create endgame play section

lib/chesstrainer/endgames/endgame.ex

@@ -36,6 +36,78 @@ defmodule Chesstrainer.Endgames.Endgame do
     |> validate_format(:key, ~r/^(?=.{5,10}$)KQ*R*[NB]*P*\sv\sKQ*R*[NB]*P*$/,
       message: "Key must follow pattern KQ.. v KQR.., max pieces 7"
     )
+    |> validate_change(:fen, fn :fen, fen ->
+      case fen_issues(fen) do
+        [] -> []
+        issues -> [fen: "is not a valid FEN: #{Enum.join(issues, "; ")}"]
+      end
+    end)
     |> unique_constraint(:fen, message: "FEN already exists")
   end
+
+  # Strict structural validation. Mirrors chess.js rules so the FEN survives
+  # both the server-side parser (Chex, permissive) and the client-side parser
+  # (chess.js, strict).
+  defp fen_issues(fen) do
+    case String.split(fen, " ") do
+      [board, active, _castling, _en_passant, halfmove, fullmove] ->
+        issues = []
+
+        issues =
+          if valid_board?(board),
+            do: issues,
+            else: ["board must be 8 ranks of 8 squares each"]
+
+        issues =
+          if active in ["w", "b"],
+            do: issues,
+            else: ["active color must be 'w' or 'b'"]
+
+        issues =
+          if non_negative_integer?(halfmove),
+            do: issues,
+            else: ["halfmove clock must be a non-negative integer"]
+
+        issues =
+          if positive_integer?(fullmove),
+            do: issues,
+            else: ["fullmove number must be a positive integer"]
+
+        issues
+
+      _ ->
+        ["must have 6 space-separated fields"]
+    end
+  end
+
+  defp valid_board?(board) do
+    ranks = String.split(board, "/")
+
+    length(ranks) == 8 and Enum.all?(ranks, &valid_rank?/1)
+  end
+
+  defp valid_rank?(rank) do
+    squares =
+      rank
+      |> String.graphemes()
+      |> Enum.reduce(0, fn
+        g, acc when byte_size(g) == 1 ->
+          c = :binary.first(g)
+
+          if c >= ?1 and c <= ?8 do
+            acc + (c - ?0)
+          else
+            acc + 1
+          end
+      end)
+
+    squares == 8
+  end
+
+  defp non_negative_integer?(str), do: integer_string?(str) and String.to_integer(str) >= 0
+  defp positive_integer?(str), do: integer_string?(str) and String.to_integer(str) >= 1
+
+  defp integer_string?(str) do
+    Regex.match?(~r/^[0-9]+$/, str)
+  end
 end