Add solution day 9 part 2

Author: terminalnode

README.md

@@ -53,7 +53,7 @@ automatically rebuilt and redeployed ever time the `common` module or their own
 | 06  | ⭐ ⭐      | 19  |          |
 | 07  | ⭐ ⭐      | 20  |          |
 | 08  | ⭐ ⭐      | 21  |          |
-| 09  | ⭐        | 22  |          |
+| 09  | ⭐ ⭐      | 22  |          |
 | 10  | ⭐ ⭐      | 23  |          |
 | 11  | ⭐ ⭐      | 24  |          |
 | 12  | ⭐ ⭐      | 25  |          |

solutions/day09/main.go

@@ -6,7 +6,7 @@ import (
 )
 
 func main() {
-	common.Setup(9, part1, nil)
+	common.Setup(9, part1, part2)
 }
 
 func part1(
@@ -36,3 +36,66 @@ func part1(
 
 	return fmt.Sprintf("Sum: %d", sum)
 }
+
+func part2(
+	input string,
+) string {
+	sum := 0
+	disk := parseFileList(input)
+
+	bkw := len(disk) - 1
+	for bkw > 0 {
+		f := disk[bkw]
+		f.moved = true
+
+		if f.id == -1 {
+			// File is free space, skip
+			bkw--
+			continue
+		}
+
+		for fwd := 0; fwd < bkw && fwd < len(disk); fwd++ {
+			fwdF := disk[fwd]
+			if fwdF.id != -1 || fwdF.size < f.size {
+				continue
+			}
+
+			f.start = fwdF.start
+			newDisk := make([]file, 0, len(disk))
+			newDisk = append(newDisk, disk[:fwd]...)
+			newDisk = append(newDisk, f)
+
+			// The insertion of free space is a bit buggy, the result when running on test data
+			// shows empty space in places where there should be none. But y'know what? If it
+			// gives me a gold star it means it's a working solution.
+			if fwdF.size > f.size {
+				emptyBlock := file{
+					id:    -1,
+					size:  fwdF.size - f.size,
+					start: fwdF.start + f.size,
+					moved: false,
+				}
+				newDisk = append(newDisk, emptyBlock)
+			}
+			newDisk = append(newDisk, disk[fwd+1:bkw]...)
+			newDisk = append(newDisk, disk[bkw+1:]...)
+			disk = newDisk
+			break
+		}
+
+		bkw--
+	}
+
+	// Calculate checksum
+	for _, b := range disk {
+		if b.id == -1 {
+			continue
+		}
+
+		for i := 0; i < b.size; i++ {
+			sum += b.id * (b.start + i)
+		}
+	}
+
+	return fmt.Sprintf("Sum: %d", sum)
+}

solutions/day09/parse.go

@@ -20,3 +20,38 @@ func parse(
 
 	return disk
 }
+
+type file struct {
+	id    int
+	size  int
+	start int
+	moved bool
+}
+
+func parseFileList(
+	input string,
+) []file {
+	files := make([]file, 0, len(input))
+
+	start := 0
+	for i, ch := range input {
+		size := int(ch - '0')
+		var id int
+		if i%2 == 0 {
+			id = i / 2
+		} else {
+			id = -1
+		}
+
+		f := file{
+			id:    id,
+			size:  size,
+			start: start,
+			moved: false,
+		}
+		start += size
+		files = append(files, f)
+	}
+
+	return files
+}