Red ["Maze generation"]

size: as-pair to-integer ask "Maze width: " to-integer ask "Maze height: "
random/seed now/time
offsetof: function [pos] [pos/y * size/x + pos/x + 1]
visited?: function [pos] [find visited pos]

newneighbour: function [pos][
   nnbs: collect [
      if all [pos/x > 0 not visited? p: pos - 1x0] [keep p]
      if all [pos/x < (size/x - 1) not visited? p: pos + 1x0] [keep p]
      if all [pos/y > 0 not visited? p: pos - 0x1] [keep p]
      if all [pos/y < (size/y - 1) not visited? p: pos + 0x1] [keep p]
   ]
   pick nnbs random length? nnbs
]
expand: function [pos][
   insert visited pos
   either npos: newneighbour pos [
      insert exploring npos
      do select [
         0x-1 [o: offsetof npos walls/:o/y: 0]
         1x0  [o: offsetof  pos walls/:o/x: 0]
         0x1  [o: offsetof  pos walls/:o/y: 0]
         -1x0 [o: offsetof npos walls/:o/x: 0]
      ] npos - pos
   ][
      remove exploring
   ]
]
visited: []
walls: append/dup [] 1x1 size/x * size/y
exploring: reduce [random size - 1x1]

until [
   expand exploring/1
   empty? exploring
]

print append/dup "" " _" size/x
repeat j size/y [
   prin "|"
   repeat i size/x [
      p: pick walls (j - 1 * size/x + i)
      prin rejoin [pick " _" 1 + p/y pick " |" 1 + p/x]
   ]
   print ""
]