Add solution day 9 part 1

terminalnode · terminalnode · commit c88478039d1f · 2024-12-14T21:32:14.000+01:00
diff --git a/README.md b/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  |          |
diff --git a/solutions/day09/main.go b/solutions/day09/main.go
@@ -1,9 +1,38 @@
 package main
 
 import (
+	"fmt"
 	"github.com/terminalnode/adventofcode2024/common"
 )
 
 func main() {
-	common.Setup(9, nil, nil)
+	common.Setup(9, part1, nil)
+}
+
+func part1(
+	input string,
+) string {
+	sum := 0
+	disk := parse(input)
+	backIndex := len(disk)
+
+	for i, id := range disk {
+		if i >= backIndex {
+			break
+		}
+		if id == -1 {
+			backIndex--
+			for disk[backIndex] == -1 {
+				backIndex--
+			}
+
+			if i < backIndex {
+				sum += i * disk[backIndex]
+			}
+		} else {
+			sum += i * id
+		}
+	}
+
+	return fmt.Sprintf("Sum: %d", sum)
 }
diff --git a/solutions/day09/parse.go b/solutions/day09/parse.go
@@ -0,0 +1,22 @@
+package main
+
+func parse(
+	input string,
+) []int {
+	disk := make([]int, 0, len(input)*9)
+	for i, ch := range input {
+		size := int(ch - '0')
+		var id int
+		if i%2 == 0 {
+			id = i / 2
+		} else {
+			id = -1
+		}
+
+		for chunkI := 0; chunkI < size; chunkI++ {
+			disk = append(disk, id)
+		}
+	}
+
+	return disk
+}
