Smooth viewshed boundary to remove SRTM elevation spikes

Apply 5-point circular moving average to reach_km values before
rendering the boundary polygon. Eliminates sharp spikes caused by
isolated terrain anomalies in SRTM data.

lib/microwaveprop/terrain/viewshed.ex

@@ -129,6 +129,7 @@ defmodule Microwaveprop.Terrain.Viewshed do
         {:exit, _} -> nil
       end)
       |> Enum.reject(&is_nil/1)
+      |> smooth_boundary(lat, lon)
 
     %{origin: %{lat: lat, lon: lon}, boundary: boundary}
   end
@@ -180,6 +181,34 @@ defmodule Microwaveprop.Terrain.Viewshed do
     Application.get_env(:microwaveprop, :srtm_tiles_dir, Path.expand("~/srtm/tiles"))
   end
 
+  # Smooth reach_km values with a circular moving average to remove spikes
+  # from SRTM elevation artifacts, then recompute boundary lat/lon.
+  defp smooth_boundary(points, _origin_lat, _origin_lon) when length(points) < 5, do: points
+
+  defp smooth_boundary(points, origin_lat, origin_lon) do
+    reaches = Enum.map(points, & &1.reach_km)
+    n = length(reaches)
+    arr = :array.from_list(reaches)
+    half = 2
+
+    smoothed_reaches =
+      Enum.map(0..(n - 1), fn i ->
+        window =
+          for offset <- -half..half do
+            :array.get(rem(i + offset + n, n), arr)
+          end
+
+        Enum.sum(window) / length(window)
+      end)
+
+    points
+    |> Enum.zip(smoothed_reaches)
+    |> Enum.map(fn {pt, reach} ->
+      {lat, lon} = destination_point(origin_lat, origin_lon, pt.bearing, reach)
+      %{pt | reach_km: reach, lat: lat, lon: lon}
+    end)
+  end
+
   defp find_first_obstructed_index(interior) do
     Enum.find_index(interior, & &1.obstructed)
   end