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[Chess] Fix stale INIT_ZOBRIST_HASH: derive it from the Zobrist tables at import time #1320

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[Chess] Fix stale INIT_ZOBRIST_HASH: derive it from the Zobrist tables at import time #1320

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15 changes: 14 additions & 1 deletion pgx/_src/games/chess.py
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Original file line number Diff line number Diff line change
Expand Up @@ -134,7 +134,20 @@
ZOBRIST_SIDE = jax.random.randint(keys[1], shape=(2,), minval=0, maxval=2**31 - 1, dtype=jnp.uint32)
ZOBRIST_CASTLING = jax.random.randint(keys[2], shape=(4, 2), minval=0, maxval=2**31 - 1, dtype=jnp.uint32)
ZOBRIST_EN_PASSANT = jax.random.randint(keys[3], shape=(65, 2), minval=0, maxval=2**31 - 1, dtype=jnp.uint32)
INIT_ZOBRIST_HASH = jnp.uint32([1455170221, 1478960862])


def _init_zobrist_hash() -> Array:
# Derived from the tables above instead of hardcoded: the tables come from jax.random,
# whose outputs are not stable across JAX versions — a stale constant silently breaks
# repetition counting for the initial position.
hash_ = ZOBRIST_SIDE # color == 0
hash_ ^= lax.reduce(ZOBRIST_BOARD[jnp.arange(64), INIT_BOARD + 6], 0, lax.bitwise_xor, (0,))
hash_ ^= lax.reduce(ZOBRIST_CASTLING, 0, lax.bitwise_xor, (0,)) # all castling rights
hash_ ^= ZOBRIST_EN_PASSANT[-1] # en_passant == -1
return hash_


INIT_ZOBRIST_HASH = _init_zobrist_hash()


class GameState(NamedTuple):
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