1 files changed, 71 insertions, 0 deletions
diff --git a/pkg/btrfs/image.go b/pkg/btrfs/image.go
new file mode 100644
index 0000000..6037c4d
--- /dev/null
+++ b/pkg/btrfs/image.go
@@ -0,0 +1,71 @@
+package btrfs
+
+import (
+	"fmt"
+	"os"
+
+	"lukeshu.com/btrfs-tools/pkg/binstruct"
+)
+
+type Img struct {
+	*os.File
+}
+
+func (img *Img) Size() (int64, error) {
+	fi, err := img.Stat()
+	if err != nil {
+		return 0, err
+	}
+	return fi.Size(), nil
+}
+
+type Ref[T any] struct {
+	img  *Img
+	addr int64
+	Data T
+}
+
+func (r *Ref[T]) Read() error {
+	size, err := binstruct.Size(r.Data)
+	if err != nil {
+		return err
+	}
+	buf := make([]byte, size)
+	if _, err := r.img.ReadAt(buf, r.addr); err != nil {
+		return err
+	}
+	return binstruct.Unmarshal(buf, &r.Data)
+}
+
+func (img *Img) Superblocks() ([]Ref[Superblock], error) {
+	const superblockSize = 0x1000
+
+	var superblockAddrs = []int64{
+		0x00_0001_0000, // 64KiB
+		0x00_0400_0000, // 64MiB
+		0x40_0000_0000, // 256GiB
+	}
+
+	sz, err := img.Size()
+	if err != nil {
+		return nil, err
+	}
+
+	var ret []Ref[Superblock]
+	for i, addr := range superblockAddrs {
+		if addr+superblockSize <= sz {
+			superblock := Ref[Superblock]{
+				img:  img,
+				addr: addr,
+			}
+			if err := superblock.Read(); err != nil {
+				return nil, fmt.Errorf("superblock %d: %w", i, err)
+			}
+			ret = append(ret, superblock)
+		}
+	}
+	if len(ret) == 0 {
+		return nil, fmt.Errorf("no superblocks")
+	}
+	return ret, nil
+}