Advent of Code 2023 - Day 07

By Eric Burden | December 7, 2023

It’s that time of year again! Just like last year, I’ll be posting my solutions to the Advent of Code puzzles. This year, I’ll be solving the puzzles in Kotlin. I’ll post my solutions and code to GitHub as well. If you haven’t given AoC a try, I encourage you to do so along with me!

Day 7 - Camel Cards

Find the problem description HERE.

The Input - Dromedary Deck

Hmm, the input looks suspiciously straightforward to parse today. It’s an odd-numbered, day, though, so I’m still cautious about the difficulty level. Regardless, here we’ve got input in lines, each one containing two strings separated by a single space, where we need a list of characters from the first string and a parsed integer from the second. That’s do-able! I’ll also be representing each card as hand type as an enum with an associated ‘strength’ that can help us when sorting later.


/**
 * This enumeration represents the various types of Camel Cards
 *
 * @param strength The strength of the card when it comes to sorting.
 */
enum class CamelCard(val strength: Int) {
    ACE(14),
    KING(13),
    QUEEN(12),
    JACK(11),
    TEN(10),
    NINE(9),
    EIGHT(8),
    SEVEN(7),
    SIX(6),
    FIVE(5),
    FOUR(4),
    THREE(3),
    TWO(2),
    JOKER(1); // For Part Two

    companion object {
        /**
         * Parse a [CamelCard] from a [Char]
         *
         * @param char The Char value to parse.
         * @return The corresponding [Camel Card].
         */
        fun fromChar(char: Char): CamelCard {
            return when (char) {
                'A' -> ACE
                'K' -> KING
                'Q' -> QUEEN
                'J' -> JACK
                'T' -> TEN
                '9' -> NINE
                '8' -> EIGHT
                '7' -> SEVEN
                '6' -> SIX
                '5' -> FIVE
                '4' -> FOUR
                '3' -> THREE
                '2' -> TWO
                else -> throw IllegalArgumentException(
                  "$char does not represent a CamelCard!"
                )
            }
        }
    }
}

/**
 * This enumeration represents the various kinds of hands
 *
 * @param strength The strength of this kind of hand when it comes to sorting.
 */
enum class HandKind(val strength: Int) {
    FiveOfAKind(7_000_000),
    FourOfAKind(6_000_000),
    FullHouse(5_000_000),
    ThreeOfAKind(4_000_000),
    TwoPair(3_000_000),
    OnePair(2_000_000),
    HighCard(1_000_000);

    companion object {
        /**
         * Classify a hand of [CamelCard]s into a [HandKind]
         *
         * Check a list of camel cards for special cases, like four of a kind, 
         * and return the highest-strength classification possible for that 
         * list of cards. The classifications are the variants of [HandKind].
         *
         * @param cards The list of cards to be classified.
         * @return The [HandKind] representing the classification of the cards.
         */
        fun classify(cards: List<CamelCard>): HandKind {
            var counts = MutableList(15) { 0 }
            cards.forEach { card -> counts[card.strength] += 1 }

            val maxCount = counts.max()
            val pairCount = counts.filter { it == 2 }.count()
            return when (maxCount) {
                5 -> FiveOfAKind
                4 -> FourOfAKind
                3 -> if (pairCount > 0) FullHouse else ThreeOfAKind
                2 -> if (pairCount == 2) TwoPair else OnePair
                else -> HighCard
            }
        }
    }
}

/**
 * This class represents a hand of [CamelCard]s
 *
 * Each hand contains five cards, and the strength of that hand can be derived 
 * from the identity and order of those cards. Each hand also includes the 
 * accompanying bid, used to calculate the final result for both parts of 
 * today's puzzle.
 *
 * @param cards The list of [CamelCard]s in hand.
 * @param bid The value of the bid.
 * @param kind The [HandKind] classification of the hand.
 * @param strength The strength of this hand when it comes to sorting.
 */
data class CamelCardHand
private constructor(val cards: List<CamelCard>, val bid: Int, val kind: HandKind) {
    companion object {
        /**
         * Parse a [CamelCardHand] from a line of the input string.
         *
         * Each line includes the cards (as a string where each character
         * represents a card) and a  bid.
         *
         * @param string The input line to be parsed.
         * @return A hand of Camel Cards.
         */
        fun fromString(string: String): CamelCardHand {
            val (cardString, bidString) = string.split(" ")
            require(cardString.length == 5) {
                throw IllegalArgumentException("A hand must contain five cards!")
            }
            val cards = cardString.map { CamelCard.fromChar(it) }
            val bid = bidString.toInt()
            val kind = HandKind.classify(cards)
            return CamelCardHand(cards, bid, kind)
        }
    }
}

class Day07(input: List<String>) {

    // With the data nicely modeled, parsing is a breeze!
    private val parsed = input
      .filter { it.isNotEmpty() }
      .map { CamelCardHand.fromString(it) }

}

That’s still a fair bit of code, but the enums tend to make it look like more than it is. This should set us up nicely for part one.

Part One - A Link in the Chain

I’ve figured it out! We’re in a Zelda game. “Fetch me that sword!” “You can have my sword, but before I give it to you, fetch me a sandwich!” “I’d love to make you a sandwich, but this bug in my house is driving me mad. What, you don’t have a bug net? Maybe the raccoon in the woods has one.” That’s what we’re doing here. I’m going to be both extremely amused and a bit exasperated if those jokers on the ground with the trebuchet have the last piece of this quest chain… Ah well, at least we get to play a lovely game of poker while traveling via camel. That actually sounds kind of fun! Let’s sort out these hands of cards!


// I made my own!
import dev.ericburden.aoc2023.Utils.pow

// enum class CamelCard(val strength: Int) { ... }

// enum class HandKind(val strength: Int) { ... }

data class CamelCardHand
private constructor(val cards: List<CamelCard>, val bid: Int, val kind: HandKind) {
    companion object { ... }
    
    /**
     * Calculate and return the total strength of this hand.
     * 
     * The strength of a hand is derived from the individual cards in it, their 
     * order, and what kind of hand is formed by those cards. Each card in the 
     * hand is worth it's own strength times its place value. The place value is 
     * 14 (the maximum card strength) raised to the place index (descending from 
     * left to right). For example, the cards [2, 2, 2, 2, 2] would have place 
     * values of [2 * 14^4, 2 * 14^3, 2 * 14^2, 2 * 14^1, 2 * 14^0]. The strength 
     * derived from the _kind_ of hand is the most influential, since hands are 
     * sorted based on kind, then on the order of the cards. For this reason, 
     * each kind contributes an extra 1_000_000 to the strength, which is 
     * greater than the maximum possible strength derived from any set of cards 
     * ([A, A, A, A, A] = 579,194), and they're nice round numbers! These values 
     * contribute to a strength score such that, when sorted by that strength, 
     * hands will be sorted first on the kind of hand then on the order of the
     * individual cards.
     * 
     * @return The total strength of this hand.
     */
    val strength: Int
        get() {
            val maxCardIdx = cards.size - 1
            val cardStrength =
                    cards.zip(maxCardIdx downTo 0).sumOf { (card, exp) ->
                        card.strength * 14.pow(exp)
                    }
            return cardStrength + kind.strength
        }
}

class Day07(input: List<String>) {

    // private val parsed = ...

    // The trickiest bit to part one was telling the difference between a hand with
    // on pair and a hand with two pair.
    fun solvePart1(): Int =
            parsed
              .sortedBy { it.strength }
              .withIndex()
              .sumOf { (idx, hand) -> (idx + 1) * hand.bid }

}

There we go! A bit of math to make sure that all the hands are scored and sorted correctly. I do feel a bit clever basically treating the different types of cards as a base-14 number and converting it to decimal. Snaps for me! I also decided to stop complaining about the lack of an exponentiation function for integers and just make my own. So there!

Part Two - Speaking of Jokers…

Now this game is about to get wild! (Yes, I did that. I’m not ashamed.) Looks like all the Jacks were really Jokers which can pretend to be anything else! While at first this seems like a major twist, it ends up being not too tricky to handle.


// enum class CamelCard(val strength: Int) { ... }


enum class HandKind(val strength: Int) {
    FiveOfAKind(7_000_000),
    FourOfAKind(6_000_000),
    FullHouse(5_000_000),
    ThreeOfAKind(4_000_000),
    TwoPair(3_000_000),
    OnePair(2_000_000),
    HighCard(1_000_000);

    companion object {
    
        // fun classify(cards: List<CamelCard>): HandKind { ... }

        /**
         * Classify a hand of cards containing Jokers
         *
         * This could have been included in the `HandKind.classify` function, 
         * but I separated it out  because it's only used in part 2. For hands 
         * that contain Jokers (and treat them as wildcards), we determine what 
         * the hand _would be_ without the jokers, then promote the 
         * classification one step for each Joker found.
         *
         * @param cards A list of cards where Joker are wild.
         * @return The [HandKind] representing the classification of the cards.
         */
        fun classifyWithJokers(cards: List<CamelCard>): HandKind {
            // What kind of hand would it be without the Jokers?
            var handKind = classify(cards.filter { it != CamelCard.JOKER })

            // For every Joker in the hand, increase the value of the hand by 
            // one step, always choosing the next possible kind with the highest
            // strength possible.
            val jokerCount = cards.filter { it == CamelCard.JOKER }.count()
            repeat(jokerCount) {
                handKind =
                        when (handKind) {
                            FiveOfAKind -> FiveOfAKind // Best we can do
                            FourOfAKind -> FiveOfAKind
                            FullHouse -> FourOfAKind
                            ThreeOfAKind -> FourOfAKind
                            TwoPair -> FullHouse
                            OnePair -> ThreeOfAKind
                            HighCard -> OnePair
                        }
            }

            return handKind
        }
    }
}

data class CamelCardHand
private constructor(val cards: List<CamelCard>, val bid: Int, val kind: HandKind) {
    // companion object { ... }

    // val strength: Int get() { ... }

    /**
     * Replace all the Jacks in a hand with Jokers
     * 
     * Recalculates the [HandKind] of this hand with Jokers included.
     * 
     * @return A copy of this hand with all Jacks replaced with Jokers.
     */
    fun replaceJacksWithJokers(): CamelCardHand {
        val cards = cards.map { if (it == CamelCard.JACK) CamelCard.JOKER else it }
        val kind = HandKind.classifyWithJokers(cards)
        return CamelCardHand(cards, bid, kind)
    }
}

class Day07(input: List<String>) {

    // private val parsed = ...

    // fun solvePart1(): Int = ...

    // The trickiest bit to part two was that one hand with all Jacks!
    fun solvePart2(): Int =
            parsed
              .map { it.replaceJacksWithJokers() }
              .sortedBy { it.strength }
              .withIndex()
              .sumOf { (idx, hand) -> (idx + 1) * hand.bid }
}

The “one weird trick” here is realizing you don’t need to actually know what card the Jokers are pretending to be, you just need to know what effect they have on the hand. And that effect is to make it one stage better than it was!

Wrap Up

Now, this was a fun puzzle! I got to explore some Kotlin features I hadn’t touched yet (enum and importing my own extension function from a ‘Utils.kt` module), do a bit of math, and generally have a good time! I think I needed that. Here’s to hoping I’m not being lulled into a false sense of security.

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