Inspired by this puzzle: Doubled Knights redux
Let's make the following change to the rules of chess. Every piece can once per game (on its day in the sun) make a double move.
Clarifications: Things allowed in the double move:
- double capture
- walking in and out of check either by moving the king or by leaving a pin and reblocking or capturing
- if the first move is castling the second move can be done by either king or rook and only that piece's double move is spent (but see special rule below)
Things not allowed:
- moving two different pieces (except castling)
- synthetic non-moves i.e. moving and immediately moving back except when the first move is a capture; this rule is meant to protect the endangered species of stalemates
Further clarifications:
- if the opponent's king has used up their double move and yours hasn't your king on its own can check and also mate the other king
- squares that are reachable by double move (of a piece that still has the ability) are considered threatened, i.e. the king cannot end its move there or castle through such a square (but see special rule below)
- a piece blocking a double move capture of its own king by a rook or bishop or queen or an about-to-be-promoted pawn is pinned; it can only move if at the end of its move its king is under no direct threat
- promotion does not reset the double move: if the pawn has spent it it's gone
- the standard pawn double move (like e2-e4) does not count as a double move
Special rule:
- castling through a threatened square is allowed but counts as a king's double move
A short example opening:
1.e4 (note how this pins both the d7 and f7 pawns) e5 2. Nc3 Bb4xc3 3. dxc3+ (because f8 is covered by the white queen and the black d-pawn is still pinned black can only defend by putting something in between:) Qe7-d6+ 4. Qxd6-a3 etc.
Your task: find the shortest possible game, i.e. the shortest sequence of legal moves from the standard opening position that ends in a mate or stalemate.
