From 32ba5de4564053014180280c758a3ff7e030bf0e Mon Sep 17 00:00:00 2001
From: Graham McIntire <graham@mcintire.me>
Date: Tue, 21 Apr 2026 13:57:47 -0500
Subject: [PATCH] test(property): add property tests for parsing + domain
 conversions

---
 test/microwaveprop/format_property_test.exs   |  57 +++++++
 .../radio/band_resolver_property_test.exs     |  97 +++++++++++
 .../radio/maidenhead_property_test.exs        | 150 ++++++++++++++++++
 3 files changed, 304 insertions(+)
 create mode 100644 test/microwaveprop/format_property_test.exs
 create mode 100644 test/microwaveprop/radio/band_resolver_property_test.exs
 create mode 100644 test/microwaveprop/radio/maidenhead_property_test.exs

diff --git a/test/microwaveprop/format_property_test.exs b/test/microwaveprop/format_property_test.exs
new file mode 100644
index 00000000..6b270975
--- /dev/null
+++ b/test/microwaveprop/format_property_test.exs
@@ -0,0 +1,57 @@
+defmodule Microwaveprop.FormatPropertyTest do
+  use ExUnit.Case, async: true
+  use ExUnitProperties
+
+  alias Microwaveprop.Format
+
+  # Any finite non-negative number is legitimate input for distance_km/1.
+  # StreamData floats default to the full representable range — we keep
+  # that but reject negatives since negative distances are meaningless
+  # (the algorithm callers always pass an already-absolute km value).
+  defp non_negative_number do
+    one_of([
+      integer(0..1_000_000_000),
+      map(float(min: 0.0), fn f -> f end),
+      map(integer(0..1_000_000), &(&1 * 1.0))
+    ])
+  end
+
+  describe "distance_km/1" do
+    property "returns a binary for nil" do
+      # Not really a property, just pinned for discoverability alongside
+      # the numeric properties.
+      assert Format.distance_km(nil) == "—"
+    end
+
+    property "always returns a non-empty binary for any non-negative number" do
+      check all(n <- non_negative_number()) do
+        result = Format.distance_km(n)
+        assert is_binary(result)
+        assert result != ""
+      end
+    end
+
+    property "output contains both `mi` and `km` units" do
+      check all(n <- non_negative_number()) do
+        result = Format.distance_km(n)
+        assert result =~ "mi"
+        assert result =~ "km"
+      end
+    end
+
+    property "accepts Decimal input without crashing" do
+      check all(n <- integer(0..10_000)) do
+        decimal = Decimal.new(n)
+        assert is_binary(Format.distance_km(decimal))
+      end
+    end
+
+    property "accepts negative distances without crashing" do
+      # Not a valid input at the call site, but we don't want arbitrary
+      # numeric data (e.g. a sign slip in a caller) to crash the view.
+      check all(n <- float(min: -1.0e6, max: -0.01)) do
+        assert is_binary(Format.distance_km(n))
+      end
+    end
+  end
+end
diff --git a/test/microwaveprop/radio/band_resolver_property_test.exs b/test/microwaveprop/radio/band_resolver_property_test.exs
new file mode 100644
index 00000000..14153952
--- /dev/null
+++ b/test/microwaveprop/radio/band_resolver_property_test.exs
@@ -0,0 +1,97 @@
+defmodule Microwaveprop.Radio.BandResolverPropertyTest do
+  use ExUnit.Case, async: true
+  use ExUnitProperties
+
+  alias Microwaveprop.Radio.BandResolver
+
+  describe "resolve/1 output contract" do
+    property "result is always nil or a member of allowed_bands/0 (strings)" do
+      check all(s <- string(:printable)) do
+        case BandResolver.resolve(s) do
+          nil -> :ok
+          mhz -> assert mhz in BandResolver.allowed_bands()
+        end
+      end
+    end
+
+    property "result is always nil or a member of allowed_bands/0 (integers)" do
+      check all(i <- integer(-1_000..500_000)) do
+        case BandResolver.resolve(i) do
+          nil -> :ok
+          mhz -> assert mhz in BandResolver.allowed_bands()
+        end
+      end
+    end
+
+    property "result is always nil or a member of allowed_bands/0 (floats)" do
+      check all(f <- float(min: -10.0, max: 500_000.0)) do
+        case BandResolver.resolve(f) do
+          nil -> :ok
+          mhz -> assert mhz in BandResolver.allowed_bands()
+        end
+      end
+    end
+
+    property "never crashes on arbitrary term input" do
+      check all(
+              term <-
+                one_of([
+                  constant(nil),
+                  constant(""),
+                  constant(:atom),
+                  constant([]),
+                  constant(%{}),
+                  string(:printable),
+                  integer(),
+                  float()
+                ])
+            ) do
+        result = BandResolver.resolve(term)
+        assert is_nil(result) or is_integer(result)
+      end
+    end
+  end
+
+  describe "resolve/1 is idempotent for canonical bands" do
+    property "every allowed band resolves to itself" do
+      check all(mhz <- member_of(BandResolver.allowed_bands())) do
+        assert BandResolver.resolve(mhz) == mhz
+        assert BandResolver.resolve(Integer.to_string(mhz)) == mhz
+      end
+    end
+
+    property "resolve_as_string/1 round-trips through Integer.to_string" do
+      check all(mhz <- member_of(BandResolver.allowed_bands())) do
+        assert BandResolver.resolve_as_string(mhz) == Integer.to_string(mhz)
+      end
+    end
+  end
+
+  describe "resolve/1 snap-to-nearest" do
+    property "frequencies within a small neighborhood of a microwave band resolve to that band" do
+      # The >= 900 MHz branch uses nearest-neighbor snapping. A small
+      # perturbation of a canonical band value must land on the same
+      # band — but we clamp the perturbed input to stay ≥ 900 so we
+      # don't cross the resolver's lower cutoff for 902 (the smallest
+      # microwave band).
+      bands_900_plus = Enum.filter(BandResolver.allowed_bands(), &(&1 >= 900))
+
+      check all(
+              mhz <- member_of(bands_900_plus),
+              delta_pct <- float(min: -0.001, max: 0.001)
+            ) do
+        perturbed = max(900.01, mhz * (1.0 + delta_pct))
+        # Only assert when the perturbed value is still unambiguously
+        # closer to `mhz` than to any other allowed band.
+        nearest_other =
+          bands_900_plus
+          |> Enum.reject(&(&1 == mhz))
+          |> Enum.min_by(&abs(&1 - perturbed))
+
+        if abs(perturbed - mhz) < abs(perturbed - nearest_other) do
+          assert BandResolver.resolve(perturbed) == mhz
+        end
+      end
+    end
+  end
+end
diff --git a/test/microwaveprop/radio/maidenhead_property_test.exs b/test/microwaveprop/radio/maidenhead_property_test.exs
new file mode 100644
index 00000000..a248753b
--- /dev/null
+++ b/test/microwaveprop/radio/maidenhead_property_test.exs
@@ -0,0 +1,150 @@
+defmodule Microwaveprop.Radio.MaidenheadPropertyTest do
+  use ExUnit.Case, async: true
+  use ExUnitProperties
+
+  alias Microwaveprop.Radio.Maidenhead
+
+  # Generators ---------------------------------------------------------
+
+  # Upper-case field chars A-R (18 divisions, the only canonical output
+  # from from_latlon/3).
+  defp field_char, do: map(integer(0..17), &<<?A + &1>>)
+
+  # Upper-case subsquare chars A-X (24 divisions). from_latlon/3 emits
+  # lower-case, so we hold grids canonical in lower-case when assembling.
+  defp subsquare_char, do: map(integer(0..23), &<<?a + &1>>)
+
+  defp digit_char, do: map(integer(0..9), &<<?0 + &1>>)
+
+  # Canonical 4-char grid: UPPER field + digits. to_latlon upcases, so
+  # staying canonical here gives us exact round-trip equality.
+  defp grid4 do
+    gen all(
+          f1 <- field_char(),
+          f2 <- field_char(),
+          d1 <- digit_char(),
+          d2 <- digit_char()
+        ) do
+      f1 <> f2 <> d1 <> d2
+    end
+  end
+
+  defp grid6 do
+    gen all(
+          g <- grid4(),
+          s1 <- subsquare_char(),
+          s2 <- subsquare_char()
+        ) do
+      g <> s1 <> s2
+    end
+  end
+
+  defp grid8 do
+    gen all(
+          g <- grid6(),
+          d1 <- digit_char(),
+          d2 <- digit_char()
+        ) do
+      g <> d1 <> d2
+    end
+  end
+
+  # Lat/lon that stay inside the representable field range. The top row
+  # (lat = 90.0) and the wrap-around longitude (180.0) sit exactly on a
+  # field boundary and land outside the A-R field set, so we stop just
+  # short.
+  defp latlon do
+    gen all(
+          lat <- float(min: -90.0, max: 89.999),
+          lon <- float(min: -180.0, max: 179.999)
+        ) do
+      {lat, lon}
+    end
+  end
+
+  # Properties ---------------------------------------------------------
+
+  describe "round-trip from_latlon/3 → to_latlon/1" do
+    property "precision 4: decoded center is within one grid cell of input" do
+      # A 4-char grid covers 2° lon × 1° lat; the center must be within
+      # half that distance of any point in the cell.
+      check all({lat, lon} <- latlon()) do
+        grid = Maidenhead.from_latlon(lat, lon, 4)
+        assert {:ok, {dlat, dlon}} = Maidenhead.to_latlon(grid)
+        assert abs(dlat - lat) <= 0.5 + 1.0e-9
+        assert abs(dlon - lon) <= 1.0 + 1.0e-9
+      end
+    end
+
+    property "precision 6: decoded center is within half a subsquare" do
+      # 6-char subsquare is 5'×2.5' → 5/60 lon, 2.5/60 lat.
+      check all({lat, lon} <- latlon()) do
+        grid = Maidenhead.from_latlon(lat, lon, 6)
+        assert {:ok, {dlat, dlon}} = Maidenhead.to_latlon(grid)
+        assert abs(dlat - lat) <= 2.5 / 60 / 2 + 1.0e-9
+        assert abs(dlon - lon) <= 5.0 / 60 / 2 + 1.0e-9
+      end
+    end
+
+    property "precision 8: decoded center is within half an extended square" do
+      check all({lat, lon} <- latlon()) do
+        grid = Maidenhead.from_latlon(lat, lon, 8)
+        assert {:ok, {dlat, dlon}} = Maidenhead.to_latlon(grid)
+        assert abs(dlat - lat) <= 2.5 / 60 / 10 / 2 + 1.0e-9
+        assert abs(dlon - lon) <= 5.0 / 60 / 10 / 2 + 1.0e-9
+      end
+    end
+  end
+
+  describe "round-trip to_latlon/1 → from_latlon/3" do
+    property "precision 4 grids come back unchanged (after upcasing)" do
+      check all(grid <- grid4()) do
+        assert {:ok, {lat, lon}} = Maidenhead.to_latlon(grid)
+        assert Maidenhead.from_latlon(lat, lon, 4) == String.upcase(grid)
+      end
+    end
+
+    property "precision 6 grids come back unchanged" do
+      # from_latlon/3 emits upper-case field/square and lower-case
+      # subsquare, matching the canonical form our generator produces.
+      check all(grid <- grid6()) do
+        assert {:ok, {lat, lon}} = Maidenhead.to_latlon(grid)
+        assert Maidenhead.from_latlon(lat, lon, 6) == grid
+      end
+    end
+
+    property "precision 8 grids come back unchanged" do
+      check all(grid <- grid8()) do
+        assert {:ok, {lat, lon}} = Maidenhead.to_latlon(grid)
+        assert Maidenhead.from_latlon(lat, lon, 8) == grid
+      end
+    end
+  end
+
+  describe "decoded bounds" do
+    property "to_latlon output stays inside valid geographic range" do
+      check all(grid <- grid8()) do
+        assert {:ok, {lat, lon}} = Maidenhead.to_latlon(grid)
+        assert lat >= -90.0 and lat <= 90.0
+        assert lon >= -180.0 and lon <= 180.0
+      end
+    end
+  end
+
+  describe "valid?/1" do
+    property "accepts every grid emitted by from_latlon/3" do
+      check all(
+              {lat, lon} <- latlon(),
+              precision <- one_of([constant(4), constant(6), constant(8)])
+            ) do
+        assert Maidenhead.valid?(Maidenhead.from_latlon(lat, lon, precision))
+      end
+    end
+
+    property "never crashes on arbitrary binary input" do
+      check all(s <- string(:printable)) do
+        assert is_boolean(Maidenhead.valid?(s))
+      end
+    end
+  end
+end