add speed filters

pkg/types/readsb/filter.go

@@ -104,6 +104,38 @@ func OnGround() AircraftFilter {
 	return func(a *Aircraft) bool { return a.AltBaro.OnGround }
 }
 
+// SpeedSource selects which speed reading a speed filter compares against. Its
+// zero value is GroundSpeed.
+type SpeedSource int
+
+const (
+	// GroundSpeed measures against GS (ground speed, knots).
+	GroundSpeed SpeedSource = iota
+	// TrueAirspeed measures against TAS (true airspeed, knots).
+	TrueAirspeed
+)
+
+func (s SpeedSource) of(a *Aircraft) float64 {
+	if s == TrueAirspeed {
+		return float64(a.TAS)
+	}
+	return a.GS
+}
+
+// MinSpeed keeps aircraft at or above knots, measured by src.
+func MinSpeed(knots float64, src SpeedSource) AircraftFilter {
+	return func(a *Aircraft) bool { return src.of(a) >= knots }
+}
+
+// MaxSpeed keeps aircraft with a positive reading at or below knots, measured by
+// src; aircraft with no reading are dropped.
+func MaxSpeed(knots float64, src SpeedSource) AircraftFilter {
+	return func(a *Aircraft) bool {
+		v := src.of(a)
+		return v > 0 && v <= knots
+	}
+}
+
 // WithinNM keeps aircraft within the given range (nautical miles) of the receiver.
 func WithinNM(nm float64) AircraftFilter {
 	return func(a *Aircraft) bool { return a.RDst > 0 && a.RDst <= nm }

pkg/types/readsb/readsb_test.go

@@ -68,6 +68,19 @@ func TestAircraftFilters(t *testing.T) {
 			t.Errorf("WithinNM kept %s at %.1f nm", a.Hex, a.RDst)
 		}
 	}
+	fast := r.Filter(MinSpeed(250, GroundSpeed))
+	for _, a := range fast {
+		if a.GS < 250 {
+			t.Errorf("MinSpeed kept %s at %.0f kt", a.Hex, a.GS)
+		}
+	}
+	// MaxSpeed drops aircraft with no reading, so every kept aircraft is positive.
+	slow := r.Filter(MaxSpeed(250, GroundSpeed))
+	for _, a := range slow {
+		if a.GS <= 0 || a.GS > 250 {
+			t.Errorf("MaxSpeed kept %s at %.0f kt", a.Hex, a.GS)
+		}
+	}
 	// Composition is AND: position AND high altitude is a subset of each.
 	both := r.Filter(WithPosition(), MinAltitude(30000))
 	if len(both) > len(pos) || len(both) > len(high) {