Function to set the foldtext

FoldText.vim

function! FoldText()
  " Description: Folding configuration
  let nucolwidth = &fdc + &number*&numberwidth
  let winwidth = winwidth(0) - nucolwidth - 3
  let foldlinecount = foldclosedend(v:foldstart) - foldclosed(v:foldstart) + 1
  let dashtext = strpart(string(v:folddashes),1,len(string(v:folddashes))-2)
  let foldinfo = "+" . dashtext . " Fold: " . string(v:foldlevel) . ", " . string(foldlinecount) . " lines "
  let firstline =  strpart(getline(v:foldstart), 0 , winwidth - len(foldinfo))
  let fillcharcount = winwidth - len(firstline) - len(foldinfo)
  return firstline . repeat(" ",fillcharcount) . foldinfo
endfunction
set foldtext=FoldText()