop