clang-format: work around vim/vim#5930 when positioning cursor

This bug (and a related predecessor) cause `:goto` and `line2bytes()`
to do the wrong thing when vim textprops are used.
This manifests as "cursor jumps around randomly after formatting" when using
plugins that use textprops for diagnostics, highlighting etc.

We happen to have all the code in an array when we need to do cursor coordinate
conversions, so we do those in vimscript instead of querying vim.

Without this patch, The newly added test fails on affected versions of vim
including both vim master and debian's 8.1.2269 (in different ways).

Vim bug tracker entry:
vim/vim#5930
Similar workaround in first-party vim integration:
llvm/llvm-project@591be7e

Author: sam-mccall

autoload/codefmt/clangformat.vim

@@ -120,32 +120,49 @@ function! codefmt#clangformat#GetFormatter() abort
       let l:cmd += ['-lines', l:startline . ':' . l:endline]
     endfor
 
+    let l:lines = getline(1, line('$'))
+
     " Version 3.4 introduced support for cursor tracking
     " http://llvm.org/releases/3.4/tools/clang/docs/ClangFormat.html
     let l:supports_cursor = s:ClangFormatHasAtLeastVersion([3, 4])
     if l:supports_cursor
-      " line2byte counts bytes from 1, and col counts from 1, so -2 
-      let l:cursor_pos = line2byte(line('.')) + col('.') - 2
+      " Avoid line2byte: https://github.com/vim/vim/issues/5930
+      let l:cursor_pos = col('.') - 1
+      for l:i in range(1, line('.') - 1)
+        let l:cursor_pos += len(l:lines[l:i - 1]) + 1
+      endfor
       let l:cmd += ['-cursor', string(l:cursor_pos)]
     endif
 
-    let l:input = join(getline(1, line('$')), "\n")
+    let l:input = join(l:lines, "\n")
     let l:result = maktaba#syscall#Create(l:cmd).WithStdin(l:input).Call()
     let l:formatted = split(l:result.stdout, "\n")
 
-    if !l:supports_cursor
-      call maktaba#buffer#Overwrite(1, line('$'), l:formatted[0:])
-    else
-      call maktaba#buffer#Overwrite(1, line('$'), l:formatted[1:])
+    if l:supports_cursor
+      " With -cursor, the first line is a JSON object.
+      let l:header = l:formatted[0]
+      let l:formatted = l:formatted[1:]
+      call maktaba#buffer#Overwrite(1, line('$'), l:formatted)
       try
-        let l:clang_format_output_json = maktaba#json#Parse(l:formatted[0])
-        let l:new_cursor_pos =
-            \ maktaba#ensure#IsNumber(l:clang_format_output_json.Cursor) + 1
-        execute 'goto' l:new_cursor_pos
+        let l:header_json = maktaba#json#Parse(l:header)
+        let l:offset = maktaba#ensure#IsNumber(l:header_json.Cursor)
+        " Compute line/col, avoid goto: https://github.com/vim/vim/issues/5930
+        let l:new_line = 0
+        for l:line in l:formatted
+          let l:offset_after_next = l:offset - len(l:line) - 1
+          if l:offset_after_next < 0
+            break
+          endif
+          let l:offset = l:offset_after_next
+          let l:new_line += 1
+        endfor
+        call cursor(l:new_line + 1, l:offset + 1)
       catch
         call maktaba#error#Warn('Unable to parse clang-format cursor pos: %s',
             \ v:exception)
       endtry
+    else
+      call maktaba#buffer#Overwrite(1, line('$'), l:formatted)
     endif
   endfunction
 

vroom/clangformat.vroom

@@ -178,7 +178,33 @@ that clang-format has a version >= 3.4).
   :echomsg getline('.')[col('.')-1]
   ~ 5
 
+This should work when with textprops set in the file, despite various functions
+like line2byte() and :goto being buggy in that case.
+(See https://github.com/vim/vim/issues/5930 for bug details)
 
+  @clear
+  % int f()     {<CR>
+  |    int i=1;<CR>
+  |   return 1234567890; }<CR>
+  :call prop_type_add('keyword', {})
+  :call prop_add(1, 1, {'length': 3, 'type': 'keyword'})
+  :call cursor(2, 10)
+  :echomsg getline('.')[col('.')-1]
+  ~ =
+  :FormatCode clang-format
+  ! clang-format -style file .* -cursor 23 .*2>.*
+  $ { "Cursor": 18 }
+  $ int f() {
+  $   int i = 1;
+  $   return 1234567890;
+  $ }
+  int f() {
+    int i = 1;
+    return 1234567890;
+  }
+  @end
+  :echomsg getline('.')[col('.')-1]
+  ~ =
 
 You might have wondered where the "-style file" above comes from. The
 clang-format tool accepts a "style" option to control the formatting style. By