" Define the function we'll use.  It's easier to do this than to faff about 
" trying to run a sequence of normal mode commands.
fun! P4merge()
  " There should be four arguments:
  " 0. The original file (tmp.xxx).
  " 1. Their version (tmp.yyy).
  " 2. My version (original filename).
  " 3. The merged version (tmp.zzz).

  if argc() != 4
    return
  endif

  let l:filename = fnameescape(fnamemodify(argv(2), ":h"))

  " Make sure the window is big enough for diffs.
  if &columns < 165
    let &columns=(&columns * 2)
  endif

  " Mark the buffer for the original file.
  set ro
  set nomodifiable
  set nobuflisted
  set buftype=help
  setlocal noswapfile
  exe "file \[ORIG:" . l:filename . "\]"
  diffthis
  1

  " Split the buffer for my version.
  split +buffer\ 3
  set ro
  set nomodifiable
  set nobuflisted
  set buftype=help
  setlocal noswapfile
  exe "file \[YOURS:" . l:filename . "\]"
  diffthis
  1

  " Split the buffer for their version and put it below.
  split +buffer\ 2
  wincmd J
  set ro
  set nomodifiable
  set nobuflisted
  set buftype=help
  set bufhidden=hide
  setlocal noswapfile
  exe "file \[THEIRS:" . l:filename . "\]"
  diffthis
  1

  " Split the buffer for the merged version and put it at the side.
  split +buffer\ 4
  wincmd L
  se modifiable
  1

  try
    au QuitPre * qa!
  endtry
  catch
endfun

" Expand the command line window so we don't have to press RETURN later.
let s:cmdheight = &cmdheight
set cmdheight=3

" Call and then discard our function.
call P4merge()
delfunction P4merge

" Restore the command window.
exe "set cmdheight=" . s:cmdheight

" Close the scratch buffer.
if version >= 700
  " Vim 7 lets us close the buffer by number.  The merge is always in #4.
  au BufWinLeave <buffer=4> qa!
else
  " Prior versions can only do it by name.  Fall back to a temporary file.
  au BufWinLeave /tmp/tmp.*.* qa!
endif