add position filter

pkg/types/readsb/filter.go

@@ -2,6 +2,7 @@ package readsb
 
 import (
 	"iter"
+	"math"
 	"slices"
 	"strings"
 	"time"
@@ -141,6 +142,28 @@ func WithinNM(nm float64) AircraftFilter {
 	return func(a *Aircraft) bool { return a.RDst > 0 && a.RDst <= nm }
 }
 
+// WithinNMOf keeps aircraft whose current position is within nm nautical miles
+// of the given point (decimal degrees). Unlike WithinNM, which measures from the
+// receiver, this measures from an arbitrary point; aircraft with no position are
+// dropped.
+func WithinNMOf(lat, lon, nm float64) AircraftFilter {
+	return func(a *Aircraft) bool {
+		return a.HasPosition() && haversineNM(lat, lon, a.Lat, a.Lon) <= nm
+	}
+}
+
+// haversineNM returns the great-circle distance in nautical miles between two
+// points given in decimal degrees.
+func haversineNM(lat1, lon1, lat2, lon2 float64) float64 {
+	const earthRadiusNM = 3440.065 // mean Earth radius in nautical miles
+	dLat, dLon := rad(lat2-lat1), rad(lon2-lon1)
+	h := math.Sin(dLat/2)*math.Sin(dLat/2) +
+		math.Cos(rad(lat1))*math.Cos(rad(lat2))*math.Sin(dLon/2)*math.Sin(dLon/2)
+	return earthRadiusNM * 2 * math.Asin(math.Sqrt(h))
+}
+
+func rad(deg float64) float64 { return deg * math.Pi / 180 }
+
 // SeenWithin keeps aircraft heard from within the given duration.
 func SeenWithin(d time.Duration) AircraftFilter {
 	return func(a *Aircraft) bool { return a.SeenFor() <= d }

pkg/types/readsb/readsb_test.go

@@ -88,6 +88,31 @@ func TestAircraftFilters(t *testing.T) {
 	}
 }
 
+func TestWithinNMOf(t *testing.T) {
+	// One degree of latitude is ~60 nm; sanity-check the haversine helper.
+	if d := haversineNM(0, 0, 1, 0); d < 59 || d > 61 {
+		t.Errorf("haversineNM(1 deg lat) = %.2f nm, want ~60", d)
+	}
+	r := decodeFile[AircraftReport](t, "aircraft.json")
+	var lat, lon float64
+	for _, a := range r.Aircraft {
+		if a.HasPosition() {
+			lat, lon = a.Lat, a.Lon
+			break
+		}
+	}
+	near := r.Filter(WithinNMOf(lat, lon, 100))
+	for _, a := range near {
+		if !a.HasPosition() || haversineNM(lat, lon, a.Lat, a.Lon) > 100 {
+			t.Errorf("WithinNMOf kept %s at %.1f nm", a.Hex, haversineNM(lat, lon, a.Lat, a.Lon))
+		}
+	}
+	// The point came from a real aircraft, so at least that one must match.
+	if len(near) == 0 {
+		t.Error("WithinNMOf returned no aircraft for an in-data point")
+	}
+}
+
 func TestAltBaroGround(t *testing.T) {
 	var a AltBaro
 	if err := json.Unmarshal([]byte(`"ground"`), &a); err != nil {