refactor: Improve parsing docs

Reveal intentions by:
- extracting magic numbers into constants
- changing function names with >1 responsibilities
- documenting non-obvious behaviors.

analyzer/udp/quic.go

@@ -36,41 +36,40 @@ type quicStream struct {
 }
 
 func (s *quicStream) Feed(rev bool, data []byte) (u *analyzer.PropUpdate, done bool) {
+	// minimal data size: protocol version (2 bytes) + random (32 bytes) +
+	//   + session ID (1 byte) + cipher suites (4 bytes) +
+	//   + compression methods (2 bytes) + no extensions
+	const minDataSize = 41
+
 	if rev {
 		// We don't support server direction for now
 		s.invalidCount++
 		return nil, s.invalidCount >= quicInvalidCountThreshold
 	}
+
 	pl, err := quic.ReadCryptoPayload(data)
-	if err != nil || len(pl) < 4 {
+	if err != nil || len(pl) < 4 { // FIXME: isn't length checked inside quic.ReadCryptoPayload? Also, what about error handling?
 		s.invalidCount++
 		return nil, s.invalidCount >= quicInvalidCountThreshold
 	}
-	// Should be a TLS client hello
-	if pl[0] != 0x01 {
-		// Not a client hello
+
+	if pl[0] != internal.TypeClientHello {
 		s.invalidCount++
 		return nil, s.invalidCount >= quicInvalidCountThreshold
 	}
+
 	chLen := int(pl[1])<<16 | int(pl[2])<<8 | int(pl[3])
-	if chLen < 41 {
-		// 2 (Protocol Version) +
-		// 32 (Random) +
-		// 1 (Session ID Length) +
-		// 2 (Cipher Suites Length) +_ws.col.protocol == "TLSv1.3"
-		// 2 (Cipher Suite) +
-		// 1 (Compression Methods Length) +
-		// 1 (Compression Method) +
-		// No extensions
-		// This should be the bare minimum for a client hello
+	if chLen < minDataSize {
 		s.invalidCount++
 		return nil, s.invalidCount >= quicInvalidCountThreshold
 	}
-	m := internal.ParseTLSClientHello(&utils.ByteBuffer{Buf: pl[4:]})
+
+	m := internal.ParseTLSClientHelloMsgData(&utils.ByteBuffer{Buf: pl[4:]})
 	if m == nil {
 		s.invalidCount++
 		return nil, s.invalidCount >= quicInvalidCountThreshold
 	}
+
 	return &analyzer.PropUpdate{
 		Type: analyzer.PropUpdateMerge,
 		M:    analyzer.PropMap{"req": m},