April 2017 Fiendish Function

The Fiendish Function posed during April 2017 had the following sample guesses and replies:

  • GUESS
  • REPLY
  • 1
  • 1
  • 2
  • 11
  • 3
  • 2
  • 4
  • 12
  • 5
  • 111
  • 10
  • 1111

The rule was: "Write the number in binary, then express as the number of '1's followed by the number of '0's, and so on".

Suppose the guess was 51:

  • Write it in binary, giving 110011.
  • Then express as the number of '1's followed by the number of '0's, and so on, giving 222.

So the result is 222.