implement stream frame splitting in StreamFrame

frames/stream_frame.go

@@ -93,3 +93,23 @@ func (f *StreamFrame) Write(b *bytes.Buffer) error {
 func (f *StreamFrame) MaxLength() int {
 	return 1 + 4 + 8 + 2 + 1
 }
+
+// MaybeSplitOffFrame removes the first n bytes and returns them as a separate frame. If n >= len(n), nil is returned and nothing is modified.
+func (f *StreamFrame) MaybeSplitOffFrame(n int) *StreamFrame {
+	if n >= f.MaxLength()-1+len(f.Data) {
+		return nil
+	}
+	n -= f.MaxLength() - 1
+
+	defer func() {
+		f.Data = f.Data[n:]
+		f.Offset += uint64(n)
+	}()
+
+	return &StreamFrame{
+		FinBit:   false,
+		StreamID: f.StreamID,
+		Offset:   f.Offset,
+		Data:     f.Data[:n],
+	}
+}

frames/stream_frame_test.go

@@ -9,7 +9,6 @@ import (
 )
 
 var _ = Describe("StreamFrame", func() {
-	Context("stream frames", func() {
 	Context("when parsing", func() {
 		It("accepts sample frame", func() {
 			b := bytes.NewReader([]byte{0xa0, 0x1, 0x06, 0x00, 'f', 'o', 'o', 'b', 'a', 'r'})
@@ -63,5 +62,35 @@ var _ = Describe("StreamFrame", func() {
 			Expect(f.MaxLength()).To(Equal(b.Len()))
 		})
 	})
+
+	Context("splitting off earlier stream frames", func() {
+		It("splits off nothing", func() {
+			f := &StreamFrame{
+				StreamID: 1,
+				Data:     []byte("bar"),
+				Offset:   3,
+			}
+			Expect(f.MaybeSplitOffFrame(1000)).To(BeNil())
+			Expect(f.Offset).To(Equal(uint64(3)))
+		})
+
+		It("splits off initial frame", func() {
+			f := &StreamFrame{
+				StreamID: 1,
+				Data:     []byte("foobar"),
+				Offset:   3,
+				FinBit:   true,
+			}
+			previous := f.MaybeSplitOffFrame(f.MaxLength() - 1 + 3)
+			Expect(previous).ToNot(BeNil())
+			Expect(previous.StreamID).To(Equal(protocol.StreamID(1)))
+			Expect(previous.Data).To(Equal([]byte("foo")))
+			Expect(previous.Offset).To(Equal(uint64(3)))
+			Expect(previous.FinBit).To(BeFalse())
+			Expect(f.StreamID).To(Equal(protocol.StreamID(1)))
+			Expect(f.Data).To(Equal([]byte("bar")))
+			Expect(f.Offset).To(Equal(uint64(6)))
+			Expect(f.FinBit).To(BeTrue())
+		})
 	})
 })