Reading and thinking you understand other people's code is trivially easy Reading and actually understanding other peoples' code is an unsolved problem

You draw an analogy from the function you wrote to a similar one. Maybe by someone who shared a social role similar to one you had in the past.

It just so happens that most times you think you understand something you aren't bit. Because bugs still exist we know that reading and understanding code can't be easier than writing. Also, in the past it would have take you less than a morning since the compiler was nicer. Anyway it sounds like most of your "writing" process was spent reading and understanding code.