Question

Is it possible to diff or even vimdiff two very similar subroutines occurring in the same file? If so, how?

I can think of copying the two subroutines in two separate files and then diff them, but is there a way to do it within the original file?

Answer 1

You cannot do this within the original file, but you can do this without using separate files, only separate buffers. This should work if you copied one subroutine in register a (for example, with "ay typed in Visual mode) and other subroutine in register b:

enew | call setline(1, split(@a, "\n")) | diffthis | vnew | call setline(1, split(@b, "\n")) | diffthis

To automate:

let g:diffed_buffers = []

function DiffText(a, b, diffed_buffers)
    enew
    setlocal buftype=nowrite
    call add(a:diffed_buffers, bufnr('%'))
    call setline(1, split(a:a, "\n"))
    diffthis
    vnew
    setlocal buftype=nowrite
    call add(a:diffed_buffers, bufnr('%'))
    call setline(1, split(a:b, "\n"))
    diffthis
endfunction

function WipeOutDiffs(diffed_buffers)
    for buffer in a:diffed_buffers
        execute 'bwipeout! ' . buffer
    endfor
endfunction

nnoremap <special> <F7> :call DiffText(@a, @b, g:diffed_buffers)<CR>
nnoremap <special> <F8> :call WipeOutDiffs(g:diffed_buffers) | let g:diffed_buffers=[]<CR>

Note that you may want to set hidden option if Vim refuses to abandon changed file (see :h abandon).