July 2017 Fiendish Function

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

• GUESS
• REPLY
• 1
• 8
• 2
• 5
• 3
• 0
• 4
• 83
• 5
• 74
• 10
• 899

The rule was: "Square the number, then convert each digit to 9 minus the digit".

Suppose the guess was 12:

• Square the number, giving 144.
• Convert each digit to 9 minus the digit, giving 855.

So the result is 855.