--- a/hgext3rd/evolve/state.py	Tue Jan 30 00:02:29 2018 +0530
+++ b/hgext3rd/evolve/state.py	Tue Jan 30 15:02:40 2018 +0530
@@ -15,12 +15,18 @@
 
 from __future__ import absolute_import
 
+import errno
+import struct
+
 from .thirdparty import cbor
 
 from mercurial import (
+    error,
     util,
 )
 
+from mercurial.i18n import _
+
 class cmdstate():
     """a wrapper class to store the state of commands like `evolve`, `grab`
 
@@ -72,3 +78,49 @@
     def exists(self):
         """check whether the evolvestate file exists or not"""
         return self._repo.vfs.exists(self.path)
+
+def _oldevolvestateread(repo):
+    """function to read the old evolvestate file
+
+    This exists for BC reasons."""
+    try:
+        f = repo.vfs('evolvestate')
+    except IOError as err:
+        if err.errno != errno.ENOENT:
+            raise
+    try:
+        versionblob = f.read(4)
+        if len(versionblob) < 4:
+            repo.ui.debug('ignoring corrupted evolvestate (file contains %i bits)'
+                          % len(versionblob))
+            return None
+        version = struct._unpack('>I', versionblob)[0]
+        if version != 0:
+            msg = _('unknown evolvestate version %i') % version
+            raise error.Abort(msg, hint=_('upgrade your evolve'))
+        records = []
+        data = f.read()
+        off = 0
+        end = len(data)
+        while off < end:
+            rtype = data[off]
+            off += 1
+            length = struct._unpack('>I', data[off:(off + 4)])[0]
+            off += 4
+            record = data[off:(off + length)]
+            off += length
+            if rtype == 't':
+                rtype, record = record[0], record[1:]
+            records.append((rtype, record))
+        state = {}
+        for rtype, rdata in records:
+            if rtype == 'C':
+                state['current'] = rdata
+            elif rtype.lower():
+                repo.ui.debug('ignore evolve state record type %s' % rtype)
+            else:
+                raise error.Abort(_('unknown evolvestate field type %r')
+                                  % rtype, hint=_('upgrade your evolve'))
+        return state
+    finally:
+        f.close()