Notification if move would result in check

assets/js/services/channel.js

@@ -34,7 +34,10 @@ class Channel {
   }
 
   sendMove(move) {
-    this.channel.push("game:move", move);
+    this.channel.push("game:move", move)
+      .receive("error", resp => {
+        alert(resp.message);
+      });
   }
 }
 

lib/chess/moves/piece.ex

@@ -54,14 +54,17 @@ defmodule Chess.Moves.Piece do
   defp _attacked?(_board, {7, _rank}, {1, _}, _), do: false
   defp _attacked?(_board, {_file, 7}, {_, 1}, _), do: false
   defp _attacked?(board, {file, rank}, {fv, rv}, pieces) do
-    {file, rank} =
     board
     |> Generator.moves({file, rank}, {fv, rv})
     |> List.last
-
+    |> case do
+      {file, rank} ->
         piece = board["#{file},#{rank}"]
 
         Enum.any?(pieces, &(match?(%{"type" => ^&1}, piece)))
+      nil ->
+        false
+    end
   end
 
   defp _attacked?(board, {file, rank}, pattern, piece_type) do

lib/chess/store/game.ex

@@ -28,6 +28,12 @@ defmodule Chess.Store.Game do
     |> foreign_key_constraint(:opponent_id)
   end
 
+  def move_changeset(struct, params \\ %{}) do
+    struct
+    |> cast(params, required_attrs())
+    |> validate_king_in_check(struct, params)
+  end
+
   def change_turn("black"), do: "white"
   def change_turn("white"), do: "black"
 
@@ -41,6 +47,20 @@ defmodule Chess.Store.Game do
       or_where: game.opponent_id == ^user_id
   end
 
+  def validate_king_in_check(changeset, %Game{turn: turn}, %{board: board}) do
+    case Board.king_in_check?(board, turn) do
+      true ->
+        changeset
+        |> add_error(
+          :board,
+          "That move would result in your king being in check"
+        )
+      _ ->
+        changeset
+    end
+  end
+  def validate_king_in_check(changeset, _, _), do: changeset
+
   def ordered(query) do
     query
     |> order_by([game], desc: game.inserted_at)

lib/chess_web/channels/game_channel.ex

@@ -24,7 +24,9 @@ defmodule ChessWeb.GameChannel do
       board: Board.transform(game.board),
       turn: game.turn
     }
-    push(socket, "game:update", payload)
+
+    socket
+    |> push("game:update", payload)
 
     {:noreply, socket}
   end
@@ -35,7 +37,7 @@ defmodule ChessWeb.GameChannel do
       |> Game.for_user_id()
       |> Repo.get!(socket.assigns.game_id)
 
-    changeset = Game.changeset(
+    changeset = Game.move_changeset(
       game, %{
         board: Board.move_piece(game.board, move_params),
         turn: Game.change_turn(game.turn)
@@ -47,6 +49,10 @@ defmodule ChessWeb.GameChannel do
         send_update(game)
 
         {:noreply, socket}
+      {:error, changeset} ->
+        {message, _} = changeset.errors[:board]
+
+        {:reply, {:error, %{message: message}}, socket}
     end
   end
 

test/chess/moves/piece_test.exs

@@ -21,6 +21,16 @@ defmodule Chess.Moves.PieceTest do
     refute Piece.attacked?(board, {4, 0})
   end
 
+  test "piece next to another piece is being attacked" do
+    board = %{
+      "4,0" => %{"type" => "king", "colour" => "white"},
+      "4,1" => %{"type" => "pawn", "colour" => "white"},
+      "7,3" => %{"type" => "bishop", "colour" => "black"},
+    }
+
+    assert Piece.attacked?(board, {4, 0})
+  end
+
   test "piece is not being attacked by piece of its own colour" do
     board = %{
       "4,5" => %{"type" => "king", "colour" => "white"},

test/features/moves_test.exs

@@ -115,6 +115,75 @@ defmodule Chess.MovesTest do
     |> assert_has(square_containing("f4-r3", "white.pawn"))
   end
 
+  test "cannot move the king into a position that would result in check",
+       %{session: session} do
+    user = insert(:user, %{
+      name: "Link",
+      email: "link@hyrule.com",
+      password: "ilovezelda"
+    })
+    opponent = insert(:user, %{
+      name: "Zelda",
+      email: "zelda@hyrule.com",
+      password: "ganonsucks"
+    })
+    insert(:game, %{
+      board: %{
+        "4,0" => %{"type" => "king",   "colour" => "white"},
+        "3,7" => %{"type" => "queen",  "colour" => "black"},
+      },
+      user_id: user.id,
+      opponent_id: opponent.id,
+      turn: "white",
+    })
+
+    session
+    |> login("link@hyrule.com", "ilovezelda")
+    |> visit("/games")
+    |> click(link("Game with Zelda"))
+
+    session
+    |> click(css("#f4-r0"))
+    |> click(css("#f3-r0"))
+    |> refute_has(square_containing("f3-r0", "white.king"))
+    |> assert_has(square_containing("f4-r0", "white.king"))
+  end
+
+  test "cannot make a move that would place the king in check",
+       %{session: session} do
+    user = insert(:user, %{
+      name: "Link",
+      email: "link@hyrule.com",
+      password: "ilovezelda"
+    })
+    opponent = insert(:user, %{
+      name: "Zelda",
+      email: "zelda@hyrule.com",
+      password: "ganonsucks"
+    })
+    insert(:game, %{
+      board: %{
+        "4,0" => %{"type" => "king", "colour" => "white"},
+        "4,1" => %{"type" => "rook", "colour" => "white"},
+        "4,7" => %{"type" => "queen", "colour" => "black"},
+      },
+      user_id: user.id,
+      opponent_id: opponent.id,
+      turn: "white",
+    })
+
+    session
+    |> login("link@hyrule.com", "ilovezelda")
+    |> visit("/games")
+    |> click(link("Game with Zelda"))
+
+    session
+    |> click(css("#f4-r1"))
+    |> click(css("#f2-r1"))
+    |> refute_has(square_containing("f2-r1", "white.rook"))
+    |> assert_has(square_containing("f4-r1", "white.rook"))
+  end
+
   defp square_selected(square) do
     css("##{square}.selected")
   end