Advent Of Code

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An unofficial home for the advent of code community on programming.dev!

Advent of Code is an annual Advent calendar of small programming puzzles for a variety of skill sets and skill levels that can be solved in any programming language you like.

AoC 2024

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🤖 - 2024 DAY 24 SOLUTIONS - 🤖 (programming.dev)

submitted 1 week ago* (last edited 1 week ago) by [email protected] to c/[email protected]

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Day 24: Crossed Wires

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[–] gedhrel 4 points 1 week ago (2 children)

Haskell bits and pieces

The nice thing about Haskell's laziness (assuming you use Data.Map rather than Data.Map.Strict) is that the laziness can do a ton of the work for you - you might've spotted a few Haskell solutions in earlier days' threads that use this kind of trick (eg for tabling/memoisation). Here's my evaluation function:

eval l =
  let
    v = l & Map.map (\case
                       Const x -> x
                       And a b -> v Map.! a && v Map.! b
                       Or a b  -> v Map.! a || v Map.! b
                       Xor a b -> v Map.! a /= v Map.! b)
  in v

For part 2, we know what the graph should look like (it's just a binary adder); I think this is a maximal common subgraph problem, but I'm still reading around that at the mo. I'd love to know if there's a trick to this.

[–] VegOwOtenks 2 points 1 week ago (1 children)

Thank you for showing this trick, I knew Haskell was lazy but this one blew my mind again.

[–] gedhrel 3 points 1 week ago

Yeah, I remember when I saw this for the first time. It's astonishing how powerful lazy evaluation can be at times.