Support for Empty Point (#172)

* Support for Empty Point

Adds support for (E)WKB and GeoJSON conversion for Empty Point.

An Empty Point in
  - in WKB is represented as `Point(NaN NaN)`.
  - in GeoJson is represented as `{"type" => "Point", "coordinates" => []}`

This PR implements an Empty Point in Geo as `%Geo.Point{coordinates: nil}`

* Make empty point happy with WKB

lib/geo/json/decoder.ex

@@ -115,6 +115,10 @@ defmodule Geo.JSON.Decoder do
     %Point{coordinates: {x, y}, srid: get_srid(crs), properties: properties}
   end
 
+  defp do_decode("Point", [], properties, crs) do
+    %Point{coordinates: nil, srid: get_srid(crs), properties: properties}
+  end
+
   defp do_decode("LineString", coordinates, properties, crs) do
     coordinates = Enum.map(coordinates, &list_to_tuple(&1))
 

lib/geo/json/encoder.ex

@@ -126,6 +126,10 @@ defmodule Geo.JSON.Encoder do
     %{"type" => "Point", "coordinates" => [x, y]}
   end
 
+  defp do_encode(%Point{coordinates: nil}) do
+    %{"type" => "Point", "coordinates" => []}
+  end
+
   defp do_encode(%PointZ{coordinates: {x, y, z}}) do
     %{"type" => "Point", "coordinates" => [x, y, z]}
   end

lib/geo/wkb/decoder.ex

@@ -317,4 +317,27 @@ defmodule Geo.WKB.Decoder do
       {%GeometryCollection{geometries: geometries, srid: srid}, rest}
     end
   end
+
+  defp do_decode(
+         @point,
+         <<0, 0, 0, 0, 0, 0, 248, 127, 0, 0, 0, 0, 0, 0, 248, 127, rest::bits>>,
+         srid,
+         1
+       ) do
+    {%Point{coordinates: nil, srid: srid}, rest}
+  end
+
+  defp do_decode(
+         @point,
+         binary,
+         srid,
+         0
+       ) do
+    little_binary =
+      binary
+      |> :binary.decode_unsigned(:big)
+      |> :binary.encode_unsigned(:little)
+
+    do_decode(@point, little_binary, srid, 1)
+  end
 end

lib/geo/wkb/encoder.ex

@@ -55,6 +55,14 @@ defmodule Geo.WKB.Encoder do
       [unquote(endian), binary, rest]
     end
 
+    def do_encode(%Point{coordinates: nil}, :ndr) do
+      {@point, [<<00, 00, 00, 00, 00, 00, 248, 127>>, <<00, 00, 00, 00, 00, 00, 248, 127>>]}
+    end
+
+    def do_encode(%Point{coordinates: nil}, :xdr) do
+      {@point, [<<127, 248, 00, 00, 00, 00, 00, 00>>, <<127, 248, 00, 00, 00, 00, 00, 00>>]}
+    end
+
     def do_encode(%Point{coordinates: {x, y}}, unquote(endian_atom)) do
       {@point, [<<x::unquote(modifier)-float-64>>, <<y::unquote(modifier)-float-64>>]}
     end

test/geo/json_test.exs

@@ -57,6 +57,16 @@ defmodule Geo.JSON.Test do
     assert(exjson == new_exjson)
   end
 
+  test "GeoJson Point without coordinates" do
+    json = "{ \"type\": \"Point\", \"coordinates\": [] }"
+    exjson = Jason.decode!(json)
+    geom = Jason.decode!(json) |> Geo.JSON.decode!()
+    assert(is_nil(geom.coordinates))
+
+    new_exjson = Geo.JSON.encode!(geom)
+    assert(exjson == new_exjson)
+  end
+
   test "GeoJson with SRID to Point and back" do
     json =
       "{\"type\":\"Point\",\"crs\":{\"type\":\"name\",\"properties\":{\"name\":\"EPSG:4326\"}},\"coordinates\":[100.0, 101.0]}"
@@ -350,6 +360,14 @@ defmodule Geo.JSON.Test do
     end
   end
 
+  test "encodes and decodes back to the correct Empty Point struct" do
+    geom = %Geo.Point{coordinates: nil}
+    json = Geo.JSON.encode!(geom) |> Jason.encode!()
+
+    assert(json == "{\"coordinates\":[],\"type\":\"Point\"}")
+    assert geom == Geo.JSON.encode!(geom) |> Geo.JSON.decode!()
+  end
+
   property "encodes and decodes back to the correct LineString struct" do
     check all(list <- list_of({float(), float()}, min_length: 1)) do
       geom = %Geo.LineString{coordinates: list}

test/geo/wkb_test.exs

@@ -500,6 +500,12 @@ defmodule Geo.WKB.Test do
     assert(point.srid == 4326)
   end
 
+  test "Decode empty Point EWKB to Point" do
+    point = Geo.WKB.decode!("0101000020E6100000000000000000F87F000000000000F87F")
+    assert(point.coordinates == nil)
+    assert(point.srid == 4326)
+  end
+
   test "Decode empty MultiPolygon EWKB to MultiPolygon" do
     multipolygon = Geo.WKB.decode!("0106000020E610000000000000")
     assert(multipolygon.coordinates == [])
@@ -703,6 +709,22 @@ defmodule Geo.WKB.Test do
     assert {:ok, "0101000000000000000000F03F000000000000F03F"} = Geo.WKB.encode(geom, :ndr)
   end
 
+  test "Encode/Decode Empty Point in big endian" do
+    geom = %Geo.Point{coordinates: nil}
+    geom_wkb = "00000000017FF80000000000007FF8000000000000"
+
+    assert Geo.WKB.encode!(geom, :xdr) == geom_wkb
+    assert Geo.WKB.decode!(geom_wkb) == geom
+  end
+
+  test "Encode/Decode Empty Point in little endian" do
+    geom = %Geo.Point{coordinates: nil}
+    geom_wkb = "00000000017FF80000000000007FF8000000000000"
+
+    assert Geo.WKB.encode!(geom, :xdr) == geom_wkb
+    assert Geo.WKB.decode!(geom_wkb) == geom
+  end
+
   property "encodes and decodes back to the correct Point struct" do
     check all(
             x <- float(),