vim s:snr() & s:sid() & s:parse_script_names() & s:path_to_snr()

sid.vim

function! s:snr()
  let sfile = expand('<sfile>')
  return matchstr(sfile, '<SNR>\zs\d\+\ze_snr$')
endfunction

function! s:sid()
  let snr = s:snr()
  return printf('<SNR>%s_', snr)
endfunction

function! s:parse_script_names()
  redir => _
  silent! scriptnames
  redir END
  let scripts = split(_, '\n')
  cal map(scripts, 'matchlist(v:val, ''^\s*\(\d*\):\s*\(\S*\)\s*$'')')
  call filter(scripts, '!empty(v:val)')
  return map(scripts, '{''snr'': v:val[1], ''path'': v:val[2]}')
endfunction

function! s:path_to_snr(path)
  let scripts = s:parse_script_names()
  call filter(scripts, 'expand(v:val.path) ==# a:path')
  return empty(scripts) ? '' : scripts[0].snr
endfunction