DEF D MAP,DIST,X,Y,W,H
 FILL DIST,99999
 DIM L1[0],L2[0]
 VAR D=0
 PUSH L2,X
 PUSH L2,Y
 DIST[X,Y]=D

 WHILE LEN(L2)
  WHILE LEN(L2) 'yes twice
    Y=POP(L2) 'note that Y is first here
    X=POP(L2)
    C MAP,DIST,L1,X+1,Y,W,H,D
    C MAP,DIST,L1,X-1,Y,W,H,D
    C MAP,DIST,L1,X,Y+1,W,H,D
    C MAP,DIST,L1,X,Y-1,W,H,D
   WEND
   SWAP L1,L2
   INC D
  WEND
END

DEF C MAP,DIST,LIST,X,Y,W,H,D
 IF X>=0&&Y>=0&&X<W&&Y<H&&MAP[X,Y]==0&&DIST[X,Y]==99999 THEN
  PUSH LIST,X
  PUSH LIST,Y
  DIST[X,Y]=D+1
 ENDIF
END