nnoremap <c-w><c-j> :call <sid>smart_split('j')<cr>
nnoremap <c-w><c-k> :call <sid>smart_split('k')<cr>
nnoremap <c-w><c-h> :call <sid>smart_split('h')<cr>
nnoremap <c-w><c-l> :call <sid>smart_split('l')<cr>

function! s:smart_split(cmd)
  " original window number
  let wn = winnr()

  " try to move to other window
  exec 'wincmd ' . a:cmd

  " do nothing if window changed
  if wn != winnr() | return | endif

  " otherwise split vertical or horizontal based on a:cmd
  exec 'wincmd ' . (a:cmd =~# '[lh]' ? 'v' : 's')

  " move to new window. You only need this 50% of time.
  exec 'wincmd ' . a:cmd
endfunction