Add stateless compression and tests.

handler.go

@@ -19,12 +19,18 @@ const (
 	BestCompression    = gzip.BestCompression
 	DefaultCompression = gzip.DefaultCompression
 	HuffmanOnly        = gzip.HuffmanOnly
+	// Stateless will do compression but without maintaining any state
+	// between Write calls, so long running responses will not take memory.
+	// There will be no memory kept between Write calls,
+	// but compression and speed will be suboptimal.
+	// Because of this, the size of actual Write calls will affect output size.
+	Stateless = gzip.StatelessCompression
 )
 
 // Config is used in Handler initialization
 type Config struct {
 	// gzip compression level to use,
-	// valid value: -2 ~ 9.
+	// valid value: -3 => 9.
 	//
 	// see https://golang.org/pkg/compress/gzip/#NewWriterLevel
 	CompressionLevel int
@@ -58,7 +64,7 @@ type Handler struct {
 //
 // config must not be modified after calling on NewHandler()
 func NewHandler(config Config) *Handler {
-	if config.CompressionLevel < HuffmanOnly || config.CompressionLevel > BestCompression {
+	if config.CompressionLevel < Stateless || config.CompressionLevel > BestCompression {
 		panic(fmt.Sprintf("gzip: invalid CompressionLevel: %d", config.CompressionLevel))
 	}
 	if config.MinContentLength <= 0 {

handler_test.go

@@ -113,7 +113,7 @@ func TestNewHandler_Checks(t *testing.T) {
 
 	assert.Panics(t, func() {
 		NewHandler(Config{
-			CompressionLevel: -3,
+			CompressionLevel: -4,
 			MinContentLength: 100,
 		})
 	})
@@ -285,6 +285,39 @@ func TestGinWithDefaultHandler(t *testing.T) {
 	}
 }
 
+func TestGinWithLevelsHandler(t *testing.T) {
+	for i := Stateless; i < 10; i++ {
+		var seq = "level_" + strconv.Itoa(i)
+		i := i
+		t.Run(seq, func(t *testing.T) {
+			g := newEchoGinInstance(bigPayload, NewHandler(Config{
+				CompressionLevel: i,
+				MinContentLength: 1,
+			}).Gin)
+
+			var (
+				w = httptest.NewRecorder()
+				r = httptest.NewRequest(http.MethodPost, "/", strings.NewReader(seq))
+			)
+
+			r.Header.Set("Accept-Encoding", "gzip")
+			g.ServeHTTP(w, r)
+
+			result := w.Result()
+			require.EqualValues(t, http.StatusOK, result.StatusCode)
+			require.Equal(t, "gzip", result.Header.Get("Content-Encoding"))
+			comp, err := ioutil.ReadAll(result.Body)
+			require.NoError(t, err)
+			reader, err := gzip.NewReader(bytes.NewReader(comp))
+			require.NoError(t, err)
+			body, err := ioutil.ReadAll(reader)
+			require.NoError(t, err)
+			require.True(t, bytes.HasPrefix(body, []byte(seq)))
+			t.Logf("%s: compressed %d => %d", seq, len(body), len(comp))
+		})
+	}
+}
+
 func TestGinWithDefaultHandler_404(t *testing.T) {
 	var (
 		g = newGinInstance(bigPayload, DefaultHandler().Gin)