AdventOfCode2020/15/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 partOne() {
	f, _ := os.Open("input")
	reader := bufio.NewReader(f)
	scanner := bufio.NewScanner(reader)

	// naive implementation
	scanner.Scan()
	numbers := strings.Split(scanner.Text(), ",")
	for {
		i := len(numbers) - 1
		lastNumber := numbers[i]
		// walk backward through the list, looking for a match
		for j := range numbers {
			if len(numbers) == 2020 {
				fmt.Println(numbers[2019])
				return
			}
			if i-j == 0 {
				// made it to the head of the list, no matches; append 0
				numbers = append(numbers, "0")
				break
			} else if numbers[i-j-1] == lastNumber {
				// a number matched; insert the difference between the last number and when we last saw it
				numbers = append(numbers, fmt.Sprintf("%d", len(numbers)-1-(i-j-1)))
				break
			}
		}
	}
}

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

	// caching implementation. It's still slow, but it at least finishes...!
	scanner.Scan()
	numbers := strings.Split(scanner.Text(), ",")
	seen := make(map[int]int)

	// load in numbers
	for i, v := range numbers[:len(numbers)-1] {
		j, _ := strconv.Atoi(v)
		seen[j] = i
	}

	i := len(numbers) - 1
	lastNumber, _ := strconv.Atoi(numbers[i])
	for {
		if seen[lastNumber] == 30000000-1 {
			fmt.Println(lastNumber)
			return
		}
		if v, ok := seen[lastNumber]; ok {
			// a number matched; insert the difference between the last number and when we last saw it
			seen[lastNumber] = i
			lastNumber = i - v
		} else {
			// never seen that before
			seen[lastNumber] = i
			lastNumber = 0
		}
		i = i + 1
	}
}