Hold only the {token} grammar's tests in format_test.go, the rest in files named for what they test, and drop the done item

render_test.go

@@ -0,0 +1,56 @@
+package fejkdata
+
+import (
+	"encoding/json"
+	"testing"
+)
+
+func TestRecursionHasNoDepthLimit(t *testing.T) {
+	// Build {format:{a}, a:[{format:{a}, a:[ ... "deep" ]]}} 50 levels deep.
+	tmpl := `"deep"`
+	for i := 0; i < 50; i++ {
+		tmpl = `{"format":"{a}","a":` + tmpl + `}`
+	}
+	if got := mustRender(t, engine(1), tmpl); got != "deep" {
+		t.Fatalf("deep recursion = %q, want deep", got)
+	}
+}
+
+// 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",
+		"{digits(2)}-{int(1,9)}{int(1,9)}-{upper(2)}-{lower(2)}",
+		"01Aa# literal",
+		"{{}} {{{x}}}",
+		"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++ {
+			var set holdSet
+			if got := len(expand(f.rand, tmpl, renderScope{set: &set})); 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)
+}