Solution for 2024, day 9

Author: lucianoq

2024/9/Makefile

@@ -0,0 +1,20 @@
+input:
+	http "https://adventofcode.com/2024/day/9/input" "Cookie:session=${AOC_SESSION};" >input
+
+main1:
+	go build -o main1 main1.go
+
+main2:
+	go build -o main2 main2.go
+
+.PHONY: run1 run2 clean
+
+run1: main1 input
+	./main1 <input
+
+run2: main2 input
+	./main2 <input
+
+clean:
+	rm -f main1 main2 input
+

2024/9/main1.go

@@ -0,0 +1,68 @@
+package main
+
+import (
+	"bufio"
+	"fmt"
+	"os"
+)
+
+const Empty = -1
+
+func main() {
+	memory := readMemory()
+	defrag(memory)
+	fmt.Println(checksum(memory))
+}
+
+func readMemory() []int {
+	scanner := bufio.NewScanner(os.Stdin)
+	scanner.Scan()
+	line := scanner.Text()
+
+	memory := make([]int, 0, len(line)*5)
+
+	for i, c := range line {
+		num := int(c - '0')
+		if i&1 == 0 {
+			for j := 0; j < num; j++ {
+				memory = append(memory, i/2)
+			}
+		} else {
+			for j := 0; j < num; j++ {
+				memory = append(memory, Empty)
+			}
+		}
+	}
+
+	return memory
+}
+
+func defrag(list []int) {
+	left, right := 0, len(list)-1
+
+	for {
+		for list[left] != Empty {
+			left++
+		}
+
+		for list[right] == Empty {
+			right--
+		}
+
+		if left >= right {
+			break
+		}
+
+		list[left], list[right] = list[right], list[left]
+	}
+}
+
+func checksum(blocks []int) int {
+	sum := 0
+	for i, b := range blocks {
+		if b != Empty {
+			sum += i * b
+		}
+	}
+	return sum
+}

2024/9/main2.go

@@ -0,0 +1,109 @@
+package main
+
+import (
+	"bufio"
+	"fmt"
+	"os"
+)
+
+func main() {
+	fs := loadFS()
+	fs.Defrag()
+	fmt.Println(fs.Checksum())
+}
+
+func loadFS() FS {
+	scanner := bufio.NewScanner(os.Stdin)
+	scanner.Scan()
+	line := scanner.Text()
+
+	files := make(map[int]*Block)
+	freeSpace := make([]*Block, 0, len(line)/2)
+
+	cursor := 0
+	for i, c := range line {
+		num := int(c - '0')
+
+		if i&1 == 0 {
+			// on odd indexes, it's a file
+			id := i / 2
+			files[id] = &Block{
+				ID:    id,
+				Start: cursor,
+				End:   cursor + num,
+			}
+			id++
+		} else {
+			// on even indexes, it's free space
+			freeSpace = append(freeSpace, &Block{
+				ID:    -1,
+				Start: cursor,
+				End:   cursor + num,
+			})
+		}
+
+		cursor += num
+	}
+
+	return FS{
+		Files:     files,
+		FreeSpace: freeSpace,
+		MaxFileID: len(line) / 2,
+	}
+}
+
+type Block struct {
+	ID, Start, End int
+}
+
+func (b Block) Size() int {
+	return b.End - b.Start
+}
+
+type FS struct {
+	Files     map[int]*Block
+	FreeSpace []*Block
+	MaxFileID int
+}
+
+func (fs FS) Defrag() {
+	for toMove := fs.MaxFileID; toMove >= 0; toMove-- {
+		f := fs.Files[toMove]
+
+		for i := 0; i < len(fs.FreeSpace); i++ {
+			currEmpty := fs.FreeSpace[i]
+
+			// Has to be big enough
+			if currEmpty.Size() < f.Size() {
+				continue
+			}
+
+			// Do not move to the right
+			if currEmpty.Start > f.Start {
+				break
+			}
+
+			// Found a valid empty space. Move file
+			f.Start, f.End = currEmpty.Start, currEmpty.Start+f.Size()
+			currEmpty.Start += f.Size()
+
+			// If the emptySpace doesn't exist anymore, clean it up
+			if currEmpty.Size() == 0 {
+				fs.FreeSpace = append(fs.FreeSpace[:i], fs.FreeSpace[i+1:]...)
+			}
+
+			// After the file is moved, go to the next file
+			break
+		}
+	}
+}
+
+func (fs FS) Checksum() int {
+	sum := 0
+	for _, f := range fs.Files {
+		for i := f.Start; i < f.End; i++ {
+			sum += f.ID * i
+		}
+	}
+	return sum
+}