AoC2020/day15/day15.go

package main

import "fmt"

func main() {
	fmt.Println(run([]int{5,1,9,18,13,8,0}, 2020))
	fmt.Println(runPart2([]int{5,1,9,18,13,8,0}, 30000000))
}

func run(startingNumber []int, length int) int{
	var band []int
	band = append(band, startingNumber...)
	for i := len(startingNumber); i < length; i++ {
		currentNumber := band[i-1]
		index := findIndex(currentNumber,band)
		if index == -1 {
			band = append(band, 0)
		}else {
			band = append(band, i-1-index)
		}
	}
	return band[len(band)-1]
}

func runPart2(startingNumber []int, length int) int{
	var band = make (map[int][]int)
	for i, val := range startingNumber {
		band[val] = []int{i,i}
	}
	lastNumber := startingNumber[len(startingNumber)-1]
	for i := len(startingNumber); i < length; i++ {
		currentNumber := lastNumber
		indexes := band[currentNumber]
		diffIndexes := indexes[1]-indexes[0]
		if diffIndexes == 0 {
			lastNumber = 0
			band[0] = []int{band[0][1],i}
		}else {
			lastNumber = diffIndexes
			if len(band[diffIndexes]) != 0 {
				band[diffIndexes] = []int{band[diffIndexes][1],i}
			}else {
				band[diffIndexes] = []int{i,i}
			}
		}
	}
	return lastNumber
}

func findIndex(number int, band []int) int{
	for i := len(band)-2; i >= 0; i-- {
		if band[i] == number {
			return i
		}
	}
	return -1
}