adventofcode-2023/day7/day7.kt

import java.io.File

typealias Hand = List<Char>

fun main() {
    val input = File("input")
    val bets = parseHands(input)

    println(part1(bets))
    println(part2(bets))
}

fun part1(bets: List<Pair<Hand, Int>>): Int {
    return bets.sortedBy { handValue(it.first) }.foldIndexed(0) { i, acc, bet ->
        acc + (i + 1) * bet.second
    }
}

fun part2(bets: List<Pair<Hand, Int>>): Int {
    return bets.sortedBy { handValueJoker(it.first) }.foldIndexed(0) { i, acc, bet ->
        acc + (i + 1) * bet.second
    }
}

fun cardValue(card: Char): Int {
    return when (card) {
        'A' -> 12
        'K' -> 11
        'Q' -> 10
        'J' -> 9
        'T' -> 8
        in '2'..'9' -> card.digitToInt() - 2
        else -> throw Exception("Invalid card")
    }
}

fun type(cards: Hand): Int {
    return cards.groupBy { it }.mapValues { it.value.count() }.entries.fold(0) { acc, entry ->
        acc +
                when (entry.value) {
                    5 -> 6000000
                    4 -> 5000000
                    3 -> 3000000
                    2 -> 1000000
                    1 -> 0
                    else -> throw Exception("Too many cards")
                }
    }
}

fun handValue(cards: Hand): Int {
    // Each hand has 5 cards, the possible card values are 2-9+TJQKA,
    // and the order matters.
    //
    // This means the hands themselves are basically a 5-digit base 13
    // number.
    //
    // In addition, particular groupings of the cards matter. This is
    // represented by the type - to condense this into an easily
    // usable number, the type is therefore encoded in digits just
    // beyond the max of the 5-digit base 13 numbers. For convenient
    // programming, this means the type is encoded by the millions
    // digit in base 10, hence the odd type return numbers.
    return type(cards) + cards.fold(0) { acc, label -> acc * 13 + cardValue(label) }
}

fun cardValueJoker(card: Char): Int {
    return when (card) {
        'A' -> 12
        'K' -> 11
        'Q' -> 10
        'T' -> 9
        in '2'..'9' -> card.digitToInt() - 1
        'J' -> 0
        else -> throw Exception("Invalid card")
    }
}

fun typeJoker(cards: Hand): Int {
    var groups: MutableMap<Char, Int> =
            cards.groupBy { it }.mapValues { it.value.count() }.toMutableMap()

    // If there is only one group, we return early with the highest
    // value (this is to catch edge cases where there are only jokers)
    if (groups.entries.size == 1) {
        return 6000000
    }

    val jokers = groups.remove('J') ?: 0

    // Jokers act as a kind of tie breaker here - similar to the full
    // scenario, the label we want to add the joker to is the one with
    // the highest card value *as well as* the highest type.
    //
    // So we sort it first by value (descending so the highest type
    // gets i == 0), and second by the value of the card (so that if
    // two hands have the same type the one with the higher card value
    // wins).
    //
    // The hands are base 13 numbers, so multiplying the value puts it
    // one digit ahead of the card value.
    return groups.entries.sortedByDescending { 13 * it.value + cardValueJoker(it.key) }.foldIndexed(
                    0
            ) { i, acc, entry ->
        acc +
                when (entry.value + if (i == 0) jokers else 0) {
                    5 -> 6000000
                    4 -> 5000000
                    3 -> 3000000
                    2 -> 1000000
                    1 -> 0
                    else -> throw Exception("Too many cards")
                }
    }
}

fun handValueJoker(cards: Hand): Int {
    // This is just a bit more complicated; the card value doesn't
    // change much, but the *type* changes quite a bit.
    //
    // Rather than just giving the best type the highest value, we now
    // need to take into account scenarios in which ties are broken by
    // jokers. To do this, we sort the groups by the value of their
    // cards and the number of them.
    //
    // Since the tie breaker in this case is again the value of the
    // card, this is kind of recursive.
    return typeJoker(cards) + cards.fold(0) { acc, label -> acc * 13 + cardValueJoker(label) }
}

fun parseHands(input: File): List<Pair<Hand, Int>> {
    return input.useLines {
        it
                .map {
                    val (hand, bid) = it.split(' ')
                    hand.toList() to bid.toInt()
                }
                .toList()
    }
}