--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/hgext3rd/evolve/evolvestate.py	Wed Jan 10 14:53:01 2018 +0530
@@ -0,0 +1,74 @@
+# This software may be used and distributed according to the terms of the
+# GNU General Public License version 2 or any later version.
+
+"""
+This file contains class to wrap the state for hg evolve command and other
+related logic.
+
+All the data related to the command state is stored as dictionary in the object.
+The class has methods using which the data can be stored to disk in
+.hg/evolvestate file.
+
+We store the data on disk in cbor, for which we use cbor library to serialize
+and deserialize data.
+"""
+
+from __future__ import absolute_import
+
+from .thirdparty import cbor
+
+from mercurial import (
+    util,
+)
+
+class evolvestate():
+    """a wrapper class to store the state of `hg evolve` command
+
+    All the data for the state is stored in the form of key-value pairs in a
+    dictionary.
+
+    The class object can write all the data to .hg/evolvestate file and also can
+    populate the object data reading that file
+    """
+
+    def __init__(self, repo, path='evolvestate', opts={}):
+        self._repo = repo
+        self.path = path
+        self.opts = opts
+
+    def __nonzero__(self):
+        return self.exists()
+
+    def __getitem__(self, key):
+        return self.opts[key]
+
+    def load(self):
+        """load the existing evolvestate file into the class object"""
+        op = self._read()
+        self.opts.update(op)
+
+    def addopts(self, opts):
+        """add more key-value pairs to the data stored by the object"""
+        self.opts.update(opts)
+
+    def save(self):
+        """write all the evolvestate data stored in .hg/evolvestate file
+
+        we use third-party library cbor to serialize data to write in the file.
+        """
+        with self._repo.vfs(self.path, 'wb', atomictemp=True) as fp:
+            cbor.dump(self.opts, fp)
+
+    def _read(self):
+        """reads the evolvestate file and returns a dictionary which contain
+        data in the same format as it was before storing"""
+        with self._repo.vfs(self.path, 'rb') as fp:
+            return cbor.load(fp)
+
+    def delete(self):
+        """drop the evolvestate file if exists"""
+        util.unlinkpath(self._repo.vfs.join(self.path), ignoremissing=True)
+
+    def exists(self):
+        """check whether the evolvestate file exists or not"""
+        return self._repo.vfs.exists(self.path)