This was one of the more involved IMO past-paper questions. I really doubt I was capable of retconning a fully-worked solution subconsciously once awake.

Hmmm. FWIW I am capable of debugging while asleep. When I wake up I have a clear image of the bug, where I hadn't noticed when I went to sleep that there was a bug, nor did I know that I recalled my source code so exactly. (Unless I subconsciously noticed it when writing the code, and that information took a while to surface.)