Series.concat handle multiple integer, float and null types (#812)

lib/explorer/series.ex

@@ -2097,20 +2097,55 @@ defmodule Explorer.Series do
   @doc type: :shape
   @spec concat([Series.t()]) :: Series.t()
   def concat([%Series{} | _t] = series) do
-    dtypes = series |> Enum.map(& &1.dtype) |> Enum.uniq()
+    {null?, dtypes_map} =
+      Enum.reduce(series, {false, %{}}, fn
+        %Series{dtype: :null}, {true, _dtypes} = acc ->
+          acc
 
-    case dtypes do
-      [_dtype] ->
-        impl!(series).concat(series)
+        %Series{dtype: :null}, {_, dtypes} ->
+          {true, dtypes}
 
-      [a, b] when K.and(is_numeric_dtype(a), is_numeric_dtype(b)) ->
-        series = Enum.map(series, &cast(&1, {:f, 64}))
-        impl!(series).concat(series)
+        %Series{dtype: dt}, {null?, dtypes} when is_atom(dt) ->
+          {null?, Map.put_new(dtypes, dt, dt)}
 
-      incompatible ->
-        raise ArgumentError,
-              "cannot concatenate series with mismatched dtypes: #{inspect(incompatible)}. " <>
-                "First cast the series to the desired dtype."
+        %Series{dtype: {type, n} = dt}, {null?, dtypes} ->
+          dtypes =
+            case dtypes[type] do
+              nil -> Map.put(dtypes, type, dt)
+              {_t, tn} when n > tn -> Map.put(dtypes, type, dt)
+              _ -> dtypes
+            end
+
+          {null?, dtypes}
+      end)
+
+    dtypes = Map.values(dtypes_map)
+
+    series =
+      series
+      |> maybe_raise_mismatched!(dtypes)
+      |> maybe_cast(null?, dtypes)
+
+    impl!(series).concat(series)
+  end
+
+  defp maybe_cast(series, true, []), do: series
+
+  defp maybe_cast(series, _, [dtype]), do: Enum.map(series, &cast(&1, dtype))
+
+  defp maybe_cast(series, _, _), do: Enum.map(series, &cast(&1, {:f, 64}))
+
+  defp maybe_raise_mismatched!(series, [_dtype]), do: series
+
+  defp maybe_raise_mismatched!(series, dtypes) do
+    if Enum.all?(dtypes, &is_numeric_dtype/1) do
+      series
+    else
+      dtypes = Enum.map(series, & &1.dtype)
+
+      raise ArgumentError,
+            "cannot concatenate series with mismatched dtypes: #{inspect(dtypes)}. " <>
+              "First cast the series to the desired dtype."
     end
   end