Advent of Code has pretty much not topped 2015 Day 22. I had a ton of fun writing the solution.

I finished Day 22 at 7pm on December 29th. This wasn't due to 7 days of rigorous coding, failing, rethinking, mind you. I didn't do any work on it Christmas Eve, Christmas Day, the 26th, 27th or 28th. Long stories there. On the 23rd and 24th I took a break from 22 because it was clear I was not going about it the right way, and did days 23 and 24. I don't know how much coding I actually put into it. The leaderboard filled up at about 4am on the 22nd, I wouldn't have been on it if I did all of the coding at one time starting at midnight, I know that much. It might have been 8 hours for me.

There were a few little things that made this puzzle very fun but also a pain! Little subtle word clues that I initially just glossed over, turned out to lead to flaws in the logic that prevented me from solving this puzzle.

The Puzzle

The puzzle returns to day 21, where Little Henry Case is playing the same role playing game, but instead with swords and armor, you're now a magician. You have 5 spells at your disposal, each with varying effects. You are tasked with defeating the boss, who's hit points and damage are the puzzle's input. Mine happened to be 58 HP and 9 DMG. You must find the "path" to defeat the boss using as little mana as possible.

The Spells

These are the spells at your disposal:

1. Magic Missile - Costs 53. Instant damage of 4.
2. Drain - Costs 73. Instant damage of 2. Instant heal of 2.
3. Shield - Costs 113. Initializes an effect that lasts 6 turns and adds 7 defense to you over those 6 turns.
4. Poison - Costs 173. Lasts 6 turns. At the start of each turn, deals 3 damage
5. Recharge - Costs 229. Lasts 5 turns. At the start of each turn, adds 101 mana to player.

Rules

The subtle wording comes in the rules and how the spells are interpreted.

• On each of your turns, you must select one of your spells to cast. If you cannot afford to cast any spell, you lose.
• Effects apply at the start of both the player's turns and the boss' turns.
• Effects are created with a timer
• At the start of each turn, after they apply any effect they have, their timer is decreased by one
• You cannot cast a spell that would start an effect which is already active
• However, effects can be started on the same turn they end.
• Do not include mana recharge effects as "spending" negative mana

The Code!

Breakdown

• Instant is a property on Effect because Instant effects are processed at the same time as reoccurring effects, and the rule states that you can't start an effect which is already active. However, if you have poison active, which causes Damage, you should be able to cast the Magic Missile effect, which also causes Damage, because it's an instant effect.
• OneTime is a property on Effect to make Shield work correctly. Shield occurs immediately, and remains, but its effects aren't processed every turn. Just once it's cast, and removed when the timer counts down.
• I use random generation to get the next spell. It will pick one based on how much mana is left on the player, and if that effect is not already active, and also checking the Instant flag on Effect, because we want to be able to cast Magic Missile even though Poison may be active.
• I was a little inconsistent in how the functions work. For instance, ApplyEffects applies the Boss and Player hit points and mana directly (use pointers), while Attack just returns the damage to apply. I'm ok with this.
• The Story struct is fun to read through, when it works, and you're verifying everything, and you say to yourself, "This might actually work!" :)

I had a lot of fun on these puzzles. I think I want to post one for Day 25 too, so check back soon for that one!