diff --git a/gen_version.py b/gen_version.py
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+++ b/gen_version.py
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+#!/usr/bin/env python
+#
+# Copyright 2015 Google Inc. All rights reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""Generate version string for NotoColorEmoji.
+
+This parses the color emoji template file and updates the lines
+containing version string info, writing a new file.
+
+The nameID 5 field in the emoji font should reflect the commit/date
+of the repo it was built from.  This will build a string of the following
+format:
+  Version 1.39;GOOG;noto-emoji:20170220:a8a215d2e889'
+
+This is intended to indicate that it was built by Google from noto-emoji
+at commit a8a215d2e889 and date 20170220 (since dates are a bit easier
+to locate in time than commit hashes).
+
+For building with external data we don't include the commit id as we
+might be using different resoruces.  Instead the version string is:
+  Version 1.39;GOOG;noto-emoji:20170518;BETA <msg>
+
+Here the date is the current date, and the message after 'BETA ' is
+provided using the '-b' flag.  There's no commit hash.  This also
+bypasses some checks about the state of the repo.
+
+The relase number should have 2 or 3 minor digits.  Right now we've been
+using 2 but at the next major relase we probably want to use 3.  This
+supports both.  It will bump the version number if none is provided,
+maintaining the minor digit length.
+"""
+
+import argparse
+import datetime
+import re
+
+from nototools import tool_utils
+
+# These are not very lenient, we expect to be applied to the noto color
+# emoji template ttx file which matches these.  Why then require the
+# input argument, you ask?  Um... testing?
+_nameid_re = re.compile(r'\s*<namerecord nameID="5"')
+_version_re = re.compile(r'\s*Version\s(\d+.\d{2,3})')
+_headrev_re = re.compile(r'\s*<fontRevision value="(\d+.\d{2,3})"/>')
+
+def _get_existing_version(lines):
+  """Scan lines for all existing version numbers, and ensure they match.
+  Return the matched version number string."""
+
+  version = None
+  def check_version(new_version):
+    if version is not None and new_version != version:
+      raise Exception(
+          'version %s and namerecord version %s do not match' % (
+              version, new_version))
+    return new_version
+
+  saw_nameid = False
+  for line in lines:
+    if saw_nameid:
+      saw_nameid = False
+      m = _version_re.match(line)
+      if not m:
+        raise Exception('could not match line "%s" in namerecord' % line)
+      version = check_version(m.group(1))
+    elif _nameid_re.match(line):
+      saw_nameid = True
+    else:
+      m = _headrev_re.match(line)
+      if m:
+        version = check_version(m.group(1))
+  return version
+
+
+def _version_to_mm(version):
+  majs, mins = version.split('.')
+  minor_len = len(mins)
+  return int(majs), int(mins), minor_len
+
+
+def _mm_to_version(major, minor, minor_len):
+  fmt = '%%d.%%0%dd' % minor_len
+  return fmt % (major, minor)
+
+
+def _version_compare(lhs, rhs):
+  lmaj, lmin, llen = _version_to_mm(lhs)
+  rmaj, rmin, rlen = _version_to_mm(rhs)
+  # if major versions differ, we don't care about the minor length, else
+  # they should be the same
+  if lmaj != rmaj:
+    return lmaj - rmaj
+  if llen != rlen:
+    raise Exception('minor version lengths differ: "%s" and "%s"' % (lhs, rhs))
+  return lmin - rmin
+
+
+def _version_bump(version):
+  major, minor, minor_len = _version_to_mm(version)
+  minor = (minor + 1) % (10 ** minor_len)
+  if minor == 0:
+    raise Exception('cannot bump version "%s", requires new major' % version)
+  return _mm_to_version(major, minor, minor_len)
+
+
+def _get_repo_version_str(beta):
+  """See above for description of this string."""
+  if beta is not None:
+    date_str = datetime.date.today().strftime('%Y%m%d')
+    return 'GOOG;noto-emoji:%s;BETA %s' % (date_str, beta)
+
+  p = tool_utils.resolve_path('[emoji]')
+  commit, date, _ = tool_utils.git_head_commit(p)
+  if not tool_utils.git_check_remote_commit(p, commit):
+    raise Exception('emoji not on upstream master branch')
+  date_re = re.compile(r'(\d{4})-(\d{2})-(\d{2})')
+  m = date_re.match(date)
+  if not m:
+    raise Exception('could not match "%s" with "%s"' % (date, date_re.pattern))
+  ymd = ''.join(m.groups())
+  return 'GOOG;noto-emoji:%s:%s' % (ymd, commit[:12])
+
+
+def _replace_existing_version(lines, version, version_str):
+  """Update lines with new version strings in appropriate places."""
+  saw_nameid = False
+  for i in range(len(lines)):
+    line = lines[i]
+    if saw_nameid:
+      saw_nameid = False
+      # preserve indentation
+      lead_ws = len(line) - len(line.lstrip())
+      lines[i] = line[:lead_ws] + version_str + '\n'
+    elif _nameid_re.match(line):
+      saw_nameid = True
+    elif _headrev_re.match(line):
+      lead_ws = len(line) - len(line.lstrip())
+      lines[i] = line[:lead_ws] + '<fontRevision value="%s"/>\n' % version
+
+
+def update_version(srcfile, dstfile, version, beta):
+  """Update version in srcfile and write to dstfile.  If version is None,
+  bumps the current version, else version must be greater than the
+  current verison."""
+
+  with open(srcfile, 'r') as f:
+    lines = f.readlines()
+  current_version = _get_existing_version(lines)
+  if not version:
+    version = _version_bump(current_version)
+  elif version and _version_compare(version, current_version) <= 0:
+    raise Exception('new version %s is <= current version %s' % (
+        version, current_version))
+  version_str = 'Version %s;%s' % (version, _get_repo_version_str(beta))
+  _replace_existing_version(lines, version, version_str)
+  with open(dstfile, 'w') as f:
+    for line in lines:
+      f.write(line)
+
+
+def main():
+  parser = argparse.ArgumentParser()
+  parser.add_argument(
+      '-v', '--version', help='version number, default bumps the current '
+      'version', metavar='ver')
+  parser.add_argument(
+      '-s', '--src', help='ttx file with name and head tables',
+      metavar='file', required=True)
+  parser.add_argument(
+      '-d', '--dst', help='name of edited ttx file to write',
+      metavar='file', required=True)
+  parser.add_argument(
+      '-b', '--beta', help='beta tag if font is built using external resources')
+  args = parser.parse_args()
+
+  update_version(args.src, args.dst, args.version, args.beta)
+
+
+if __name__ == '__main__':
+  main()