chess2plyrs"
In chess2plyrs: Chess Game Creation and Tools

chess2plyrs is an R package which allows to create a chess game and add moves, verify the status of the game and list legal moves, as well as read and write FENs. It is also possible to plot the current board position. A simple chess engine based on minimax is also implemented.

library(chess2plyrs)

Initialise a game

A new game can be created with the newgame function:

g1 <- newgame()
g1

We can see that the object we create is a lists which contains

board: the current position
turn: 1 if it is white's turn, -1 if it is black's turn
history: the previous moves played (in a primitive style)
fen_history: the FEN representation of the previous moves

We can add one move using the chess_move function, which takes in input:

the game being played
the piece label (p for pawns, N for knights, B for bishops, R for rooks, Q for queens, K for kings)
the initial square of the piece to be moved
the final square

chess_move(game = g1,
            piece = "p",
            initialposition = "e2",
            finalposition = "e4")

We can also make use of the pipe operator for a smoother insertion of the moves:

g2 <- newgame() |>
  chess_move("p", "e2", "e4")  |>
  chess_move("p", "e7", "e5")  |>
  chess_move("N", "g1", "f3")  |>
  chess_move("N", "b8", "c6")  |>
  chess_move("B", "f1", "b5")  |>
  chess_move("N", "g8", "f6")  |>
  chess_move("K", "e1", "0-0") |>
  chess_move("N", "f6", "e4")

If we try to add an illegal move, a note tells us that the move is not legal and the action is not performed:

x <- newgame() |>
  chess_move("p", "e2", "d5") # illegal move

Chess plotting

First, we can plot the current board with the function chessplot. The style option allows for different piece styles:

style = 1 is the default option: unicode encoding is used
style = 2 plots pieces with their labels instead of the unicode. As unicode may yield an error on some platforms, here we use this option

chessplot(g2, style = 2)

Score list

We can get the moves list in scientific notation using the moves_scoresheet function

moves_scoresheet(g2)

Game result

The function game_result tells whether the game is still ongoing or if there is a checkmate or stalemate.

game_result(g2)

We can see that in the current position there are still legal moves, hence the game is not finished.

Legal moves

All legal moves in a given position can be listed using the function legalmoves

legalmoves(g2)

Takeback

The function takeback allows to remove the last move played. It can also be used sequentially.

takeback(g2)

Engine

Along with a simple random mover engine, which can be called using the function random_mover, a slightly more sophisticated engine which uses minimax and alpha-beta pruning is present in this package. For more information have a look at

https://en.wikipedia.org/wiki/Minimax
https://www.r-bloggers.com/2022/07/programming-a-simple-minimax-chess-engine-in-r/

We can set a simple position and see the move chosen by each engine with different depths:

ck4 <- newgame() |>

  chess_move("N", "b1", "a3") |>
  chess_move("p", "e7", "e6") |>

  chess_move("p", "f2", "f4") |>
  chess_move("p", "a7", "a6") |>
  chess_move("p", "h2", "h3") |>
  chess_move("p", "h7", "h6") |>
  chess_move("N", "a3", "b5")

chessplot(ck4, style = 2)

You can see that black is to move and has a mate starting with Qh4+ (g3, Qxg3#). You can verify that with depth equal to 3 the engine indeed selects that as the move of choice, while at lower depth it prefers to take the Knight in b5.

#engine2(ck4, 1) # "pa6b5" is the chosen move (5 seconds needed)
#engine2(ck4, 2) # "pa6b5" is the chosen move (2/3 minutes needed)
#engine2(ck4, 3) # "Qd8h4" is the chosen move (it takes some minutes to find it)