4 files changed, 95 insertions, 0 deletions
diff --git a/obnamlib/__init__.py b/obnamlib/__init__.py
index 1c7ae9ae..df719903 100644
--- a/obnamlib/__init__.py
+++ b/obnamlib/__init__.py
@@ -191,6 +191,8 @@ from .fmt_ga import (
     CowLeaf,
     CowTree,
     LeafList,
+    CowDelta,
+    removed_key,
 )
 
 
diff --git a/obnamlib/fmt_ga/__init__.py b/obnamlib/fmt_ga/__init__.py
index 5803cbe1..3874a768 100644
--- a/obnamlib/fmt_ga/__init__.py
+++ b/obnamlib/fmt_ga/__init__.py
@@ -20,6 +20,7 @@ from .chunk_store import GAChunkStore
 from .leaf_store import InMemoryLeafStore, LeafStore
 from .leaf import CowLeaf
 from .leaf_list import LeafList
+from .cowdelta import CowDelta, removed_key
 from .cowtree import CowTree
 from .indexes import GAChunkIndexes
 from .dirobj import GADirectory, GAImmutableError, create_gadirectory_from_dict
diff --git a/obnamlib/fmt_ga/cowdelta.py b/obnamlib/fmt_ga/cowdelta.py
new file mode 100644
index 00000000..a5413d54
--- /dev/null
+++ b/obnamlib/fmt_ga/cowdelta.py
@@ -0,0 +1,40 @@
+# Copyright 2017  Lars Wirzenius
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+#
+# =*= License: GPL-3+ =*=
+
+
+class CowDelta(object):
+
+    def __init__(self):
+        self._dict = {}
+
+    def __contains__(self, key):
+        return key in self._dict
+
+    def keys(self):
+        return self._dict.keys()
+
+    def set(self, key, value):
+        self._dict[key] = value
+
+    def get(self, key):
+        return self._dict[key]
+
+    def remove(self, key):
+        self._dict[key] = removed_key
+
+
+removed_key = object()
diff --git a/obnamlib/fmt_ga/cowdelta_tests.py b/obnamlib/fmt_ga/cowdelta_tests.py
new file mode 100644
index 00000000..f0a5a5e6
--- /dev/null
+++ b/obnamlib/fmt_ga/cowdelta_tests.py
@@ -0,0 +1,52 @@
+# Copyright 2017  Lars Wirzenius
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+#
+# =*= License: GPL-3+ =*=
+
+
+import unittest
+
+
+import obnamlib
+
+
+class CowDeltaTests(unittest.TestCase):
+
+    def test_is_empty_initially(self):
+        delta = obnamlib.CowDelta()
+        self.assertEqual(delta.keys(), [])
+        self.assertFalse('key' in delta)
+
+    def test_remembers_key_value_pair(self):
+        delta = obnamlib.CowDelta()
+        delta.set('key', 'value')
+        self.assertEqual(delta.keys(), ['key'])
+        self.assertEqual(delta.get('key'), 'value')
+        self.assertTrue('key' in delta)
+
+    def test_remembers_second_value_for_key(self):
+        delta = obnamlib.CowDelta()
+        delta.set('key', 'old-value')
+        delta.set('key', 'new-value')
+        self.assertEqual(delta.keys(), ['key'])
+        self.assertEqual(delta.get('key'), 'new-value')
+        self.assertTrue('key' in delta)
+
+    def test_remembers_removed_key(self):
+        delta = obnamlib.CowDelta()
+        delta.remove('key')
+        self.assertEqual(delta.keys(), ['key'])
+        self.assertEqual(delta.get('key'), obnamlib.removed_key)
+        self.assertTrue('key' in delta)