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tromp / ChessPositionRanking

Software suite for ranking chess positions and accurately estimating the number of legal chess positions
MIT License
136 stars 7 forks source link

2n2b1R/3Nr3/rQn1n3/1P1Kb1nR/kQP2N2/Pbq4B/2bBP3/N2QN3 b - - 0 1 #905

Closed tromp closed 3 years ago

tromp commented 3 years ago

2n2b1R/3Nr3/rQn1n3/1P1Kb1nR/kQP2N2/Pbq4B/2bBP3/N2QN3 b - - 0 1

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Single Check wx 0 wp 4 wpr 4 wpx 0 maxuwp 4 minopp 0 bx 4 bp 0 bpr 4 bpx 4 maxubp 0 minopp 0

ruedigerj commented 3 years ago

4 black pawns were promoted. Since no white piece was captured, all 4 pawns moved straight to their promotion square. So the opposite white pawns must have cleared the way by capturing a black pawn and thus opening their file. This must have happened for the d and f pawns and either the g or h pawn. But this allows only for 3 black promotions. The missing fourth promotion should have happened on file a or b, but there are still two white pawns which prevent that possibility. Therefore we can conclude that this position is illegal.

tromp commented 3 years ago

Correct; 12 possible promotions from 4 captured black pawns requires one capture between a&b files, contradicting the white a and b pawns.