Detect TeX program from %! TeX program= directive

dtp.vim

function! DetectTexProgram()

  let l:pat = '^\c\s*%\s*!\?\s*tex\s\+\(TS-\)\?program\s*=\s*\zs.*\ze\s*$'
  let l:engine_list = {
      \ 'pdflatex'         : '',
      \ 'lualatex'         : '-lualatex',
      \ 'xelatex'          : '-xelatex',
      \ 'context (luatex)' : '-pdflatex=context',
      \ 'context (xetex)'  : '-pdflatex=''texexec --xtx''',
      \ 'context (pdftex)' : '-pdflatex=texexec',
      \  }

      " \ 'pdftex'           : 'pdftex',
      " \ 'luatex'           : 'luatex',
      " \ 'xetex'            : 'xetex',

  for l:line in getline(1, 5)
    let l:engine = matchstr(l:line, l:pat)
    if len(l:engine) > 0
      if has_key(l:engine_list, tolower(l:engine)) > 0
        let l:opt = ' '.l:engine_list[tolower(l:engine)].' '
        let g:vimtex_latexmk_options .= l:opt
      else
        echoerr 'Unknown engine '.l:engine
      endif
    endif
  endfor

endfunction

autocmd FileType tex call DetectTexProgram()

https://github.com/lervag/vimtex/blob/62b3bf014694e80648a7e7da7b567836ad8b4707/autoload/vimtex.vim#L605
https://github.com/TeXworks/texworks/blob/45ed91cb8bd3cb816d7e26e034b0f891f56df207/src/TWApp.cpp#L884
https://github.com/TeXworks/texworks/blob/master/src/TeXDocument.cpp#L2984