I'm really enjoying the advent of compiler optimization from 
@mattgodbolt, but Day 11 ("Counting Bits") has been my favorite so far, just for the trick I'd never heard of:
// Clear the least-significant set bit
value &= value - 1
With 30 seconds of thought and a little scribbling, it's obvious why it works. But… wow. Yeah; it really is that easy. Sometimes I think I know a lot of things, and sometimes I discover how many things I was this old when I learned.
