1 dir(west, -1, 0).
   2 dir(east,  1, 0).
   3 dir(north, 0,-1).
   4 dir(south, 0, 1).
   5 
   6 dl(west, -1).
   7 dl(north,-1).
   8 dl(east,  1).
   9 dl(south, 1).
  10 
  11 dir(west, row).
  12 dir(east, row).
  13 dir(north,col).
  14 dir(south,col).
  15 
  16 dir(D) :- dir(D,_).
  17 
  18 robot(R) :- pos(R,_,_).
  19 
  20 pos(R,row,I,0) :- pos(R,I,_).
  21 pos(R,col,J,0) :- pos(R,_,J).
  22 
  23 barrier(I+1,J,west ) :- barrier(I,J,east ), dim(I), dim(J), dim(I+1).
  24 barrier(I,J+1,north) :- barrier(I,J,south), dim(I), dim(J), dim(J+1).
  25 barrier(I-1,J,east ) :- barrier(I,J,west ), dim(I), dim(J), dim(I-1).
  26 barrier(I,J-1,south) :- barrier(I,J,north), dim(I), dim(J), dim(I-1).
  27 
  28 conn(D,I,J) :- dir(D,col), dir(D,_,DJ), not barrier(I,J,D), dim(I), dim(J), dim(J+DJ).
  29 conn(D,J,I) :- dir(D,row), dir(D,DI,_), not barrier(I,J,D), dim(I), dim(J), dim(I+DI).
  30 
  31 step(1).
  32 step(X+1) :- step(X), length(L), X < L.
  33 %step(1..X) :- length(X).
  34 
  35 
  36 1<= { selectRobot(R,T) : robot(R) } <=1 :- step(T).
  37 1<= { selectDir(D,O,T) : dir(D,O) } <=1 :- step(T).
  38 
  39 go(R,D,O,T) :- selectRobot(R,T), selectDir(D,O,T).
  40 go_(R,O,T)   :- go(R,_,O,T).
  41 go(R,D,T) :- go(R,D,_,T).
  42 
  43 sameLine(R,D,O,RR,T)  :- go(R,D,O,T), pos(R,-O,L,T-1), pos(RR,-O,L,T-1), R != RR.
  44 blocked(R,D,O,I+DI,T) :- go(R,D,O,T), pos(R,-O,L,T-1), not conn(D,L,I), dl(D,DI), dim(I), dim(I+DI).
  45 blocked(R,D,O,L,T)    :- sameLine(R,D,O,RR,T), pos(RR,O,L,T-1).
  46 
  47 reachable(R,D,O,I,   T) :- go(R,D,O,T), pos(R,O,I,T-1).
  48 reachable(R,D,O,I+DI,T) :- reachable(R,D,O,I,T), not blocked(R,D,O,I+DI,T), dl(D,DI), dim(I+DI).
  49 
  50 :- go(R,D,O,T), pos(R,O,I,T-1), blocked(R,D,O,I+DI,T), dl(D,DI).
  51 :- go(R,D,O,T), go(R,DD,O,T-1).
  52 
  53 pos(R,O,I,T) :- reachable(R,D,O,I,T), not reachable(R,D,O,I+DI,T), dl(D,DI).
  54 pos(R,O,I,T) :- pos(R,O,I,T-1), not go_(R,O,T), step(T).
  55 
  56 selectDir(O,T) :- selectDir(D,O,T).
  57 
  58 :- target(R,I,_), not pos(R,row,I,X), length(X).
  59 :- target(R,_,J), not pos(R,col,J,X), length(X).