Therefore the holdings keys are not XORed but added to the total key,
after all other key terms have been taken into account through XOR.
<p>
-The move encoding on large boards is fromSquare*boardSize + toSquare + boardSize*boardSize*promotionType.
+The move encoding on non-8x8 boards is generalized to fromSquare*boardSize + toSquare + boardSize*boardSize*promotionType
+(where boardSize = boardLength*boardWidth).
For large boards this might leave a shortage of promotion types,
but fortunately promotions are not very common in opening theory.
The meaning of the promotionType code can be variant-dependent.
(E.g. in Grand Chess the board is 10x10, so the promotion-type multiplier is 10,000,
while the 16-bit move encoding only runs upto 65,535.
So there is only room for 6 promotion types, while 7 would be needed for the 6 possible piece types and non-promotion.)
-In asymmetric variants (e.g. Spartan Chess) the same code need not indicate the same piece type for both sides.
+In asymmetric variants (e.g. Spartan Chess) the same code needs not indicate the same piece type for both sides.
+<p>
+Special moves are indicated by using a (non-existent) promotion choice as an 'escape',
+in which case the from-square number isn't really a from-square, but identifies the special move.
+This system is used (with promotionType = 9, when available) for drops, where the from-square contains the ID of the piece to be dropped
+(A=1, B=2 etc.).
+It is also used to indicate double-moves of a Chu-Shogi Lion (promotionType = 10, when available, but 2 in Chu Shogi),
+where every combination of first and second step gets a unique code 0-63, which is then stored in the from-square.
+</p>
+<p>
+For really large boards (say larger than 144 squares) a more compact encoding system should be used,
+which indicates the from-square relative to the to-square in polar coordinates:
+toSquare + boardSize*directionCode + 4*boardSize*distanceCode.
+Here boardLength is the largest linear dimension of the board (be it height or width),
+and the distanceCodes < boardLength-1 indicate the distance along the indicated ray to the from-square,
+such that the move goes in the direction forward, right, right-forward or left-forward as seen from the player that makes the move.
+When this lies off-board, the distanceCode should be subtracted from boardLength+1 to obtain the distance in the opposite direction.
+The rayCodes 0-3 represent the directions vertical, horizontal, diagonal and anti-diagonal (N, E, NE, SE), respectively.
+The remaining distanceCodes (>= boardLength-1) represent oblique moves, special moves and promotions.
+They always start with the two Knight jumps landing just right of the ray, in the positive and negative direction,
+followed by two (1,3) and two (2,3) jumps.
+</p><p>
+After the Knight jumps the meaning of the distanceCodes becomes variant dependent.
+By default they would contain promotions to various piece types,
+in the order of their piece code (i.e. N, B, R, Q, K, other) in so far they participate in the variant.
+These codes imply a single step in the forward direction along the ray.
+In variants with Shogi-like promotions, where all pieces can promote, but the only choice is promote or not,
+the entire range of distanceCode meanings will be repeated, this time meaning the move will be accompanied by a promotion.
+</p><p>
+The meaning of these variant-dependent distanceCodes can be overruled through the book header.
+After a line with the word "Special" in this header,
+a number of lines can follow, specifying the move as it would be for the vertical ray in the forward direction, starting from e4.
+
+
+After that follow codes to indicate additional oblique moves: a pair of Camel jumps, a pair of Zebra jumps, etc.
+With Shogi-like promotions, the 16 promotion choices instead code for two-step Lion moves that start off along the ray.
+Promotions are indicated by adding a number to the distanceCode one larger than the highest code used to indicate any oblique move.
+E.g. for Tenjiku Shogi with its 16x16 board codes 0-14 indicate true (positiove and negative) distances along the ray,
+15 & 16 are Knight jumps, 17-32 are Lion double moves, 33-36 are C and Z jumps (for the area moves),
+and promotions add 37.
+As the board has 256 squares, there are only 64 distance codes available,
+so the highest code for which a promoted version is available is 26 (26 + 37 = 63).
+So there is no room to specify promotion on C and Z jumps, and with part of the Lion double-moves.
+But the only pieces in Tenjiku Shogi that can make C and Z moves do not promote,
+and the Lion double-moves can be ordered such that only the backward moves cannot promote.
+</p><p>
+For Maka Dai Dai Shogi the 8 codes after the Lion double moves
+(36-43, as the board is 19x19 there, shifting up everything 3 places compared to Tenjiku)
+specify Lion Dog linear triple-moves,
+(2-out-1-in, 2+1-out, 1+2-out and 1+1+1-out in both directions along the ray)
+and 44 specifies the moves was made by a 'Universal Leaper' (the ray number indicating which one).
+The latter necessitates the decoding routine to find the corresponding piece on the board.
+('Universal Leaper' in this context means a piece that is not bound to rays,
+and can move almost anywhere, such as Hook Mover, encoded in ray 0, Capricorn (ray 1) or Emperor (ray 2).)
+With 45 distance codes, 4 rays and 361 board squares all available 16-bit codes ar almost used up,
+so there is no way to indicate promotion.
+This is serious, as this variant has promotion on capture,
+making promotion early in the opening quite common.
+Fortunately the pieces are clearly divided into those that promote to upward-compatible pieces
+(replacing all step moves by unlimited sliding in that direction),
+for which you would always promote,
+and those that 'promote' to a worthless Gold, which you would virtually never promote.
+So promotion / deferral is decided by which class the piece belongs to,
+and only moves that do the opposite from what you would normally do cannot be incorporated in the book.
+This again need consulting of the board position before the move can be decoded.
</p>
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