diff --git a/tools/releasetools/common.py b/tools/releasetools/common.py
index d0ee6ae..d09f60c 100644
--- a/tools/releasetools/common.py
+++ b/tools/releasetools/common.py
@@ -25,6 +25,7 @@
 import re
 import shlex
 import shutil
+import string
 import subprocess
 import sys
 import tempfile
@@ -34,6 +35,7 @@
 from hashlib import sha1, sha256
 
 import blockimgdiff
+import sparse_img
 
 class Options(object):
   def __init__(self):
@@ -124,6 +126,11 @@
   return subprocess.Popen(args, **kwargs)
 
 
+def RoundUpTo4K(value):
+  rounded_up = value + 4095
+  return rounded_up - (rounded_up % 4096)
+
+
 def CloseInheritedPipes():
   """ Gmake in MAC OS has file descriptor (PIPE) leak. We close those fds
   before doing other work."""
@@ -618,6 +625,56 @@
   return tmp, zipfile.ZipFile(filename, "r")
 
 
+def GetSparseImage(which, tmpdir, input_zip):
+  """Returns a SparseImage object suitable for passing to BlockImageDiff.
+
+  This function loads the specified sparse image from the given path, and
+  performs additional processing for OTA purpose. For example, it always adds
+  block 0 to clobbered blocks list. It also detects files that cannot be
+  reconstructed from the block list, for whom we should avoid applying imgdiff.
+
+  Args:
+    which: The partition name, which must be "system" or "vendor".
+    tmpdir: The directory that contains the prebuilt image and block map file.
+    input_zip: The target-files ZIP archive.
+
+  Returns:
+    A SparseImage object, with file_map info loaded.
+  """
+  assert which in ("system", "vendor")
+
+  path = os.path.join(tmpdir, "IMAGES", which + ".img")
+  mappath = os.path.join(tmpdir, "IMAGES", which + ".map")
+
+  # The image and map files must have been created prior to calling
+  # ota_from_target_files.py (since LMP).
+  assert os.path.exists(path) and os.path.exists(mappath)
+
+  # In ext4 filesystems, block 0 might be changed even being mounted R/O. We add
+  # it to clobbered_blocks so that it will be written to the target
+  # unconditionally. Note that they are still part of care_map. (Bug: 20939131)
+  clobbered_blocks = "0"
+
+  image = sparse_img.SparseImage(path, mappath, clobbered_blocks)
+
+  # block.map may contain less blocks, because mke2fs may skip allocating blocks
+  # if they contain all zeros. We can't reconstruct such a file from its block
+  # list. Tag such entries accordingly. (Bug: 65213616)
+  for entry in image.file_map:
+    # "/system/framework/am.jar" => "SYSTEM/framework/am.jar".
+    arcname = string.replace(entry, which, which.upper(), 1)[1:]
+    # Skip artificial names, such as "__ZERO", "__NONZERO-1".
+    if arcname not in input_zip.namelist():
+      continue
+
+    info = input_zip.getinfo(arcname)
+    ranges = image.file_map[entry]
+    if RoundUpTo4K(info.file_size) > ranges.size() * 4096:
+      ranges.extra['incomplete'] = True
+
+  return image
+
+
 def GetKeyPasswords(keylist):
   """Given a list of keys, prompt the user to enter passwords for
   those which require them.  Return a {key: password} dict.  password