Day 22 solution.

2020/day22.go

@@ -0,0 +1,122 @@
+package main
+
+import (
+	"fmt"
+	"os"
+	"strings"
+
+	"git.annabunch.es/annabunches/adventofcode/2020/lib/util"
+)
+
+func parseInput(input []string) [][]int {
+	decks := make([][]int, 2)
+	decks[0] = make([]int, 0)
+	decks[1] = make([]int, 0)
+	player := 0
+	for _, line := range input {
+		if strings.HasPrefix(line, "Player") {
+			continue
+		}
+		if line == "" {
+			player++
+			continue
+		}
+
+		decks[player] = append(decks[player], util.MustAtoi(line))
+	}
+
+	return decks
+}
+
+func playGame(decks [][]int, step string) (int, [][]int) {
+	gameStates := make(map[string]struct{})
+	found := struct{}{}
+
+	for len(decks[0]) > 0 && len(decks[1]) > 0 {
+		state := serializeGame(decks)
+		if _, ok := gameStates[state]; ok && step == "2" {
+			return 0, decks
+		}
+		gameStates[state] = found
+
+		var winner int
+		// take the top cards
+		card0 := decks[0][0]
+		card1 := decks[1][0]
+		decks[0] = decks[0][1:]
+		decks[1] = decks[1][1:]
+
+		// determine the winner, put the cards in order
+		if card0 > card1 {
+			winner = 0
+		} else {
+			winner = 1
+		}
+
+		// if each card is >= the number of cards in that deck
+		if step == "2" && card0 <= len(decks[0]) && card1 <= len(decks[1]) {
+			newDecks := make([][]int, 2)
+			newDecks[0] = make([]int, card0)
+			newDecks[1] = make([]int, card1)
+			copy(newDecks[0], decks[0][:card0])
+			copy(newDecks[1], decks[1][:card1])
+			// Recursive Combat throws out the "normal" winner above
+			winner, _ = playGame(newDecks, step)
+		}
+
+		// stack the cards for adding to the winner's bottom
+		cards := make([]int, 2)
+		if winner == 0 {
+			cards[0] = card0
+			cards[1] = card1
+		} else {
+			cards[0] = card1
+			cards[1] = card0
+		}
+
+		// and add them to the winner's bottom
+		decks[winner] = append(decks[winner], cards...)
+	}
+
+	if len(decks[0]) > 0 {
+		return 0, decks
+	}
+
+	return 1, decks
+}
+
+func serializeGame(decks [][]int) string {
+	out := ""
+	for _, deck := range decks {
+		for _, card := range deck {
+			out = out + string(card) + ","
+		}
+		out = out + "|"
+	}
+	return out
+}
+
+func calculateScore(decks [][]int) int {
+	score := 0
+	var index int
+	if len(decks[0]) > 0 {
+		index = 0
+	} else {
+		index = 1
+	}
+	deckSize := len(decks[index])
+	for i := deckSize - 1; i >= 0; i-- {
+		score += decks[index][i] * (deckSize - i)
+	}
+
+	return score
+}
+
+func main() {
+	step := os.Args[1]
+	values := util.InputParserStrings(os.Args[2])
+	input := parseInput(values)
+
+	_, decks := playGame(input, step)
+	fmt.Println(calculateScore(decks))
+}