From 84202206dbd54c47fe6d2d92b11431fa0c0f580b Mon Sep 17 00:00:00 2001
From: Sotetsu KOYAMADA <koyamada-s@sys.i.kyoto-u.ac.jp>
Date: Mon, 8 Jan 2024 21:05:43 +0900
Subject: [PATCH] [ConnectFour] Separate game specific methods (#1152)

---
 pgx/connect_four.py | 80 +++++++++++++++++++++++++++++----------------
 1 file changed, 51 insertions(+), 29 deletions(-)

diff --git a/pgx/connect_four.py b/pgx/connect_four.py
index 244cb4eb5..81938fc45 100644
--- a/pgx/connect_four.py
+++ b/pgx/connect_four.py
@@ -34,7 +34,7 @@ class GameState:
     #  [28, 29, 30, 31, 32, 33, 34],
     #  [35, 36, 37, 38, 39, 40, 41]]
     _board: Array = -jnp.ones(42, jnp.int32)  # -1 (empty), 0, 1
-    _blank_row: Array = jnp.full(7, 5)
+    winner: Array = jnp.int32(-1)
 
 
 @dataclass
@@ -115,47 +115,69 @@ def _init(rng: PRNGKey) -> State:
     return State(current_player=current_player)  # type:ignore
 
 
+def _step_game_state(state: GameState, action: Array) -> GameState:
+    board2d = state._board.reshape(6, 7)
+    num_filled = (board2d[:, action] >= 0).sum()
+    board2d = board2d.at[5 - num_filled, action].set(state._turn)
+    won = _win_check(board2d.flatten(), state._turn)
+    winner = jax.lax.select(won, state._turn, -1)
+    return state.replace(  # type: ignore
+        _turn=1 - state._turn,
+        _board=board2d.flatten(),
+        winner=winner,
+    )
+
+
 def _step(state: State, action: Array) -> State:
-    board = state._x._board
-    row = state._x._blank_row[action]
-    blank_row = state._x._blank_row.at[action].set(row - 1)
-    board = board.at[_to_idx(row, action)].set(state._x._turn)
-    won = _win_check(board, state._x._turn)
-    reward = jax.lax.cond(
-        won,
-        lambda: jnp.float32([-1, -1]).at[state.current_player].set(1),
-        lambda: jnp.zeros(2, jnp.float32),
+    x = _step_game_state(state._x, action)
+    state = state.replace(  # type: ignore
+        current_player=1 - state.current_player,
+        _x=x,
     )
+    terminated = is_terminal(state._x)
+    rewards = returns(state._x)
+    should_flip = state.current_player != state._x._turn
+    rewards = jax.lax.select(should_flip, jnp.flip(rewards), rewards)
+    rewards = jax.lax.select(terminated, rewards, jnp.zeros(2, jnp.float32))
     return state.replace(  # type: ignore
-        current_player=1 - state.current_player,
-        legal_action_mask=blank_row >= 0,
-        terminated=won | jnp.all(blank_row == -1),
-        rewards=reward,
-        _x=state._x.replace(  # type: ignore
-            _turn=1 - state._x._turn,
-            _board=board,
-            _blank_row=blank_row,
-        ),
+        legal_action_mask=legal_action_mask(state._x),
+        rewards=rewards,
+        terminated=terminated,
+    )
+
+
+def returns(state: GameState) -> Array:
+    return jax.lax.cond(
+        state.winner >= 0,
+        lambda: jnp.float32([-1, -1]).at[state.winner].set(1),
+        lambda: jnp.zeros(2, jnp.float32),
     )
 
 
-def _to_idx(row, col):
-    return row * 7 + col
+def is_terminal(state: GameState) -> Array:
+    board2d = state._board.reshape(6, 7)
+    return (state.winner >= 0) | jnp.all((board2d >= 0).sum(axis=0) == 6)
+
+
+def legal_action_mask(state: GameState) -> Array:
+    board2d = state._board.reshape(6, 7)
+    return (board2d >= 0).sum(axis=0) < 6
 
 
 def _win_check(board, turn) -> Array:
     return ((board[IDX] == turn).all(axis=1)).any()
 
 
-def _observe(state: State, player_id: Array) -> Array:
-    turns = jnp.int32([state._x._turn, 1 - state._x._turn])
-    turns = jax.lax.cond(
-        player_id == state.current_player,
-        lambda: turns,
-        lambda: jnp.flip(turns),
-    )
+def _observe_game_state(state: GameState, color: Array) -> Array:
+    turns = jax.lax.select(color == 0, jnp.int32([0, 1]), jnp.int32([1, 0]))
 
     def make(turn):
-        return state._x._board.reshape(6, 7) == turn
+        return state._board.reshape(6, 7) == turn
 
     return jnp.stack(jax.vmap(make)(turns), -1)
+
+
+def _observe(state: State, player_id: Array) -> Array:
+    curr_color = state._x._turn
+    my_color = jax.lax.select(player_id == state.current_player, curr_color, 1 - curr_color)
+    return _observe_game_state(state._x, my_color)