1 files changed, 89 insertions, 0 deletions
diff --git a/u1db/vectorclock.py b/u1db/vectorclock.py
new file mode 100644
index 00000000..42bceaa8
--- /dev/null
+++ b/u1db/vectorclock.py
@@ -0,0 +1,89 @@
+# Copyright 2011 Canonical Ltd.
+#
+# This file is part of u1db.
+#
+# u1db is free software: you can redistribute it and/or modify
+# it under the terms of the GNU Lesser General Public License version 3
+# as published by the Free Software Foundation.
+#
+# u1db 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 Lesser General Public License for more details.
+#
+# You should have received a copy of the GNU Lesser General Public License
+# along with u1db.  If not, see <http://www.gnu.org/licenses/>.
+
+"""VectorClockRev helper class."""
+
+
+class VectorClockRev(object):
+    """Track vector clocks for multiple replica ids.
+
+    This allows simple comparison to determine if one VectorClockRev is
+    newer/older/in-conflict-with another VectorClockRev without having to
+    examine history. Every replica has a strictly increasing revision. When
+    creating a new revision, they include all revisions for all other replicas
+    which the new revision dominates, and increment their own revision to
+    something greater than the current value.
+    """
+
+    def __init__(self, value):
+        self._values = self._expand(value)
+
+    def __repr__(self):
+        s = self.as_str()
+        return '%s(%s)' % (self.__class__.__name__, s)
+
+    def as_str(self):
+        s = '|'.join(['%s:%d' % (m, r) for m, r
+                      in sorted(self._values.items())])
+        return s
+
+    def _expand(self, value):
+        result = {}
+        if value is None:
+            return result
+        for replica_info in value.split('|'):
+            replica_uid, counter = replica_info.split(':')
+            counter = int(counter)
+            result[replica_uid] = counter
+        return result
+
+    def is_newer(self, other):
+        """Is this VectorClockRev strictly newer than other.
+        """
+        if not self._values:
+            return False
+        if not other._values:
+            return True
+        this_is_newer = False
+        other_expand = dict(other._values)
+        for key, value in self._values.iteritems():
+            if key in other_expand:
+                other_value = other_expand.pop(key)
+                if other_value > value:
+                    return False
+                elif other_value < value:
+                    this_is_newer = True
+            else:
+                this_is_newer = True
+        if other_expand:
+            return False
+        return this_is_newer
+
+    def increment(self, replica_uid):
+        """Increase the 'replica_uid' section of this vector clock.
+
+        :return: A string representing the new vector clock value
+        """
+        self._values[replica_uid] = self._values.get(replica_uid, 0) + 1
+
+    def maximize(self, other_vcr):
+        for replica_uid, counter in other_vcr._values.iteritems():
+            if replica_uid not in self._values:
+                self._values[replica_uid] = counter
+            else:
+                this_counter = self._values[replica_uid]
+                if this_counter < counter:
+                    self._values[replica_uid] = counter