cpplint: improve root path for header guard behavior

This is lifted from the cpplint 1.6 release at
https://github.com/cpplint/cpplint, with minor
modifications to match the existing docstring style.

Change-Id: I20aff08805e044883717001856d99942690b45c4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3648533
Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Commit-Queue: David Sanders <dsanders11@ucsbalum.com>

cpplint.py

@@ -527,6 +527,7 @@ _error_suppressions = {}
 # The root directory used for deriving header guard CPP variable.
 # This is set by --root flag.
 _root = None
+_root_debug = False
 
 # The project root directory. Used for deriving header guard CPP variable.
 # This is set by --project_root flag. Must be an absolute path.
@@ -1735,6 +1736,32 @@ def GetIndentLevel(line):
     return 0
 
 
+def PathSplitToList(path):
+  """Returns the path split into a list by the separator.
+
+  Args:
+    path: An absolute or relative path (e.g. '/a/b/c/' or '../a')
+
+  Returns:
+    A list of path components (e.g. ['a', 'b', 'c]).
+  """
+  lst = []
+  while True:
+    (head, tail) = os.path.split(path)
+    if head == path:  # absolute paths end
+      lst.append(head)
+      break
+    if tail == path:  # relative paths end
+      lst.append(tail)
+      break
+
+    path = head
+    lst.append(tail)
+
+  lst.reverse()
+  return lst
+
+
 def GetHeaderGuardCPPVariable(filename):
   """Returns the CPP variable that should be used as a header guard.
 
@@ -1756,8 +1783,59 @@ def GetHeaderGuardCPPVariable(filename):
 
   fileinfo = FileInfo(filename)
   file_path_from_root = fileinfo.RepositoryName()
-  if _root:
-    file_path_from_root = re.sub('^' + _root + os.sep, '', file_path_from_root)
+
+  def FixupPathFromRoot():
+    if _root_debug:
+      sys.stderr.write("\n_root fixup, _root = '%s', repository name = '%s'\n"
+          % (_root, fileinfo.RepositoryName()))
+
+    # Process the file path with the --root flag if it was set.
+    if not _root:
+      if _root_debug:
+        sys.stderr.write("_root unspecified\n")
+      return file_path_from_root
+
+    def StripListPrefix(lst, prefix):
+      # f(['x', 'y'], ['w, z']) -> None  (not a valid prefix)
+      if lst[:len(prefix)] != prefix:
+        return None
+      # f(['a, 'b', 'c', 'd'], ['a', 'b']) -> ['c', 'd']
+      return lst[(len(prefix)):]
+
+    # root behavior:
+    #   --root=subdir , lstrips subdir from the header guard
+    maybe_path = StripListPrefix(PathSplitToList(file_path_from_root),
+                                 PathSplitToList(_root))
+
+    if _root_debug:
+      sys.stderr.write(("_root lstrip (maybe_path=%s, file_path_from_root=%s," +
+          " _root=%s)\n") % (maybe_path, file_path_from_root, _root))
+
+    if maybe_path:
+      return os.path.join(*maybe_path)
+
+    #   --root=.. , will prepend the outer directory to the header guard
+    full_path = fileinfo.FullName()
+    # adapt slashes for windows
+    root_abspath = os.path.abspath(_root).replace('\\', '/')
+
+    maybe_path = StripListPrefix(PathSplitToList(full_path),
+                                 PathSplitToList(root_abspath))
+
+    if _root_debug:
+      sys.stderr.write(("_root prepend (maybe_path=%s, full_path=%s, " +
+          "root_abspath=%s)\n") % (maybe_path, full_path, root_abspath))
+
+    if maybe_path:
+      return os.path.join(*maybe_path)
+
+    if _root_debug:
+      sys.stderr.write("_root ignore, returning %s\n" % (file_path_from_root))
+
+    #   --root=FAKE_DIR is ignored
+    return file_path_from_root
+
+  file_path_from_root = FixupPathFromRoot()
   return re.sub(r'[^a-zA-Z0-9]', '_', file_path_from_root).upper() + '_'