feat: move stuff to numerics

internal/numerics/motion_test.go

@@ -0,0 +1,58 @@
+package numerics
+
+import (
+	"math"
+	"testing"
+)
+
+func TestAddGeo(t *testing.T) {
+	// Rates sum component-wise with no longitude wrapping.
+	got := AddGeo(GeoVec{Lat: 1, Lng: 350, Altitude: 2}, GeoVec{Lat: 3, Lng: 20, Altitude: 4})
+	want := GeoVec{Lat: 4, Lng: 370, Altitude: 6}
+	if got != want {
+		t.Errorf("AddGeo = %+v, want %+v (no wrap on rates)", got, want)
+	}
+}
+
+func TestWindToGeoRate(t *testing.T) {
+	// Pure eastward 10 m/s at the equator, sea level.
+	dLat, dLng := WindToGeoRate(10, 0, 0, 0)
+	wantLng := (180.0 / math.Pi) * 10.0 / EarthRadius
+	if math.Abs(dLat) > 1e-15 {
+		t.Errorf("dLat = %v, want 0", dLat)
+	}
+	if math.Abs(dLng-wantLng) > 1e-15 {
+		t.Errorf("dLng = %v, want %v", dLng, wantLng)
+	}
+
+	// Northward 5 m/s at 60°N: dLat independent of longitude scaling.
+	dLat, _ = WindToGeoRate(0, 5, 60, 0)
+	wantLat := (180.0 / math.Pi) * 5.0 / EarthRadius
+	if math.Abs(dLat-wantLat) > 1e-15 {
+		t.Errorf("dLat at 60N = %v, want %v", dLat, wantLat)
+	}
+
+	// cos(lat) factor makes eastward motion span more degrees nearer the poles.
+	_, dLngEq := WindToGeoRate(10, 0, 0, 0)
+	_, dLng60 := WindToGeoRate(10, 0, 60, 0)
+	if dLng60 <= dLngEq {
+		t.Errorf("eastward deg/s should grow with latitude: eq=%v 60N=%v", dLngEq, dLng60)
+	}
+}
+
+func TestDragTerminalVelocity(t *testing.T) {
+	// Descent is downward (negative) and faster (more negative) at altitude
+	// where the air is thinner.
+	sea := DragTerminalVelocity(5, 0)
+	high := DragTerminalVelocity(5, 20000)
+	if sea >= 0 {
+		t.Errorf("sea-level rate = %v, want negative (downward)", sea)
+	}
+	if high >= sea {
+		t.Errorf("expected faster descent at altitude: sea=%v high=%v", sea, high)
+	}
+	// Sanity: at sea level rho≈1.225, so v ≈ -5*1.1045/sqrt(1.225) ≈ -4.99 m/s.
+	if math.Abs(sea-(-5*1.1045/math.Sqrt(NasaDensity(0)))) > 1e-12 {
+		t.Errorf("sea-level formula mismatch: %v", sea)
+	}
+}