Perft figures for a few variants

[TonyJH]

Thanks. I just found out why my perft result is different in this position. In shogi, unlike chess, a player who has no legal moves loses the game. So a "stalemate" is treated as checkmate (although it would be very rare to matter in real games). Checkmating with a pawn drop is not allowed, but my engine was allowing a pawn drop to stalemate the opponent to legally win the game. When I changed it to disallow pawn drop stalemate, I get the same perft result as you. I haven't seen this rule explained very clearly. Now that I think about it, I think my engine is probably wrong to allow this (since when the rules mention "checkmate" it should probably be interpreted as including both checkmate / stalemate) ?

[ilari]

Interesting. I hadn't even considered the case of stalemate yet. The Shogi page at Wikipedia doesn't address it at all, but here it's stated a bit more clearly: http://www.ricoh.co.jp/SHOGI/rules/erules.html
It says "Attacking the King by dropping a Pawn on the square in front of him is not allowed if the King cannot prevent being captured on the following move. To use the Chess expression: checkmating the King with a Pawn drop is prohibited." So maybe it's legal to stalemate with a pawn drop.

Is there any official document that describes Shogi rules?

[TonyJH]

Ok, based on that description it seems to be legal. I just tested this rule in Shogidokoro GUI in human-vs-human mode, and also with Lesserkai shogi engine, and both programs also consider it legal.
I agree it would be nice to have an official detailed description of the rules that was more explicit about this.