test: property tests for PathCompute loss/power budget + Viewshed destination

- PathCompute: compute_loss_budget/5 and compute_power_budget/2 made
  public with @doc false; property tests covering fspl, o2, h2o, rain,
  diffraction across all 13 known bands + varying distances
- Viewshed: property tests for destination_point back-bearing round-trip
  and east-west bearing at equator (2 new properties)

Coverage: 79.07% → 79.06% (PathCompute 60→63%)

lib/microwaveprop/propagation/path_compute.ex

@@ -380,7 +380,8 @@ defmodule Microwaveprop.Propagation.PathCompute do
     end
   end
 
-  defp compute_loss_budget(dist_km, freq_ghz, band_config, terrain_result, conditions) do
+  @doc false
+  def compute_loss_budget(dist_km, freq_ghz, band_config, terrain_result, conditions) do
     freq_mhz = freq_ghz * 1000
     fspl = 20 * :math.log10(max(dist_km, 0.001)) + 20 * :math.log10(freq_mhz) + 32.44
 
@@ -423,7 +424,8 @@ defmodule Microwaveprop.Propagation.PathCompute do
     }
   end
 
-  defp compute_power_budget(station_params, loss_budget) do
+  @doc false
+  def compute_power_budget(station_params, loss_budget) do
     tx_power_dbm = station_params.tx_power_dbm
     eirp_dbm = tx_power_dbm + station_params.src_gain_dbi
     rx_power_dbm = eirp_dbm - loss_budget.total + station_params.dst_gain_dbi

test/microwaveprop/propagation/path_compute_test.exs

@@ -0,0 +1,120 @@
+defmodule Microwaveprop.Propagation.PathComputeTest do
+  use ExUnit.Case, async: true
+  use ExUnitProperties
+
+  alias Microwaveprop.Propagation.BandConfig
+  alias Microwaveprop.Propagation.PathCompute
+
+  describe "compute_loss_budget/5" do
+    test "fspl increases with distance" do
+      band = BandConfig.get(10_000)
+      near = PathCompute.compute_loss_budget(10.0, 10.0, band, nil, nil)
+      far = PathCompute.compute_loss_budget(100.0, 10.0, band, nil, nil)
+      assert near.fspl < far.fspl
+      assert near.total < far.total
+    end
+
+    test "fspl increases with frequency" do
+      band_10g = BandConfig.get(10_000)
+      band_24g = BandConfig.get(24_000)
+      low = PathCompute.compute_loss_budget(50.0, 10.0, band_10g, nil, nil)
+      high = PathCompute.compute_loss_budget(50.0, 24.0, band_24g, nil, nil)
+      assert low.fspl < high.fspl
+    end
+
+    test "gas absorption losses with default humidity when conditions absent" do
+      band = BandConfig.get(10_000)
+      result = PathCompute.compute_loss_budget(50.0, 10.0, band, nil, nil)
+      assert result.o2 >= 0
+      assert result.h2o >= 0
+    end
+
+    test "higher bands have more h2o absorption" do
+      band_10g = BandConfig.get(10_000)
+      band_75g = BandConfig.get(75_000)
+      low = PathCompute.compute_loss_budget(50.0, 10.0, band_10g, nil, nil)
+      high = PathCompute.compute_loss_budget(50.0, 75.0, band_75g, nil, nil)
+      assert low.h2o <= high.h2o
+    end
+
+    test "rain loss is zero when there's no rain" do
+      band = BandConfig.get(10_000)
+      conditions = %{rain_rate_mmhr: 0.0, abs_humidity: 7.5}
+      result = PathCompute.compute_loss_budget(50.0, 10.0, band, nil, conditions)
+      assert result.rain == 0.0
+    end
+
+    test "rain loss is positive when it's raining" do
+      band = BandConfig.get(10_000)
+      conditions = %{rain_rate_mmhr: 5.0, abs_humidity: 7.5}
+      result = PathCompute.compute_loss_budget(50.0, 10.0, band, nil, conditions)
+      assert result.rain > 0
+    end
+
+    test "diffraction loss is included when terrain result is present" do
+      band = BandConfig.get(10_000)
+      terrain = %{analysis: %{diffraction_db: 15.0}}
+      result = PathCompute.compute_loss_budget(50.0, 10.0, band, terrain, nil)
+      assert result.diffraction == 15.0
+    end
+
+    test "diffraction loss is zero when no terrain result" do
+      band = BandConfig.get(10_000)
+      result = PathCompute.compute_loss_budget(50.0, 10.0, band, nil, nil)
+      assert result.diffraction == 0.0
+    end
+  end
+
+  describe "compute_loss_budget/5 property" do
+    property "total is always >= fspl" do
+      check all dist_km <- float(min: 0.1, max: 500),
+                band_config = BandConfig.get(10_000) do
+        result = PathCompute.compute_loss_budget(dist_km, 10.0, band_config, nil, nil)
+        assert result.total >= result.fspl
+        assert result.total > 0
+      end
+    end
+
+    property "all loss components are non-negative across known bands" do
+      bands = [50, 144, 222, 432, 902, 1_296, 2_304, 3_400, 5_760, 10_000, 24_000, 47_000, 75_000]
+
+      check all dist_km <- float(min: 0.1, max: 500),
+                band_mhz <- member_of(bands) do
+        band_config = BandConfig.get(band_mhz)
+        freq_ghz = band_mhz / 1000
+        result = PathCompute.compute_loss_budget(dist_km, freq_ghz, band_config, nil, nil)
+        assert result.fspl >= 0
+        assert result.o2 >= 0
+        assert result.h2o >= 0
+        assert result.rain >= 0
+        assert result.diffraction >= 0
+      end
+    end
+  end
+
+  describe "compute_power_budget/2" do
+    test "computes rx power and margins" do
+      loss = %{total: 140.0, fspl: 130.0, o2: 3.0, h2o: 5.0, rain: 0.0, diffraction: 2.0}
+      params = %{tx_power_dbm: 30.0, src_gain_dbi: 20.0, dst_gain_dbi: 20.0}
+
+      result = PathCompute.compute_power_budget(params, loss)
+
+      # EIRP = tx_power + src_gain = 50 dBm
+      assert_in_delta result.eirp_dbm, 50.0, 0.1
+      # rx_power = eirp - loss + dst_gain
+      assert_in_delta result.rx_power_dbm, -70.0, 0.1
+      # margin_cw = rx_power - (-140)
+      assert_in_delta result.margin_cw, 70.0, 0.1
+    end
+
+    test "higher tx power improves margins" do
+      loss = %{total: 140.0, fspl: 130.0, o2: 3.0, h2o: 5.0, rain: 0.0, diffraction: 2.0}
+
+      low = PathCompute.compute_power_budget(%{tx_power_dbm: 20.0, src_gain_dbi: 10.0, dst_gain_dbi: 10.0}, loss)
+      high = PathCompute.compute_power_budget(%{tx_power_dbm: 30.0, src_gain_dbi: 20.0, dst_gain_dbi: 20.0}, loss)
+
+      assert low.rx_power_dbm < high.rx_power_dbm
+      assert low.margin_cw < high.margin_cw
+    end
+  end
+end

test/microwaveprop/terrain/viewshed_test.exs

@@ -236,6 +236,32 @@ defmodule Microwaveprop.Terrain.ViewshedTest do
     end
   end
 
+  describe "destination_point/4 property" do
+    use ExUnitProperties
+
+    property "short bearings round-trip for CONUS-mid latitudes" do
+      check all lat <- float(min: 25, max: 50),
+                lon <- float(min: -125, max: -65),
+                bearing <- float(min: 0, max: 360),
+                dist_km <- float(min: 0.1, max: 20) do
+        {lat2, lon2} = Viewshed.destination_point(lat, lon, bearing, dist_km)
+        back_bearing = if(bearing < 180, do: bearing + 180, else: bearing - 180)
+        {lat3, lon3} = Viewshed.destination_point(lat2, lon2, back_bearing, dist_km)
+        assert_in_delta lat3, lat, 0.5
+        assert_in_delta lon3, lon, 0.5
+      end
+    end
+
+    property "east-west bearing preserves latitude at equator" do
+      check all lon <- float(min: -170, max: 170),
+                dist_km <- float(min: 1, max: 50) do
+        {lat2, lon2} = Viewshed.destination_point(0.0, lon, 90, dist_km)
+        assert_in_delta lat2, 0.0, 0.01
+        assert lon2 != lon
+      end
+    end
+  end
+
   describe "analyse_ray/5" do
     test "returns full range for flat terrain with antenna heights" do
       profile =