Basic invariants are very helpful for developing data structures and algorithms if you can come up with good ones. The more the better, as long as they can be c

Basic invariants are very helpful for developing data structures and algorithms if you can come up with good ones. The more the better, as long as they can be checked reasonably quickly. For example, I was implementing a dynamic spatial data structure a while ago, so one of the first things I did is implement a validation function that checks the entire structure for consistency errors. Then as I implemented new features, I added calls to the validation after every change, and it caught way more bugs and edge cases than I expected. Easy to accidentally swap a <= for a <, and have it complete successfully, but produce subtly incorrect results that you'd never spot by hand.