Add capability to format the diff on save to clang-format's vim integration

[djasper]

Summary

With this patch, one can configure a BufWrite hook that will make the clang-format integration diff the current buffer with the file that's on disk and format all changed lines. This should create a zero-overhead auto-format solution that doesn't require the file to already be clang-format clean to avoid spurious diffs.

[djasper]

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[klimek]

Lg

docs/ClangFormat.rst

124

Pronoun missing.

[djasper]

Submitted as r305665.

docs/ClangFormat.rst

 With this integration you can press the bound key and clang-format will
 format the current line in NORMAL and INSERT mode or the selected region in
 VISUAL mode. The line or region is extended to the next bigger syntactic
 entity.
 
 It operates on the current, potentially unsaved buffer and does not create
 or save any files. To revert a formatting, just undo.
 
+An alternative option is to format changes when saving a file and thus to
+have a zero-effort integration into coding workflow. To do this, add this to
+your `.vimrc`:
+
+.. code-block:: vim
+
+  function! Formatonsave()
+    let l:formatdiff = 1
+    pyf ~/llvm/tools/clang/tools/clang-format/clang-format.py
+  endfunction
+  autocmd BufWritePre *.h,*.cc,*.cpp call Formatonsave()
+
 
 Emacs Integration
 =================
 
 Similar to the integration for :program:`vim`, there is an integration for
 :program:`emacs`. It can be found at `clang/tools/clang-format/clang-format.el`
 and used by adding this to your `.emacs`:
 

tools/clang-format/clang-format.py

def main():
   # Get the current text.
   encoding = vim.eval("&encoding")
   buf = get_buffer(encoding)
   text = '\n'.join(buf)
 
   # Determine range to format.
   if vim.eval('exists("l:lines")') == '1':
     lines = vim.eval('l:lines')
+  elif vim.eval('exists("l:formatdiff")') == '1':
+    with open(vim.current.buffer.name, 'r') as f:
+      ondisk = f.read().splitlines();
+    sequence = difflib.SequenceMatcher(None, ondisk, vim.current.buffer)
+    lines = []
+    for op in reversed(sequence.get_opcodes()):
+      if op[0] not in ['equal', 'delete']:
+        lines += ['-lines', '%s:%s' % (op[3] + 1, op[4])]
+    if lines == []:
+      return
   else:
-    lines = '%s:%s' % (vim.current.range.start + 1, vim.current.range.end + 1)
+    lines = ['-lines', '%s:%s' % (vim.current.range.start + 1,
+                                  vim.current.range.end + 1)]
 
   # Determine the cursor position.
   cursor = int(vim.eval('line2byte(line("."))+col(".")')) - 2
   if cursor < 0:
     print('Couldn\'t determine cursor position. Is your file empty?')
     return
 
   # Avoid flashing an ugly, ugly cmd prompt on Windows when invoking clang-format.
   startupinfo = None
   if sys.platform.startswith('win32'):
     startupinfo = subprocess.STARTUPINFO()
     startupinfo.dwFlags |= subprocess.STARTF_USESHOWWINDOW
     startupinfo.wShowWindow = subprocess.SW_HIDE
 
   # Call formatter.
   command = [binary, '-style', style, '-cursor', str(cursor)]
   if lines != 'all':
-    command.extend(['-lines', lines])
+    command += lines
   if fallback_style:
     command.extend(['-fallback-style', fallback_style])
   if vim.current.buffer.name:
     command.extend(['-assume-filename', vim.current.buffer.name])
   p = subprocess.Popen(command,
                        stdout=subprocess.PIPE, stderr=subprocess.PIPE,
                        stdin=subprocess.PIPE, startupinfo=startupinfo)
   stdout, stderr = p.communicate(input=text.encode(encoding))