diff --git a/tools/releasetools/sparse_img.py b/tools/releasetools/sparse_img.py
new file mode 100644
index 0000000..ef7ff92
--- /dev/null
+++ b/tools/releasetools/sparse_img.py
@@ -0,0 +1,184 @@
+import bisect
+import os
+import sys
+import struct
+import pprint
+from hashlib import sha1
+
+from rangelib import *
+
+class SparseImage(object):
+  """Wraps a sparse image file (and optional file map) into an image
+  object suitable for passing to BlockImageDiff."""
+
+  def __init__(self, simg_fn, file_map_fn=None):
+    self.simg_f = f = open(simg_fn, "rb")
+
+    header_bin = f.read(28)
+    header = struct.unpack("<I4H4I", header_bin)
+
+    magic = header[0]
+    major_version = header[1]
+    minor_version = header[2]
+    file_hdr_sz = header[3]
+    chunk_hdr_sz = header[4]
+    self.blocksize = blk_sz = header[5]
+    self.total_blocks = total_blks = header[6]
+    total_chunks = header[7]
+    image_checksum = header[8]
+
+    if magic != 0xED26FF3A:
+      raise ValueError("Magic should be 0xED26FF3A but is 0x%08X" % (magic,))
+    if major_version != 1 or minor_version != 0:
+      raise ValueError("I know about version 1.0, but this is version %u.%u" %
+                       (major_version, minor_version))
+    if file_hdr_sz != 28:
+      raise ValueError("File header size was expected to be 28, but is %u." %
+                       (file_hdr_sz,))
+    if chunk_hdr_sz != 12:
+      raise ValueError("Chunk header size was expected to be 12, but is %u." %
+                       (chunk_hdr_sz,))
+
+    print("Total of %u %u-byte output blocks in %u input chunks."
+          % (total_blks, blk_sz, total_chunks))
+
+    pos = 0   # in blocks
+    care_data = []
+    self.offset_map = offset_map = []
+
+    for i in range(total_chunks):
+      header_bin = f.read(12)
+      header = struct.unpack("<2H2I", header_bin)
+      chunk_type = header[0]
+      reserved1 = header[1]
+      chunk_sz = header[2]
+      total_sz = header[3]
+      data_sz = total_sz - 12
+
+      if chunk_type == 0xCAC1:
+        if data_sz != (chunk_sz * blk_sz):
+          raise ValueError(
+              "Raw chunk input size (%u) does not match output size (%u)" %
+              (data_sz, chunk_sz * blk_sz))
+        else:
+          care_data.append(pos)
+          care_data.append(pos + chunk_sz)
+          offset_map.append((pos, chunk_sz, f.tell()))
+          pos += chunk_sz
+          f.seek(data_sz, os.SEEK_CUR)
+
+      elif chunk_type == 0xCAC2:
+        raise ValueError("Fill chunks are not supported")
+
+      elif chunk_type == 0xCAC3:
+        if data_sz != 0:
+          raise ValueError("Don't care chunk input size is non-zero (%u)" %
+                           (data_sz))
+        else:
+          pos += chunk_sz
+
+      elif chunk_type == 0xCAC4:
+        raise ValueError("CRC32 chunks are not supported")
+
+      else:
+        raise ValueError("Unknown chunk type 0x%04X not supported" %
+                         (chunk_type,))
+
+    self.care_map = RangeSet(care_data)
+    self.offset_index = [i[0] for i in offset_map]
+
+    if file_map_fn:
+      self.LoadFileBlockMap(file_map_fn)
+    else:
+      self.file_map = {"__DATA": self.care_map}
+
+  def ReadRangeSet(self, ranges):
+    return [d for d in self._GetRangeData(ranges)]
+
+  def TotalSha1(self):
+    """Return the SHA-1 hash of all data in the 'care' regions of this image."""
+    h = sha1()
+    for d in self._GetRangeData(self.care_map):
+      h.update(d)
+    return h.hexdigest()
+
+  def _GetRangeData(self, ranges):
+    """Generator that produces all the image data in 'ranges'.  The
+    number of individual pieces returned is arbitrary (and in
+    particular is not necessarily equal to the number of ranges in
+    'ranges'.
+
+    This generator is stateful -- it depends on the open file object
+    contained in this SparseImage, so you should not try to run two
+    instances of this generator on the same object simultaneously."""
+
+    f = self.simg_f
+    for s, e in ranges:
+      to_read = e-s
+      idx = bisect.bisect_right(self.offset_index, s) - 1
+      chunk_start, chunk_len, filepos = self.offset_map[idx]
+
+      # for the first chunk we may be starting partway through it.
+      p = filepos + ((s - chunk_start) * self.blocksize)
+      remain = chunk_len - (s - chunk_start)
+
+      f.seek(p, os.SEEK_SET)
+      this_read = min(remain, to_read)
+      yield f.read(this_read * self.blocksize)
+      to_read -= this_read
+
+      while to_read > 0:
+        # continue with following chunks if this range spans multiple chunks.
+        idx += 1
+        chunk_start, chunk_len, filepos = self.offset_map[idx]
+        f.seek(filepos, os.SEEK_SET)
+        this_read = min(chunk_len, to_read)
+        yield f.read(this_read * self.blocksize)
+        to_read -= this_read
+
+  def LoadFileBlockMap(self, fn):
+    remaining = self.care_map
+    self.file_map = out = {}
+
+    with open(fn) as f:
+      for line in f:
+        fn, ranges = line.split(None, 1)
+        ranges = RangeSet.parse(ranges)
+        out[fn] = ranges
+        assert ranges.size() == ranges.intersect(remaining).size()
+        remaining = remaining.subtract(ranges)
+
+    # For all the remaining blocks in the care_map (ie, those that
+    # aren't part of the data for any file), divide them into blocks
+    # that are all zero and blocks that aren't.  (Zero blocks are
+    # handled specially because (1) there are usually a lot of them
+    # and (2) bsdiff handles files with long sequences of repeated
+    # bytes especially poorly.)
+
+    zero_blocks = []
+    nonzero_blocks = []
+    reference = '\0' * self.blocksize
+
+    f = self.simg_f
+    for s, e in remaining:
+      for b in range(s, e):
+        idx = bisect.bisect_right(self.offset_index, b) - 1
+        chunk_start, chunk_len, filepos = self.offset_map[idx]
+        filepos += (b-chunk_start) * self.blocksize
+        f.seek(filepos, os.SEEK_SET)
+        data = f.read(self.blocksize)
+
+        if data == reference:
+          zero_blocks.append(b)
+          zero_blocks.append(b+1)
+        else:
+          nonzero_blocks.append(b)
+          nonzero_blocks.append(b+1)
+
+    out["__ZERO"] = RangeSet(data=zero_blocks)
+    out["__NONZERO"] = RangeSet(data=nonzero_blocks)
+
+  def ResetFileMap(self):
+    """Throw away the file map and treat the entire image as
+    undifferentiated data."""
+    self.file_map = {"__DATA": self.care_map}