AdventOfCode2020/22/main.go

package main

import (
	"bufio"
	"fmt"
	"os"
	"strconv"
	"time"
)

func main() {
	start := time.Now()
	partOne()
	duration := time.Since(start)
	partTwo()
	duration2 := time.Since(start)
	fmt.Printf("p1: %s, p2: %s\n", duration, duration2-duration)
}

type deck struct {
	cards []int
}

type cacheKey struct {
	one int
	two int
}

func countWinningDeck(deck []int) int {
	sum := 0
	for i := range deck {
		sum = sum + ((len(deck) - i) * deck[i])
	}
	return sum
}

// return values is who won/their score: true for player one, false for player two
func playCombat(one, two []int) (bool, int) {
	round := 0
	for len(one) > 0 && len(two) > 0 {
		round = round + 1
		if one[0] > two[0] {
			one = append(one[1:], one[0], two[0])
			two = two[1:]
		} else if one[0] < two[0] {
			two = append(two[1:], two[0], one[0])
			one = one[1:]
		}
	}
	if len(one) == 0 {
		return false, countWinningDeck(two)
	}
	return true, countWinningDeck(one)
}

func playRecursiveCombat(one, two []int) (bool, int) {
	loopCache := make(map[cacheKey]bool, 0)
	round := 0
	for len(one) > 0 && len(two) > 0 {
		// check for loops
		if loopCache[cacheKey{one: countWinningDeck(one), two: countWinningDeck(two)}] {
			return true, countWinningDeck(one)
		}
		loopCache[cacheKey{one: countWinningDeck(one), two: countWinningDeck(two)}] = true

		// otherwise, play a round
		round = round + 1
		if len(one[1:]) >= one[0] && len(two[1:]) >= two[0] {
			// fuckin' go slice internals
			subDeckOne := make([]int, len(one))
			subDeckTwo := make([]int, len(two))
			copy(subDeckOne, one)
			copy(subDeckTwo, two)
			winner, _ := playRecursiveCombat(subDeckOne[1:one[0]+1], subDeckTwo[1:two[0]+1])
			if winner { // p1 wins
				one = append(one[1:], one[0], two[0])
				two = two[1:]
			} else { // p2 wins
				two = append(two[1:], two[0], one[0])
				one = one[1:]
			}
		} else if one[0] > two[0] {
			one = append(one[1:], one[0], two[0])
			two = two[1:]
		} else if one[0] < two[0] {
			two = append(two[1:], two[0], one[0])
			one = one[1:]
		}
	}
	if len(one) == 0 {
		return false, countWinningDeck(two)
	}
	return true, countWinningDeck(one)
}

func partOne() {
	f, _ := os.Open("input")
	reader := bufio.NewReader(f)
	scanner := bufio.NewScanner(reader)

	// deal the decks
	playerone := deck{}
	for scanner.Scan() {
		line := scanner.Text()
		if line == "" {
			break
		}
		i, err := strconv.Atoi(line)
		if err == nil {
			playerone.cards = append(playerone.cards, i)
		}
	}
	playertwo := deck{}
	for scanner.Scan() {
		line := scanner.Text()
		i, err := strconv.Atoi(line)
		if err == nil {
			playertwo.cards = append(playertwo.cards, i)
		}
	}

	// play a game and determine a winner
	_, score := playCombat(playerone.cards, playertwo.cards)
	fmt.Println(score)
}

func partTwo() {
	f, _ := os.Open("input")
	reader := bufio.NewReader(f)
	scanner := bufio.NewScanner(reader)

	// deal the decks
	playerone := deck{}
	for scanner.Scan() {
		line := scanner.Text()
		if line == "" {
			break
		}
		i, err := strconv.Atoi(line)
		if err == nil {
			playerone.cards = append(playerone.cards, i)
		}
	}
	playertwo := deck{}
	for scanner.Scan() {
		line := scanner.Text()
		i, err := strconv.Atoi(line)
		if err == nil {
			playertwo.cards = append(playertwo.cards, i)
		}
	}

	_, score := playRecursiveCombat(playerone.cards, playertwo.cards)
	fmt.Println(score)
}