An NP-hard problem somewhat tamed by preprocessing and a small instance of the problem.

Coordinate geometry and graph processing. What's not to love?

Solving a discrete maths puzzle rather than a programming puzzle.

RWS monads, and reverse-engineering the puzzle input.

Using the type system keeps me on track, and lenses and monoids keep things simple.

A day of two halves: parsing and calculation.

A lesson on not overthinking things, except when I used phantom types.

First use of pattern synonyms.

The parsing got to me eventually.

Finding loops, and off-by-many errors.

Lots of use of standard list functions.

A good choice for part 1 pays dividends for part 2.

The day to practise search.

The return of the Difference Engine

A reverse-engineering puzzle, not my favourite type.

Explicit data types save the day.

A day where you really don't need to overthink things.

Brute force almost works, but a different representation pays off.

A puzzle that rewards a little bit of thought about what's actually important

A lot of scaffolding to get where I wanted to be.

Declarative code and fun with monoids!

The importance of comprehensive examples.