calculate correct MinLength for QUIC 34 ACK frames with ACK ranges

ref #182

frames/ack_frame_new.go

@@ -237,12 +237,17 @@ func (f *AckFrameNew) MinLength(version protocol.VersionNumber) (protocol.ByteCo
 	length = 1 + 2 + 1 + 1 + 4 // 1 TypeByte, 2 ACK delay time, 1 Num Timestamp, 1 Delta Largest Observed, 4 FirstTimestamp
 	length += protocol.ByteCount(protocol.GetPacketNumberLength(f.LargestObserved))
 	// for the first ACK block length
-	length += protocol.ByteCount(protocol.PacketNumberLen6)
+
+	missingSequenceNumberDeltaLen := protocol.ByteCount(protocol.PacketNumberLen6)
+
+	if f.HasMissingRanges() {
+		length += (1 + missingSequenceNumberDeltaLen) * protocol.ByteCount(len(f.AckRanges))
+	} else {
+		length += missingSequenceNumberDeltaLen
+	}
 
 	length += (1 + 2) * 0 /* TODO: num_timestamps */
-	if f.HasMissingRanges() {
-		panic("NACKs not yet implemented")
-	}
+
 	return length, nil
 }
 

frames/ack_frame_new_test.go

@@ -249,7 +249,8 @@ var _ = Describe("AckFrame", func() {
 				f := &AckFrameNew{
 					LargestObserved: 1,
 				}
-				f.Write(b, 0)
+				err := f.Write(b, 0)
+				Expect(err).ToNot(HaveOccurred())
 				Expect(f.MinLength(0)).To(Equal(protocol.ByteCount(b.Len())))
 			})
 
@@ -257,7 +258,22 @@ var _ = Describe("AckFrame", func() {
 				f := &AckFrameNew{
 					LargestObserved: 0xDEADBEEFCAFE,
 				}
-				f.Write(b, 0)
+				err := f.Write(b, 0)
+				Expect(err).ToNot(HaveOccurred())
+				Expect(f.MinLength(0)).To(Equal(protocol.ByteCount(b.Len())))
+			})
+
+			It("has the proper min length for an ACK with missing packets", func() {
+				f := &AckFrameNew{
+					LargestObserved: 2000,
+					AckRanges: []AckRange{
+						AckRange{FirstPacketNumber: 1000, LastPacketNumber: 2000},
+						AckRange{FirstPacketNumber: 50, LastPacketNumber: 900},
+						AckRange{FirstPacketNumber: 1, LastPacketNumber: 23},
+					},
+				}
+				err := f.Write(b, 0)
+				Expect(err).ToNot(HaveOccurred())
 				Expect(f.MinLength(0)).To(Equal(protocol.ByteCount(b.Len())))
 			})
 		})