Support arbitrary-precision Maidenhead grid squares

Extend grid validation and decoding from 4/6 chars to unlimited
even-length grids (8, 10, etc). Recursive decoder handles alternating
letter(/24) and digit(/10) subdivision pairs after the field+square.
Validated against gridmap-web's Maidenhead implementation.

lib/microwaveprop/radio/maidenhead.ex

@@ -1,16 +1,24 @@
 defmodule Microwaveprop.Radio.Maidenhead do
   @moduledoc false
 
+  # Maidenhead grid: Field (2 letters A-R) + Square (2 digits) + optional
+  # repeating pairs of Subsquare (2 letters) and Extended Square (2 digits).
+  # Minimum 4 chars, always even length. E.g. EM12, EM12kp, EM12kp37, EM12kp37ab
   @spec valid?(any()) :: boolean()
   def valid?(nil), do: false
 
   def valid?(grid) when is_binary(grid) do
-    String.match?(grid, ~r/^[A-Ra-r]{2}[0-9]{2}$/) or
-      String.match?(grid, ~r/^[A-Ra-r]{2}[0-9]{2}[A-Xa-x]{2}$/)
+    len = byte_size(grid)
+    len >= 4 and rem(len, 2) == 0 and String.match?(grid, grid_regex())
   end
 
   def valid?(_), do: false
 
+  # Field: [A-Ra-r]{2}, Square: [0-9]{2}, then alternating letter/digit pairs
+  defp grid_regex do
+    ~r/^[A-Ra-r]{2}[0-9]{2}(?:[A-Xa-x]{2}(?:[0-9]{2}(?:[A-Xa-x]{2}(?:[0-9]{2})*)*)*)*$/
+  end
+
   @spec to_latlon(any()) :: {:ok, {float(), float()}} | :error
   def to_latlon(nil), do: :error
 
@@ -20,22 +28,42 @@ defmodule Microwaveprop.Radio.Maidenhead do
     if valid?(grid) do
       <<f1, f2, s1, s2>> <> rest = grid
 
-      lon = (f1 - ?A) * 20 + (s1 - ?0) * 2 - 180
-      lat = (f2 - ?A) * 10 + (s2 - ?0) * 1 - 90
+      # Field: 18 divisions (A-R), 20° lon / 10° lat each
+      lon = (f1 - ?A) * 20.0 - 180.0
+      lat = (f2 - ?A) * 10.0 - 90.0
 
-      case rest do
-        <<ss1, ss2>> ->
-          lon = lon + (ss1 - ?A) * (5 / 60) + 5 / 120
-          lat = lat + (ss2 - ?A) * (2.5 / 60) + 2.5 / 120
-          {:ok, {lat, lon}}
+      # Square: 10 divisions (0-9), 2° lon / 1° lat each
+      lon = lon + (s1 - ?0) * 2.0
+      lat = lat + (s2 - ?0) * 1.0
 
-        "" ->
-          {:ok, {lat + 0.5, lon + 1.0}}
-      end
+      chars = String.to_charlist(rest)
+      {lat, lon, lat_size, lon_size} = decode_pairs(chars, lat, lon, 1.0, 2.0, :letter)
+      {:ok, {lat + lat_size / 2, lon + lon_size / 2}}
     else
       :error
     end
   end
 
   def to_latlon(_), do: :error
+
+  # Decode remaining alternating letter(24)/digit(10) pairs after field+square
+  defp decode_pairs([c1, c2 | rest], lat, lon, lat_size, lon_size, :letter) do
+    cell_lon = lon_size / 24
+    cell_lat = lat_size / 24
+    lon = lon + (c1 - ?A) * cell_lon
+    lat = lat + (c2 - ?A) * cell_lat
+    decode_pairs(rest, lat, lon, cell_lat, cell_lon, :digit)
+  end
+
+  defp decode_pairs([c1, c2 | rest], lat, lon, lat_size, lon_size, :digit) do
+    cell_lon = lon_size / 10
+    cell_lat = lat_size / 10
+    lon = lon + (c1 - ?0) * cell_lon
+    lat = lat + (c2 - ?0) * cell_lat
+    decode_pairs(rest, lat, lon, cell_lat, cell_lon, :letter)
+  end
+
+  defp decode_pairs([], lat, lon, lat_size, lon_size, _type) do
+    {lat, lon, lat_size, lon_size}
+  end
 end