Advent of Code F# – Day 21

Yan Cui

I help clients go faster for less using serverless technologies.

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The source code for this post (both Part 1 and Part 2) is available here and you can click here to see my solutions for the other Advent of Code challenges.

Description for today’s challenge is here.


Modelling and simulating a RPG, this challenge totally got my nerd brain into overdrive, which is probably why I went totally overboard with modelling the domain! (you’ll see soon enough..)


First we have characters in the game (you and the boss) with hit points (HP), damage and armor scores:


Then we have the equipments, which have costs, as well as damage and armor stats:


Then we also have shops, who sell these equipments:



Initially, you have 100 Hit Points, but no damage or armor stats (these come from equipments).

The state of the boss comes from your input:


We also need to configure the shop’s inventory according to the description of the challenge:



From the description, we know that:

  • you have to pick one weapon
  • you can have 0-1 armor
  • you can have 0-2 rings

So, to make combining equipments easier later on (we want your selection of equipments to be a simple Equipment[]), let’s create a few sequences of Equipment[] to represent your choices of weapons, armors and rings.

For weapons and armors, this is pretty straightforward:


For rings, it’s slightly more involved:


Because the ordering of the rings are not important – [| ring1; ring2 |] is the same as [| ring2; ring1 |] – so here we work out all combinations of 2 rings from the available 6.

Next, let’s work out all possible equipment combinations we can buy from the shop:


We’ll also need to be able to simulate a game between you and the boss:


Notice in the inner recursive loop above, we swap the role of the attacker and defender on every iteration (whilst also deducting the defender’s HP) and return the name of the winner when the defender’s HP reaches 0<HP>.

Now that all the pieces are in place, we can answer the challenge by iterating over all equipment combos, and sending you in battle with the boss with each. For the combos that results in you winning, we simply work out the cost and find the min:


job done!


Part 2

Only a minor tweak is required here – change the winner to “boss” and look for max cost instead of min:


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2 thoughts on “Advent of Code F# – Day 21”

  1. Yan, I’ve been really enjoying the advent of code blog posts but I can’t find where the challenges are coming from. Could you share the link?

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