What's the implementation of Vim's default 'tabline' function?

Question

In Vim, the text that forms the row of tabs at the top of the screen (when using tabs) is configured with the tabline option.

I'd like to make a few minor adjustments to the default tab pages line, such as replacing the number of windows in the tab with the index of the tab. Unfortunately, the default version of this (which is active when tabline is unset) is complicated and undocumented. There's nothing for me to tweak.

Is there a piece of Vim script that provides the default implementation which I could adjust to my needs?

Answer 1

I use a custom function to reset the tab number and viewport numbers, from here (see Tonymec's comment). You can play with it to change how you display the tabs.

Here's what I have in my .vimrc. It's only a slightly modified version, that changes how the tab# and viewport# are displayed.

"Rename tabs to show tab# and # of viewports
if exists("+showtabline")
    function! MyTabLine()
        let s = ''
        let wn = ''
        let t = tabpagenr()
        let i = 1
        while i <= tabpagenr('$')
            let buflist = tabpagebuflist(i)
            let winnr = tabpagewinnr(i)
            let s .= '%' . i . 'T'
            let s .= (i == t ? '%1*' : '%2*')
            let s .= ' '
            let wn = tabpagewinnr(i,'$')

            let s .= (i== t ? '%#TabNumSel#' : '%#TabNum#')
            let s .= i
            if tabpagewinnr(i,'$') > 1
                let s .= '.'
                let s .= (i== t ? '%#TabWinNumSel#' : '%#TabWinNum#')
                let s .= (tabpagewinnr(i,'$') > 1 ? wn : '')
            end

            let s .= ' %*'
            let s .= (i == t ? '%#TabLineSel#' : '%#TabLine#')
            let bufnr = buflist[winnr - 1]
            let file = bufname(bufnr)
            let buftype = getbufvar(bufnr, 'buftype')
            if buftype == 'nofile'
                if file =~ '\/.'
                    let file = substitute(file, '.*\/\ze.', '', '')
                endif
            else
                let file = fnamemodify(file, ':p:t')
            endif
            if file == ''
                let file = '[No Name]'
            endif
            let s .= file
            let s .= (i == t ? '%m' : '')
            let i = i + 1
        endwhile
        let s .= '%T%#TabLineFill#%='
        return s
    endfunction
    set stal=2
    set tabline=%!MyTabLine()
endif

And here are the colors that are defined in my function

set tabpagemax=15
hi TabLineSel term=bold cterm=bold ctermfg=16 ctermbg=229
hi TabWinNumSel term=bold cterm=bold ctermfg=90 ctermbg=229
hi TabNumSel term=bold cterm=bold ctermfg=16 ctermbg=229

hi TabLine term=underline ctermfg=16 ctermbg=145
hi TabWinNum term=bold cterm=bold ctermfg=90 ctermbg=145
hi TabNum term=bold cterm=bold ctermfg=16 ctermbg=145

Answer 2

This is not the answer you are asking for but I will share my own tabline with you.

Did it with help from the wikia page, here is my version.

This is where the first tab has three windows open in it, with two open on one edited file.

(sorry about the 8-space tabs)

set showtabline=1  " 1 to show tabline only when more than one tab is present
set tabline=%!MyTabLine()  " custom tab pages line
function! MyTabLine() " acclamation to avoid conflict
        let s = '' " complete tabline goes here
        " loop through each tab page
        for t in range(tabpagenr('$'))
                " set highlight
                if t + 1 == tabpagenr()
                        let s .= '%#TabLineSel#'
                else
                        let s .= '%#TabLine#'
                endif
                " set the tab page number (for mouse clicks)
                let s .= '%' . (t + 1) . 'T'
                let s .= ' '
                " set page number string
                let s .= t + 1 . ' '
                " get buffer names and statuses
                let n = ''      "temp string for buffer names while we loop and check buftype
                let m = 0       " &modified counter
                let bc = len(tabpagebuflist(t + 1))     "counter to avoid last ' '
                " loop through each buffer in a tab
                for b in tabpagebuflist(t + 1)
                        " buffer types: quickfix gets a [Q], help gets [H]{base fname}
                        " others get 1dir/2dir/3dir/fname shortened to 1/2/3/fname
                        if getbufvar( b, "&buftype" ) == 'help'
                                let n .= '[H]' . fnamemodify( bufname(b), ':t:s/.txt$//' )
                        elseif getbufvar( b, "&buftype" ) == 'quickfix'
                                let n .= '[Q]'
                        else
                                let n .= pathshorten(bufname(b))
                        endif
                        " check and ++ tab's &modified count
                        if getbufvar( b, "&modified" )
                                let m += 1
                        endif
                        " no final ' ' added...formatting looks better done later
                        if bc > 1
                                let n .= ' '
                        endif
                        let bc -= 1
                endfor
                " add modified label [n+] where n pages in tab are modified
                if m > 0
                        let s .= '[' . m . '+]'
                endif
                " select the highlighting for the buffer names
                " my default highlighting only underlines the active tab
                " buffer names.
                if t + 1 == tabpagenr()
                        let s .= '%#TabLineSel#'
                else
                        let s .= '%#TabLine#'
                endif
                " add buffer names
                if n == ''
                        let s.= '[New]'
                else
                        let s .= n
                endif
                " switch to no underlining and add final space to buffer list
                let s .= ' '
        endfor
        " after the last tab fill with TabLineFill and reset tab page nr
        let s .= '%#TabLineFill#%T'
        " right-align the label to close the current tab page
        if tabpagenr('$') > 1
                let s .= '%=%#TabLineFill#%999Xclose'
        endif
        return s
endfunction