1 files changed, 43 insertions, 0 deletions
diff --git a/object/tree/mode/parse.go b/object/tree/mode/parse.go
new file mode 100644
index 00000000..7839a89a
--- /dev/null
+++ b/object/tree/mode/parse.go
@@ -0,0 +1,43 @@
+package mode
+
+import (
+	"errors"
+	"fmt"
+)
+
+// ErrInvalidMode indicates a malformed or unsupported tree entry mode.
+var ErrInvalidMode = errors.New("object/tree/mode: invalid mode")
+
+// Parse decodes a canonical octal tree entry mode.
+//
+// It accepts only the modes Git itself writes
+// and rejects malformed, unsupported, and zero-padded encodings.
+func Parse(raw []byte) (Mode, error) {
+	if len(raw) == 0 {
+		return 0, fmt.Errorf("%w: empty mode", ErrInvalidMode)
+	}
+
+	if raw[0] == '0' {
+		return 0, fmt.Errorf("%w: zero-padded mode %q", ErrInvalidMode, raw)
+	}
+
+	if len(raw) > maxModeDigits {
+		return 0, fmt.Errorf("%w: mode %q too long", ErrInvalidMode, raw)
+	}
+
+	var mode Mode
+
+	for _, c := range raw {
+		if c < '0' || c > '7' {
+			return 0, fmt.Errorf("%w: non-octal byte in mode %q", ErrInvalidMode, raw)
+		}
+
+		mode = mode<<3 | Mode(c-'0')
+	}
+
+	if !mode.IsValid() {
+		return 0, fmt.Errorf("%w: unsupported mode %q", ErrInvalidMode, raw)
+	}
+
+	return mode, nil
+}