1 files changed, 80 insertions, 0 deletions
diff --git a/format/commitgraph/layer_open.go b/format/commitgraph/layer_open.go
new file mode 100644
index 00000000..857b3022
--- /dev/null
+++ b/format/commitgraph/layer_open.go
@@ -0,0 +1,80 @@
+package commitgraph
+
+import (
+	"os"
+	"syscall"
+
+	"codeberg.org/lindenii/furgit/internal/intconv"
+	"codeberg.org/lindenii/furgit/objectid"
+)
+
+func openLayer(root *os.Root, relPath string, algo objectid.Algorithm) (*layer, error) {
+	file, err := root.Open(relPath)
+	if err != nil {
+		return nil, err
+	}
+
+	info, err := file.Stat()
+	if err != nil {
+		_ = file.Close()
+
+		return nil, err
+	}
+
+	size := info.Size()
+	if size < int64(headerSize+fanoutSize+algo.Size()) {
+		_ = file.Close()
+
+		return nil, &ErrMalformed{Path: relPath, Reason: "file too short"}
+	}
+
+	mapLen, err := intconv.Int64ToUint64(size)
+	if err != nil {
+		_ = file.Close()
+
+		return nil, err
+	}
+
+	mapLenInt, err := intconv.Uint64ToInt(mapLen)
+	if err != nil {
+		_ = file.Close()
+
+		return nil, err
+	}
+
+	fd, err := intconv.UintptrToInt(file.Fd())
+	if err != nil {
+		_ = file.Close()
+
+		return nil, err
+	}
+
+	data, err := syscall.Mmap(fd, 0, mapLenInt, syscall.PROT_READ, syscall.MAP_PRIVATE)
+	if err != nil {
+		_ = file.Close()
+
+		return nil, err
+	}
+
+	out := &layer{
+		path: relPath,
+		file: file,
+		data: data,
+	}
+
+	parseErr := parseLayer(out, algo)
+	if parseErr != nil {
+		_ = out.close()
+
+		return nil, parseErr
+	}
+
+	verifyErr := verifyTrailerHash(out.data, algo, relPath)
+	if verifyErr != nil {
+		_ = out.close()
+
+		return nil, verifyErr
+	}
+
+	return out, nil
+}