aoc2024/22/code.go

package main

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

func readInput(file *os.File) []int {
	scanner := bufio.NewScanner(file)
	var numbers []int

	for scanner.Scan() {
		line := scanner.Text()
		if line == "" {
			break
		}

		number, err := strconv.Atoi(line)
		if err != nil {
			log.Fatalf("Can't parse a number %s: %s", line, err)
		}

		numbers = append(numbers, number)
	}

	return numbers
}

func calculate(number int) int {
	a := number * 64
	number = (number ^ a) % 16777216

	b := number / 32
	number = (number ^ b) % 16777216

	c := number * 2048
	number = (number ^ c) % 16777216

	return number
}

func generateNumber(number int, iterations int) (int, []int) {
	lastDigits := make([]int, iterations+1)
	lastDigits[0] = number % 10

	for i := 0; i < iterations; i++ {
		number = calculate(number)
		lastDigits[i+1] = number % 10
	}

	return number, lastDigits
}

func part1(numbers []int, iterations int) (int, [][]int) {
	var result int
	var allLastDigits [][]int
	for _, number := range numbers {
		newNumber, lastDigits := generateNumber(number, iterations)
		result += newNumber

		allLastDigits = append(allLastDigits, lastDigits)
	}

	return result, allLastDigits
}

func sequenceKey(sequence [4]int) string {
	return fmt.Sprintf("%d_%d_%d_%d", sequence[0], sequence[1], sequence[2], sequence[3])
}

func highestSum(allLastDigits [][]int, iterations int) int {
	sums := make(map[string]int)
	for _, lastDigits := range allLastDigits {
		sequence := [4]int{lastDigits[1] - lastDigits[0], lastDigits[2] - lastDigits[1],
			lastDigits[3] - lastDigits[2], lastDigits[4] - lastDigits[3]}
		checked := make(map[string]bool)
		for i := 5; i < iterations; i++ {
			sequence[0], sequence[1], sequence[2] = sequence[1], sequence[2], sequence[3]
			sequence[3] = lastDigits[i] - lastDigits[i-1]
			if sequence[3] <= 0 {
				continue
			}

			key := sequenceKey(sequence)
			if !checked[key] {
				sums[key] += lastDigits[i]
				checked[key] = true
			}
		}
	}

	var highest int
	for _, value := range sums {
		if value > highest {
			highest = value
		}
	}

	return highest
}

func main() {
	if len(os.Args) < 2 {
		log.Fatal("You need to specify a file!")
	}

	filePath := os.Args[1]
	file, err := os.Open(filePath)
	if err != nil {
		log.Fatalf("Failed to open %s!\n", filePath)
	}

	numbers := readInput(file)
	part1Result, allLastDigits := part1(numbers, 2000)
	fmt.Println("Part1:", part1Result)
	fmt.Println("Part2:", highestSum(allLastDigits, 2001))
}