Author: Eugene Nalimov
Date: 11:00:26 11/18/98
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On November 18, 1998 at 06:04:49, Jouni Uski wrote: >They should be about 50 % of older format, but which way is this improvement >achieved? Was it some kind of symmetry reductions or what? Several tricks, mostly already known (Ernst A. Heinz wrote an excellent article, maybe it'll be published in ICCA journal; Carsten Kossendey sent a good letter to Crafty discussion group some time ago, too). Each one trick saves not much, or works only for some endgame subclasses, but net result is 50% reduction in used disk space. Brief summary: 1. For pawnless ending, you can restrict one piece to a1-d1-d4 triangle (that was done in SJE's generator). But if that piece happens to be on a1-d4 diagonal, you can restrict other piece to 'large' a1-h1-h8 triangle (exploit one more symmetry). 2. When you place second piece on the board, one square is occupied already, so there are only 63 possible squares; for third - only 62, etc. 3. If there are 2 identical pieces (e.g. as in knnkp), you can order their locations - e.g. force second piece to occupy square with smaller number. Then you have not N*(N-1) total combinations, but N*(N+1)/2. 4. Pawns usually occupies ranks 2-7. Even if you want to handle en passant captures, there are better ways to do that than to reserve entire rank (or two) for possible en passant target. 5. Kings never can be near each other, so there are only 3612 ways to place 2 kings on the empty board (do not using symmetries), versus 4096 when using SJE's format. 6. You cannot capture enemy's king, so, if you know where it's located, there are some forbidden squares for the pieces of the side-to-move. Also you can use minimal bits encoding - for 5-man ending, 7 bits usually enough. That will save additional 12.5%. I decided against it because I hope to use much better on-the-fly decompression schema. Eugene
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