Pin the builtin registry shape and grow as a lower bound, and correct stale comments

template_test.go

@@ -226,3 +226,40 @@ func TestFakePathNavigation(t *testing.T) {
 		t.Error("Fake(missing) = nil error, want unknown-category error")
 	}
 }
+
+// TestGrowIsALowerBound pins the property that makes the pre-sized render buffer
+// safe: grow must never exceed what expand emits. render multiplies it by repeat,
+// so an over-estimate would amplify up to a million-fold.
+func TestGrowIsALowerBound(t *testing.T) {
+	f := engine(3)
+	for _, format := range []string{
+		"",
+		"plain literal",
+		"00-11-AA-aa",
+		"#0#1#A#a## literal",
+		"Ö dag åäö 日本語",
+		"{x}{x}{x}",
+		"{hex(8)}-{int(10,99)}-{nanoid(5)}",
+		"9{d}{luhn()}",
+		"{a|b} and {a|b}",
+	} {
+		src := `{"format":` + quote(format) + `,"x":["1"],"a":["A"],"b":["B"],"d":["012345678901234"]}`
+		tmpl, ok := compiled(t, src).(*template)
+		if !ok {
+			t.Fatalf("format %q did not compile to a template", format)
+		}
+		for i := 0; i < 50; i++ {
+			if got := len(expand(f.rand, tmpl)); got < tmpl.grow {
+				t.Errorf("format %q: expand emitted %d bytes, below grow %d", format, got, tmpl.grow)
+			}
+		}
+	}
+}
+
+func quote(s string) string {
+	b, err := json.Marshal(s)
+	if err != nil {
+		panic(err)
+	}
+	return string(b)
+}