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chess_ai.py
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chess_ai.py
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"""
Generates an AI move. The AI ratings from 1500 to 1700 are played by the
a negamax algorithim with alpha-beta pruning, while the rest of the rating ranges
are played by the stockfish engine.
"""
import chess.engine
import position_status
ROW_DIM = 8
# These piece-square tables represent the positional advantages each piece
# has at certain squares depending on their mobility and control options.
# Values from: (https://www.chessprogramming.net/using-excel-to-help-create-piece-square-tables/)
white_pawn_values = [
[0, 0, 0, 0, 0, 0, 0, 0],
[18, 22, 34, 50, 50, 34, 22, 18],
[6, 12, 25, 40, 40, 25, 12, 6],
[-3, 3, 17, 28, 28, 17, 3, -3],
[-10, -5, 10, 20, 20, 10, -5, -10],
[-10, -5, 5, 15, 15, 5, -5, -10],
[-10, -5, 5, 10, 10, 5, -5, -10],
[0, 0, 0, 0, 0, 0, 0, 0]
]
black_pawn_values = white_pawn_values[::-1]
white_knight_values = [
[-60, -40, -30, -30,-30, -30, -40, -60],
[-40, 20, 40, 40, 40, 40, 20, -40],
[-40, 45, 60, 70, 70, 60, 45, -40],
[-40, 40, 50, 50, 50, 50, 40, -40],
[-40, 10, 40, 35, 35, 40, 10, -40],
[-40, 0, 30, 20, 20, 30, 0, -40],
[-55, -40, -10, 10, 10, -10, -40, -55],
[-70, -20, -25, -15, -15, -25, -20, -70]
]
black_knight_values = white_knight_values[::-1]
white_bishop_values = [
[-10, -8, -6, -4, -4, -6, -8, -10],
[0, 20, 10, 10, 10, 10, 20, 0],
[10, 20, 30, 35, 35, 30, 20, 10],
[10, 30, 30, 35, 35, 30, 30, 10],
[10, 10, 25, 30, 30, 25, 10, 10],
[-15, 15, 25, 20, 20, 25, 15, -15],
[-18, 20, 15, 10, 10, 15, 20, -18],
[-20, -15, -10, -10, -10, -10, -15, -20]
]
black_bishop_values = white_bishop_values[::-1]
white_rook_values = [
[-10, -8, 0, 5, 5, 0, -8, -10],
[0, 0, 5, 10, 10, 5, 0, 0],
[-10, -8, 4, 8, 8, 4, -8, -10],
[-10, -8, 4, 6, 6, 4, -8, -10],
[-10, -8, 4, 5, 5, 4, -8, -10],
[-10, -8, 4, 5, 5, 4, -8, -10],
[-10, -8, 0, 5, 5, 0, -8, -10],
[-10, -8, 0, 5, 5, 0, -8, -10],
]
black_rook_values = white_rook_values[::-1]
white_queen_values = [
[0, 0, 0, 0, 0, 0, 0, 0],
[0, 0, 0, 0, 0, 0, 0, 0],
[0, 0, 0, 0, 0, 0, 0, 0],
[0, 0, 0, 0, 0, 0, 0, 0],
[0, 0, 0, 0, 0, 0, 0, 0],
[-10, -10, -5, 0, 0, -5, -10, -10],
[-20, -15, -5, 0, 0, -5, -15, -20],
[-30, -20, -10, 0, 0, -10, -20, -30]
]
black_queen_values = white_queen_values[::-1]
white_king_values = [
[-55, -55, -60, -70, -70, -60, -55, -55],
[-55, -55, -60, -70, -70, -60, -55, -55],
[-55, -55, -60, -70, -70, -60, -55, -55],
[-55, -55, -60, -70, -70, -60, -55, -55],
[-50, -50, -55, -60, -60, -55, -50, -50],
[-40, -40, -45, -50, -50, -45, -40, -40],
[-30, -30, -30, -35, -35, -30, -30, -30],
[-3, 0, 0, -10, -10, -8, 0, -3]
]
black_king_values = white_king_values[::-1]
# Plays the best ai move depending on what difficulty/rating was set. The more advanced rating
# levels use the stockfish engine.
def ai_move(difficulty, turn, fen_board):
engine = chess.engine.SimpleEngine.popen_uci("./Stockfish/stockfish")
copied_fen_board = fen_board.copy()
if (fen_board.is_checkmate()):
return fen_board
if difficulty == 1500:
negamax(2, copied_fen_board, turn, copied_fen_board.legal_moves, 2, -9999, 9999)
if best_move in fen_board.legal_moves:
fen_board.push(best_move)
else:
legal_moves = list(fen_board.legal_moves)
fen_board.push(legal_moves[0])
elif difficulty == 1600:
negamax(3, copied_fen_board, turn, copied_fen_board.legal_moves, 3, -9999, 9999)
if best_move in fen_board.legal_moves:
fen_board.push(best_move)
else:
legal_moves = list(fen_board.legal_moves)
fen_board.push(legal_moves[0])
elif difficulty == 1700:
negamax(4, copied_fen_board, turn, copied_fen_board.legal_moves, 4, -9999, 9999)
if best_move in fen_board.legal_moves:
fen_board.push(best_move)
else:
legal_moves = list(fen_board.legal_moves)
fen_board.push(legal_moves[0])
elif difficulty == 1800:
limit = chess.engine.Limit(depth=1)
result = engine.play(fen_board, limit)
fen_board.push(result.move)
elif difficulty == 1900:
limit = chess.engine.Limit(depth=2)
result = engine.play(fen_board, limit)
fen_board.push(result.move)
elif difficulty == 2000:
limit = chess.engine.Limit(depth=3)
result = engine.play(fen_board, limit)
fen_board.push(result.move)
elif difficulty == 2100:
limit = chess.engine.Limit(depth=3)
result = engine.play(fen_board, limit)
fen_board.push(result.move)
elif difficulty == 2200:
limit = chess.engine.Limit(depth=4)
result = engine.play(fen_board, limit)
fen_board.push(result.move)
engine.quit()
return fen_board
# Apply negamax algorithim recursively with alpha beta pruning to
# evaluate root positions for a certain depth
def negamax(curr_depth, fen_board, turn, moves, max_depth, alpha, beta):
global best_move
board = position_status.fen_to_board(fen_board.fen())
if curr_depth == 0:
if turn == 'w':
return evaluate_board_score(fen_board, board, turn)
else:
return -1 * evaluate_board_score(fen_board, board, turn)
max = -9999
if not fen_board.is_checkmate():
for move in moves:
fen_board.push(move)
if turn == 'b':
turn = 'w'
else:
turn = 'b'
next_moves = fen_board.legal_moves
position_score = -negamax(curr_depth - 1, fen_board, turn, next_moves, max_depth, -beta, -alpha)
if position_score > max:
max = position_score
if curr_depth == max_depth:
best_move = move
fen_board.pop()
if max > alpha:
alpha = max
if alpha >= beta:
break
return max
# Determines piece value based on its raw value (e.g. white pawn is always
# worth 100 points) along with which square it is on the board (positional
# value). White values are positive, and black values are negative. This
# function returns their sum.
def evaluate_board_score(fen_board, board, turn):
white_value = 0
black_value = 0
for row in range(ROW_DIM):
for column in range(ROW_DIM):
piece = board[row][column]
if fen_board.is_checkmate():
if turn == 'w':
return 9999
else:
return -9999
if piece == 'e':
pass
else:
if (piece[0] == 'w'):
if (piece[1] == 'p'):
white_value += 100 + white_pawn_values[row][column]
elif (piece[1] == 'b'):
white_value += 300 + white_bishop_values[row][column]
elif (piece[1] == 'n'):
white_value += 300 + white_knight_values[row][column]
elif (piece[1] == 'r'):
white_value += 500 + white_rook_values[row][column]
elif (piece[1] == 'q'):
white_value += 900 + white_queen_values[row][column]
elif (piece[1] == 'k'):
white_value += 9000 + white_king_values[row][column]
elif (piece[0] == 'b'):
if (piece[1] == 'p'):
black_value += 100+ black_pawn_values[row][column]
elif (piece[1] == 'b'):
black_value += 300 + black_bishop_values[row][column]
elif (piece[1] == 'n'):
black_value += 300 + black_knight_values[row][column]
elif (piece[1] == 'r'):
black_value += 500 + black_rook_values[row][column]
elif (piece[1] == 'q'):
black_value += 900 + black_queen_values[row][column]
elif (piece[1] == 'k'):
black_value += 9000 + black_king_values[row][column]
return white_value - black_value