!YS-v0

defn main(steps=511):
  data [minx maxx miny maxy] =:
    loop i 0 [x y], [0 0], data {}, [minx maxx miny maxy] [0 0 0 0]:
      dir =: i-to-dir(i)
      nx ny =: +
        [(x + condp(== dir 0 1 2 -1 0))
         (y + condp(== dir 1 1 3 -1 0))]
      ob ib =: dir.nth([[8 4][2 1][4 8][1 2]])
      data =: update-in(data [y x] safe-bit-or ob)
             .update-in([ny nx] safe-bit-or ib)
      bbox =: +
        [min(minx nx) max(maxx nx)
         min(miny ny) max(maxy ny)]
      if i < steps:
        recur: i.++, [nx ny], data, bbox
        vector: data bbox

  each y (miny .. maxy):
    say: !:join
      each x (minx .. maxx):
        get-in(data [y x] 0).nth(" ╵╷│╴┘┐┤╶└┌├─┴┬┼")

defn i-to-dir(n):
  n.bit-xor(bit-shift-right(n 1)):Long/bitCount % 4

defn safe-bit-or(v bit):
  (v || 0).bit-or(bit)