From 7d3e591aaed7aa78016613254184ef2638a3c597 Mon Sep 17 00:00:00 2001
From: Jason Playne <jason@jasonplayne.com>
Date: Thu, 30 Nov 2023 21:39:37 +0800
Subject: [PATCH] when aircraft send us @@@@@@@@ as their flight
 number/callsign - ignore them

---
 Makefile                         |  2 +-
 cmd/df_example_finder/main.go    | 12 ++++++------
 lib/tracker/mode_s/decode-bds.go |  2 +-
 lib/tracker/mode_s/decode.go     |  6 ++++++
 lib/tracker/mode_s/describe.go   |  5 ++---
 lib/tracker/mode_s/frame.go      |  7 +++++++
 lib/tracker/plane.go             |  3 +++
 7 files changed, 26 insertions(+), 11 deletions(-)

diff --git a/Makefile b/Makefile
index ee62295c..1e5890be 100644
--- a/Makefile
+++ b/Makefile
@@ -1,6 +1,6 @@
 export GOBIN=$(shell pwd)/bin
 export GOAMD64=v3
-
+export CGO_ENABLED=0
 .PHONY: all
 
 all: tidy vet test build
diff --git a/cmd/df_example_finder/main.go b/cmd/df_example_finder/main.go
index bbdedc52..397d04d8 100644
--- a/cmd/df_example_finder/main.go
+++ b/cmd/df_example_finder/main.go
@@ -198,9 +198,9 @@ func showTypes(c *cli.Context) error {
 				frame.IcaoStr(),
 				frame.RawString(),
 				frame.DownLinkFormat(),
-				frame.MessageTypeString(),
+				"",
 				frame.FlightNumber(),
-				fmt.Sprint(frame.SquawkIdentity()),
+				frame.SquawkIdentityStr(),
 			}
 		case 17, 18, 19:
 			fields = []string{
@@ -212,7 +212,7 @@ func showTypes(c *cli.Context) error {
 				frame.DownLinkFormat(),
 				frame.MessageTypeString(),
 				frame.FlightNumber(),
-				fmt.Sprint(frame.SquawkIdentity()),
+				frame.SquawkIdentityStr(),
 			}
 		case 20, 21:
 			fields = []string{
@@ -222,9 +222,9 @@ func showTypes(c *cli.Context) error {
 				frame.IcaoStr(),
 				frame.RawString(),
 				frame.DownLinkFormat(),
-				frame.MessageTypeString(),
+				frame.DescribeBds(),
 				frame.FlightNumber(),
-				fmt.Sprint(frame.SquawkIdentity()),
+				frame.SquawkIdentityStr(),
 			}
 		default:
 			fields = []string{
@@ -236,7 +236,7 @@ func showTypes(c *cli.Context) error {
 				frame.DownLinkFormat(),
 				frame.MessageTypeString(),
 				frame.FlightNumber(),
-				fmt.Sprint(frame.SquawkIdentity()),
+				frame.SquawkIdentityStr(),
 			}
 		}
 		if !export {
diff --git a/lib/tracker/mode_s/decode-bds.go b/lib/tracker/mode_s/decode-bds.go
index c03f28de..a0668bda 100644
--- a/lib/tracker/mode_s/decode-bds.go
+++ b/lib/tracker/mode_s/decode-bds.go
@@ -139,7 +139,7 @@ func inferCommBMessageType(mb []byte) (byte, byte, error) {
 	if mb[0] == 0b0010_0000 {
 		// bits 9-56 are call sign, should not contain any ? chars from aisCharset
 		callsign := string(decodeFlightNumber(mb[1:7]))
-		if !strings.Contains(callsign, "?") {
+		if callsign != "" && !strings.Contains(callsign, "?") {
 			return 2, 0, nil
 		}
 	}
diff --git a/lib/tracker/mode_s/decode.go b/lib/tracker/mode_s/decode.go
index 87722d39..ffa94e6e 100644
--- a/lib/tracker/mode_s/decode.go
+++ b/lib/tracker/mode_s/decode.go
@@ -500,6 +500,12 @@ func decodeFlightNumber(b []byte) []byte {
 	callsign[5] = aisCharset[((b[3]&3)<<4)|(b[4]>>4)]
 	callsign[6] = aisCharset[((b[4]&15)<<2)|(b[5]>>6)]
 	callsign[7] = aisCharset[b[5]&63]
+
+	// because planes have sent us things like A90004A0200000000000007D8DB4
+	// we need
+	if string(callsign) == "@@@@@@@@" {
+		callsign = nil
+	}
 	return callsign
 }
 
diff --git a/lib/tracker/mode_s/describe.go b/lib/tracker/mode_s/describe.go
index a660cd24..b5092e4f 100644
--- a/lib/tracker/mode_s/describe.go
+++ b/lib/tracker/mode_s/describe.go
@@ -875,7 +875,7 @@ func (f *Frame) showAlert(output io.Writer) {
 	f.showSpecial(output)
 }
 func (f *Frame) showSpecial(output io.Writer) {
-	if "" != f.special {
+	if f.special != "" {
 		fprintf(output, "  special           : %s\n", f.special)
 	}
 }
@@ -884,9 +884,8 @@ func (f *Frame) showFlightNumber(output io.Writer) {
 	fprintf(output, "    flight Number   : %s\n", f.FlightNumber())
 }
 
-// determines what type of mode S Message this frame is
+// DownLinkFormat determines what type of mode S Message this frame is
 func (f *Frame) DownLinkFormat() string {
-
 	if description, ok := downlinkFormatTable[f.downLinkFormat]; ok {
 		return description
 	}
diff --git a/lib/tracker/mode_s/frame.go b/lib/tracker/mode_s/frame.go
index 1d8d9966..a0c259b6 100644
--- a/lib/tracker/mode_s/frame.go
+++ b/lib/tracker/mode_s/frame.go
@@ -749,6 +749,13 @@ func (f *Frame) SquawkIdentity() uint32 {
 	return f.identity
 }
 
+func (f *Frame) SquawkIdentityStr() string {
+	if nil == f || f.identity == 0 {
+		return ""
+	}
+	return fmt.Sprintf("%04d", f.identity)
+}
+
 func (f *Frame) OnGround() (bool, error) {
 	if f.VerticalStatusValid() {
 		return f.onGround, nil
diff --git a/lib/tracker/plane.go b/lib/tracker/plane.go
index 1cbfcb52..0fe77c69 100644
--- a/lib/tracker/plane.go
+++ b/lib/tracker/plane.go
@@ -493,6 +493,9 @@ func (p *Plane) Registration() *string {
 
 // setFlightNumber is the flights identifier/number
 func (p *Plane) setFlightNumber(flightIdentifier string) bool {
+	if flightIdentifier == "" {
+		return false
+	}
 	p.rwLock.Lock()
 	defer p.rwLock.Unlock()
 	hasChanged := p.flight.identifier != flightIdentifier