Avoid repeatedly clearing and adding highlights

Partially addresses natebosch#31

- Add a check comparing w:lsc_highlights_version and file against
  w:lsc_diagnotics_version and only recreate matches when there is a
  change.
- Change order of calls so that highlights are always updated after the
  diagnostics in the location list so that these variables are set and
  checked in the right order.

CHANGELOG.md

@@ -2,6 +2,8 @@
 
 - Send file updates after re-reading already open file. Fixes some cases where
   the server has a different idea of the file content than the editor.
+- Avoid clearing and readding the same diagnostic matches each time diagnostics
+  change in some other file.
 
 # 0.2.5
 

autoload/lsc/diagnostics.vim

@@ -98,8 +98,8 @@ function! lsc#diagnostics#setForFile(file_path, diagnostics) abort
   else
     let s:diagnostic_versions[a:file_path] = 1
   endif
-  call lsc#highlights#updateDisplayed()
   call lsc#diagnostics#updateLocationList(a:file_path)
+  call lsc#highlights#updateDisplayed()
 endfunction
 
 " Updates location list for all windows showing [file_path].

autoload/lsc/highlights.vim

@@ -6,6 +6,7 @@ endfunction
 
 " Refresh highlight matches in the current window.
 function! lsc#highlights#update() abort
+  if s:CurrentWindowIsFresh() | return | endif
   call lsc#highlights#clear()
   if !has_key(g:lsc_server_commands, &filetype) || &diff
     return
@@ -16,6 +17,7 @@ function! lsc#highlights#update() abort
       call add(w:lsc_diagnostic_matches, match)
     endfor
   endfor
+  call s:MarkCurrentWindowFresh()
 endfunction!
 
 " Remove all highlighted matches in the current window.
@@ -40,3 +42,16 @@ function! s:DeferForSelect() abort
   endif
   return v:false
 endfunction
+
+" Whether the diagnostic highlights for the current window are up to date.
+function! s:CurrentWindowIsFresh() abort
+  if !exists('w:lsc_diagnostics_version') | return v:true | endif
+  if !exists('w:lsc_highlights_version') | return v:false | endif
+  return w:lsc_highlights_version == w:lsc_diagnostics_version &&
+      \ w:lsc_highlights_file == w:lsc_diagnostics_file
+endfunction
+
+function! s:MarkCurrentWindowFresh() abort
+  let w:lsc_highlights_version = w:lsc_diagnostics_version
+  let w:lsc_highlights_file = w:lsc_diagnostics_file
+endfunction

plugin/lsc.vim

@@ -92,8 +92,8 @@ function! LSCEnsureCurrentWindowState() abort
     endif
     return
   endif
-  call lsc#highlights#update()
   call lsc#diagnostics#updateLocationList(expand('%:p'))
+  call lsc#highlights#update()
 endfunction
 
 " Run `function` if LSC is enabled for the current filetype.