aoc-2024/cmd/day_9/main.go

package main

import (
	"bufio"
	"errors"
	"fmt"
	"io"
	"log"
	"os"
)

const MAX_LOOP = 1_000_000

func main() {
	blocks := make([]int, 0, 8192)
	r := bufio.NewReader(os.Stdin)

	// Read input
	for i := 0; i < MAX_LOOP; i++ {
		b, err := r.ReadByte()
		if errors.Is(err, io.EOF) {
			break
		}
		if err != nil {
			log.Fatalf("While reading input: %v", err)
		}
		switch {
		case b == '\n':
			break
		case '0' <= b || b <= '9':
			size := int(b - 48)

			fid := -1
			if i%2 == 0 { // If file block
				fid = int(i / 2)
			}
			for j := 0; j < size; j++ {
				blocks = append(blocks, fid)
			}
		default:
			log.Fatalf("unexpected input block '%q' encountered at column %d", b, i+1)
		}
	}

	// Compact
	free := 0
	for i := len(blocks) - 1; free+1 < i; i-- {
		if blocks[i] == -1 {
			continue
		}
		for blocks[free] != -1 {
			free++
		}
		blocks[free] = blocks[i]
		blocks[i] = -1
	}
	printBlocks(blocks)

	// Calculate checksum
	sum := 0
	for i, b := range blocks {
		if b == -1 {
			continue
		}
		sum += i * b
	}
	fmt.Printf("%d\n", sum)
}

func printBlocks(blocks []int) {
	for _, b := range blocks {
		if b == -1 {
			fmt.Printf(".")
			continue
		}
		fmt.Printf("%d", b)
	}
	fmt.Println()
}