# two-piece-cycle.cql

// Download two-piece-cycle.cql// PGN output when run on sample.pgn

/* Find games in which there is a "two-piece cycle" in which two pieces, piece1 (a black piece) and piece2 (a white piece), each traverse the same set of squares. Piece1 starts, then piece2 moves to a square, then piece1 moves to a new square, and so on, until each piece's final move ends on the same square as that piece's initial square. Thus, the path of each piece forms a cycle. The moves must be consecutive. Games are sorted by length of cycle. */ cql (input hhdbvi.pgn) /* The function insert_square takes a single square Square and a set of squares Set If Square is in Set, the function does not match. Otherwise, Square is inserted into Set. */ function insert_square (Square Set){ not Square in Set Set = Square | Set // insert Square into Set } piece piece1 in a piece piece2 in A{ initial1 =piece1 // piece1 start initial2=piece2 // piece2 start cycle1 = initial1 // piece1 cycle cycle2 = initial2 // piece2 cycle line --> move from piece1 --> ( insert_square(piece1 cycle1) --> insert_square(piece2 cycle2) ) + // repeat the pattern of the last 2 moves --> piece1==initial1 // piece1 completes its cycle --> {piece2==initial2 // piece2 completes its cycle cycle1 == cycle2} sort "cycle length: " cycle1 }