fixes dlclark#31 grow stack spaces when performing a jump operation b…

…ackward AND to the same address

regexp_test.go

@@ -1058,3 +1058,37 @@ func TestBadGroupConstruct(t *testing.T) {
 		}
 	}
 }
+
+func TestEmptyCaptureLargeRepeat(t *testing.T) {
+	// a bug would cause our track to not grow and eventually panic
+	// with large numbers of repeats of a non-capturing group (>16)
+
+	// the issue was that the jump occured to the same statement over and over
+	// and the "grow stack/track" logic only triggered on jumps that moved
+	// backwards
+
+	r := MustCompile(`(?:){40}`, 0)
+	m, err := r.FindStringMatch("1")
+	if err != nil {
+		t.Fatalf("Unexpected error: %v", err)
+	}
+	if want, got := 0, m.Index; want != got {
+		t.Errorf("First Match Index wanted %v got %v", want, got)
+	}
+	if want, got := 0, m.Length; want != got {
+		t.Errorf("First Match Length wanted %v got %v", want, got)
+	}
+
+	m, _ = r.FindNextMatch(m)
+	if want, got := 1, m.Index; want != got {
+		t.Errorf("Second Match Index wanted %v got %v", want, got)
+	}
+	if want, got := 0, m.Length; want != got {
+		t.Errorf("Second Match Length wanted %v got %v", want, got)
+	}
+
+	m, _ = r.FindNextMatch(m)
+	if m != nil {
+		t.Fatal("Expected 2 matches, got more")
+	}
+}

runner.go

@@ -951,8 +951,8 @@ func (r *runner) advance(i int) {
 }
 
 func (r *runner) goTo(newpos int) {
-	// when branching backward, ensure storage
-	if newpos < r.codepos {
+	// when branching backward or in place, ensure storage
+	if newpos <= r.codepos {
 		r.ensureStorage()
 	}