Tweak possible move calculations

The move must be valid on the current board, but not necessarily on both
boards. This means that a piece may teleport into a position to block
check, since the king was not in check on their original board and thus
their move was valid on their original board.

Changelog.md

@@ -8,6 +8,12 @@
   - According to the linked rules (should have read it thoroughly!), there were
     some potentially incorrect moves (either missing or unnecessarily present)
     that could have been generated.
+- Tweaked possible move calculations: move must be valid on current board, but
+  not necessarily on both boards
+  - According to the linked rules (should have read it thoroughly!!!), a piece
+    may teleport into a position to block check, since the move was valid on the
+    board it was on (the king was not in check on that board), and the move is
+    legal because the king is no longer in check.
 
 ## v2.1.0
 

README.md

@@ -18,6 +18,10 @@ Here is a [description of the concept and rules][rules].
 
 Notable game rules:
 
+- A piece's move must be valid on the board it is on, which means that a piece
+  on Board B can block a check on Board A after teleporting (since the move was
+  valid on Board B, and the move overall was legal because the king is not
+  staying in check). See the above link for a more detailed argument on this.
 - **Castling**: A king and rook may only castle if neither has moved already,
   the king is not in check, the squares they will move to are vacant on both
   boards, and the king does not move through check (on Board A) or into check.

src/alicechess/_moves_calculator.py

@@ -351,8 +351,12 @@ def _move_in_check(
             check_pos = here_target_pos
         else:
             check_pos = king.pos
-        if _is_threatened(board, enemy_color, *check_pos):
-            return True
+        if moving_king or pos.bn == king.pos.bn:
+            # if the king is moving, can't move into check
+            # otherwise, if on the same board as the king, can't let the
+            # king be in check
+            if _is_threatened(board, enemy_color, *check_pos):
+                return True
         # make sure move is valid after teleporting
         board[there_target_pos] = board.pop(here_target_pos).move_to(
             there_target_pos