AdventOfCode2020/23/main.go

package main

import (
	"bufio"
	"fmt"
	"os"
	"strconv"
	"strings"
	"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)
}

func cupInList(cup int, cups []int) bool {
	for _, c := range cups {
		if c == cup {
			return true
		}
	}
	return false
}

func generateFullCupset(cups []int, totalCups int) []int {
	for i := len(cups); i < totalCups; i = i + 1 {
		cups = append(cups, i)
	}
	return cups
}

func findTargetCupIndex(cup int, cups []int) int {
	for i, c := range cups {
		if c == cup {
			return i
		}
	}
	return 0 // uh-oh
}

func step(cups []int) []int {
	cupCount := len(cups)
	currentCup := cups[0]
	removedCups := cups[1:4]
	newList := make([]int, 0, cupCount)
	newList = append([]int{cups[0]}, cups[4:]...)
	var targetCup int
	if currentCup == 1 {
		targetCup = cupCount
	} else {
		targetCup = currentCup - 1
	}
	for cupInList(targetCup, removedCups) {
		targetCup = targetCup - 1
		if targetCup == 0 {
			targetCup = cupCount
		}
	}
	tc := findTargetCupIndex(targetCup, newList)
	result := make([]int, 0, cupCount)
	result = append(result, newList[1:tc+1]...)
	result = append(result, removedCups...)
	result = append(result, newList[tc+1:]...)
	result = append(result, newList[0])
	return result
}

func runabout(cups []int) string {
	oneIndex := findTargetCupIndex(1, cups)
	result := ""
	for i := oneIndex + 1; i < len(cups); i = i + 1 {
		result = result + strconv.Itoa(cups[i])
	}
	for i := 0; i < len(cups[:oneIndex]); i = i + 1 {
		result = result + strconv.Itoa(cups[i])
	}
	return result
}

func nextTwo(cups []int) (int, int) {
	oneIndex := findTargetCupIndex(1, cups)
	if oneIndex+1 > len(cups) {
		return cups[0], cups[1]
	}
	if oneIndex+2 > len(cups) {
		return cups[len(cups)], cups[0]
	}
	return cups[oneIndex+1], cups[oneIndex+2]
}

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

	cups := []int{}

	scanner.Scan()
	for _, s := range strings.Split(scanner.Text(), "") {
		i, _ := strconv.Atoi(s)
		cups = append(cups, i)
	}

	for i := 0; i < 100; i = i + 1 {
		cups = step(cups)
	}

	fmt.Println(runabout(cups))

}

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

	cups := []int{}

	scanner.Scan()
	for _, s := range strings.Split(scanner.Text(), "") {
		i, _ := strconv.Atoi(s)
		cups = append(cups, i)
	}

	allCups := generateFullCupset(cups, 10000000)
	for i := 0; i < 10000000; i = i + 1 {
		allCups = step(allCups)
		fmt.Println(i)
	}

	fmt.Println(nextTwo(allCups))
}