day15.go

package main

import (
	"fmt"
	"io/ioutil"
	"os"
	"strconv"
	"strings"
)

func main() {
	i, err := ioutil.ReadFile("input.txt")
	if err != nil {
		os.Exit(1)
	}
	input := string(i)
	fmt.Printf("Part 1: %v\n", SolveDay15Part1(input))
	fmt.Printf("Part 2: %v\n", SolveDay15Part2(input))
}

//SolveDay15Part1 plays the Elves memory game and returns the 2020th spoken number
func SolveDay15Part1(i string) (s int) {
	return playElvesGame(i, 2020)
}

//SolveDay15Part2 plays the Elves memory game and returns the 30000000th spoken number
func SolveDay15Part2(i string) (s int) {
	return playElvesGameR(i, 30000000)
}

//playElvesGame plays the Elves game until the given iteration
func playElvesGame(input string, iteration int) int {
	said := []int{0}
	lastTime, beforeLastTime := make(map[int]int), make(map[int]int)
	for i, numbers := range strings.Split(input, ",") {
		say, err := strconv.Atoi(numbers)
		if err != nil {
			return 0
		}
		said = append(said, say)
		beforeLastTime[say] = lastTime[say]
		lastTime[say] = i + 1
	}

	for i := len(said); i <= iteration; i++ {
		last := said[i-1]
		say := 0
		if beforeLastTime[last] != 0 {
			say = lastTime[last] - beforeLastTime[last]
		}
		said = append(said, say)
		beforeLastTime[say] = lastTime[say]
		lastTime[say] = i
	}
	return said[iteration]
}

//playElvesGameR plays the Elves game until the given iteration - refactor
func playElvesGameR(input string, iteration int) int {
	splitNumbers := strings.Split(input, ",")
	if len(splitNumbers) > iteration {
		return 0
	}

	var lastSaid int
	lastTime, beforeLastTime := make(map[int]int), make(map[int]int)

	for i, numbers := range splitNumbers {
		lastSaid, err := strconv.Atoi(numbers)
		if err != nil {
			return 0
		}
		beforeLastTime[lastSaid] = lastTime[lastSaid]
		lastTime[lastSaid] = i + 1
	}

	for i := len(splitNumbers) + 1; i <= iteration; i++ {
		if beforeLastTime[lastSaid] != 0 {
			lastSaid = lastTime[lastSaid] - beforeLastTime[lastSaid]
		} else {
			lastSaid = 0
		}
		beforeLastTime[lastSaid] = lastTime[lastSaid]
		lastTime[lastSaid] = i
	}
	return lastSaid
}