Use guessProjectRoot from rking/ag.vim.

Author: kaethorn

plugin/side-search.vim

@@ -151,6 +151,24 @@ function! SideSearchWinnr()
   return s:my_buffer_winnr()
 endfunction
 
+function! s:guessProjectRoot()
+  let l:splitsearchdir = split(getcwd(), "/")
+
+  while len(l:splitsearchdir) > 2
+    let l:searchdir = '/'.join(l:splitsearchdir, '/').'/'
+    for l:marker in ['.rootdir', '.git', '.hg', '.svn', 'bzr', '_darcs', 'build.xml']
+      " found it! Return the dir
+      if filereadable(l:searchdir.l:marker) || isdirectory(l:searchdir.l:marker)
+        return l:searchdir
+      endif
+    endfor
+    let l:splitsearchdir = l:splitsearchdir[0:-2] " Splice the list to get rid of the tail directory
+  endwhile
+
+  " Nothing found, fallback to current working dir
+  return getcwd()
+endfunction
+
 " The public facing function.
 " Accept 1 or 2 arguments which basically get passed directly
 " to the `ag` command.
@@ -173,8 +191,10 @@ function! SideSearch(args) abort
 
   call s:append_guide()
 
+  " determine root directory
+  let l:cwd = s:guessProjectRoot()
   " execute showing summary of stuff read (without silent)
-  let b:cmd = g:side_search_prg . ' ' . a:args
+  let b:cmd = g:side_search_prg . ' ' . a:args . ' ' . l:cwd
   " Thanks: https://github.com/rking/ag.vim/blob/master/autoload/ag.vim#L154
   let query = matchstr(a:args, "\\v(-)\@<!(\<)\@<=\\w+|['\"]\\zs.{-}\\ze['\"]")
   let b:escaped_query = shellescape(query)