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1ed9e4d 70df921 1ed9e4d | 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 | """Make a digitized board placement into a legal, playable position.
Digitization can misclassify squares, producing impossible boards (a pawn on the
back rank, two kings of a colour, too many pieces). This fixes the common cases
so chess.js / python-chess accept the position, and reports what it changed.
"""
from __future__ import annotations
import chess
MAX_PER_SIDE = 16
MAX_PAWNS = 8
COLOR_NAME = {chess.WHITE: "white", chess.BLACK: "black"}
def _parse(placement: str) -> dict[int, chess.Piece]:
pm: dict[int, chess.Piece] = {}
rank, file = 7, 0
for ch in placement.split()[0]:
if ch == "/":
rank, file = rank - 1, 0
elif ch.isdigit():
file += int(ch)
elif 0 <= rank <= 7 and 0 <= file <= 7:
try:
pm[chess.square(file, rank)] = chess.Piece.from_symbol(ch)
except ValueError:
pass
file += 1
return pm
def sanitize(placement: str) -> dict:
"""Returns {fen, placement, corrections, valid}."""
pm = _parse(placement)
fixes: list[str] = []
# 1. pawns can't be on rank 1 or 8
for sq, p in list(pm.items()):
if p.piece_type == chess.PAWN and chess.square_rank(sq) in (0, 7):
del pm[sq]
fixes.append(f"removed pawn on {chess.square_name(sq)} (impossible rank)")
# 2. exactly one king per colour
for color in (chess.WHITE, chess.BLACK):
kings = [sq for sq, p in pm.items() if p.piece_type == chess.KING and p.color == color]
for extra in kings[1:]:
del pm[extra]
fixes.append(f"removed extra {COLOR_NAME[color]} king on {chess.square_name(extra)}")
if not kings:
home = chess.E1 if color == chess.WHITE else chess.E8
target = home if home not in pm else next(s for s in chess.SQUARES if s not in pm)
pm[target] = chess.Piece(chess.KING, color)
fixes.append(f"added missing {COLOR_NAME[color]} king on {chess.square_name(target)}")
# 3. cap pawns (<=8) and total pieces (<=16) per side
for color in (chess.WHITE, chess.BLACK):
pawns = [sq for sq, p in pm.items() if p.piece_type == chess.PAWN and p.color == color]
for sq in pawns[MAX_PAWNS:]:
del pm[sq]
fixes.append(f"removed extra {COLOR_NAME[color]} pawn on {chess.square_name(sq)}")
own = [sq for sq, p in pm.items() if p.color == color]
if len(own) > MAX_PER_SIDE:
removable = [s for s in own if pm[s].piece_type != chess.KING]
for sq in removable[: len(own) - MAX_PER_SIDE]:
del pm[sq]
fixes.append(f"removed excess {COLOR_NAME[color]} piece on {chess.square_name(sq)}")
# 4. bishops: at most one per square colour (the natural pair → max 2 per side)
for color in (chess.WHITE, chess.BLACK):
seen = set()
for sq, p in list(pm.items()):
if p.piece_type == chess.BISHOP and p.color == color:
sq_color = (chess.square_file(sq) + chess.square_rank(sq)) % 2 # 0 dark, 1 light
if sq_color in seen:
del pm[sq]
fixes.append(f"removed extra {COLOR_NAME[color]} bishop on "
f"{chess.square_name(sq)} (same square colour)")
else:
seen.add(sq_color)
# build board, drop castling/en-passant (can't be inferred from an image)
board = chess.Board(None)
for sq, p in pm.items():
board.set_piece_at(sq, p)
board.castling_rights = 0
# 4. choose a side to move that is legal (the side not to move must not be in check)
board.turn = chess.WHITE
if not board.is_valid():
board.turn = chess.BLACK
return {
"fen": board.fen(),
"placement": board.board_fen(),
"corrections": fixes,
"valid": board.is_valid(),
}
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