blob: add weird StringMinusOne method

Change-Id: If1cb450e2131002eaa80e7f5ee2a449c745b99a6

pkg/blob/ref.go

@@ -88,6 +88,21 @@ func (r Ref) String() string {
 	return string(r.appendString(buf))
 }
 
+// StringMinusOne returns the first string that's before String.
+func (r Ref) StringMinusOne() string {
+	if r.digest == nil {
+		return "<invalid-blob.Ref>"
+	}
+	// TODO: maybe memoize this.
+	dname := r.digest.digestName()
+	bs := r.digest.bytes()
+	buf := getBuf(len(dname) + 1 + len(bs)*2)[:0]
+	defer putBuf(buf)
+	buf = r.appendString(buf)
+	buf[len(buf)-1]-- // no need to deal with carrying underflow (no 0 bytes ever)
+	return string(buf)
+}
+
 func (r Ref) appendString(buf []byte) []byte {
 	dname := r.digest.digestName()
 	bs := r.digest.bytes()

pkg/blob/ref_test.go

@@ -188,6 +188,14 @@ func TestSizedBlobRefString(t *testing.T) {
 	}
 }
 
+func TestRefStringMinusOne(t *testing.T) {
+	br := MustParse("abc-1234")
+	want := "abc-1233"
+	if got := br.StringMinusOne(); got != want {
+		t.Errorf("StringMinusOne = %q; want %q", got, want)
+	}
+}
+
 func TestMarshalBinary(t *testing.T) {
 	br := MustParse("abc-00ff4869")
 	data, _ := br.MarshalBinary()