diff --git a/zipline/gens/sort.py b/zipline/gens/sort.py
new file mode 100644
index 00000000..14745fde
--- /dev/null
+++ b/zipline/gens/sort.py
@@ -0,0 +1,118 @@
+"""
+Generator version of Feed.
+"""
+from collections import deque
+from zipline import ndict
+from zipline.gens.utils import  \
+    assert_datasource_unframe_protocol, \
+    assert_sort_protocol
+
+def date_sort(stream_in, source_ids):
+    """
+    A generator that takes a generator and a list of source_ids.  We
+    maintain an internal queue for each id in source_ids. While we
+    have messages pending from all sources, we pull the earliest
+    message and yield it.
+    """
+
+    assert isinstance(source_ids, list)
+
+    # Set up an internal queue for each expected source.
+    sources = {}
+    for id in source_ids:
+        assert isinstance(id, basestring), "Bad source_id %s" % id
+        sources[id] = deque()
+
+    # Process incoming streams.
+    for message in stream_in:
+        # Incoming messages should be the output of DATASOURCE_UNFRAME.
+        assert_datasource_unframe_protocol(message), \
+            "Bad message in date_sort: %s" % message
+
+        # Only allow messages from sources we expect.
+        assert message.source_id in sources, "Unexpected source: %s" % message
+
+        sources[message.source_id].append(message)
+
+        # Only pop messages when we have a pending message from
+        # all datasources. Stop if all sources have signalled done.
+
+        while ready(sources) and not done(sources):
+            message = pop_oldest(sources)
+            assert_sort_protocol(message)
+            yield message
+
+    # We should have only a done message left in each queue.
+    for queue in sources.itervalues():
+        assert len(queue) == 1, "Bad queue in date_sort on exit: %s" % queue
+        assert queue[0].dt == "DONE", \
+            "Bad last message in date_sort on exit: %s" % queue
+
+def ready(sources):
+    """
+    Feed is ready when every internal queue has at least one
+    message. Note that this include DONE messages, so done(sources) is
+    True only if ready(sources).
+    """
+    assert isinstance(sources, dict)
+    return all( (queue_is_ready(source) for source in sources.itervalues()) )
+
+def queue_is_ready(queue):
+    assert isinstance(queue, deque)
+    return len(queue) > 0
+
+def done(sources):
+    """Feed is done when all internal queues have only a "DONE" message."""
+    assert isinstance(sources, dict)
+    return all( (queue_is_done(source) for source in sources.itervalues()) )
+
+def queue_is_done(queue):
+    assert isinstance(queue, deque)
+    if len(queue) == 0:
+        return False
+    if queue[0].dt == "DONE":
+        assert len(queue) == 1, "Message after DONE in date_sort: %s" % queue
+        return True
+    else:
+        return False
+
+def pop_oldest(sources):
+
+    oldest_event = None
+
+    # Iterate over the dict, checking internal queues for the oldest
+    # pending event.
+
+    for queue in sources.itervalues():
+        current_event = queue[0]
+        # Skip queues that are done.
+        if current_event.dt == "DONE":
+            continue
+        # Any event is older than nothing.
+        elif oldest_event == None:
+            oldest_event = current_event
+        # Keep the older event.  Break ties by source_id. This will
+        # trip an assert if we have duplicate sources.
+        else:
+            oldest_event = older(oldest_event, current_event)
+
+    # Pop the oldest event we found from its queue and return it.
+    return sources[oldest_event.source_id].popleft()
+
+# Return the event with the older timestamp.  Break ties by source_id.
+def older(oldest, current):
+    assert isinstance(oldest, ndict)
+    assert isinstance(oldest, ndict)
+
+    # Try to compare by dt.
+    if oldest.dt < current.dt:
+        return oldest
+    elif oldest.dt > current.dt:
+        return current
+    # Break ties by source_id.
+    elif oldest.source_id < current.source_id:
+        return oldest
+    elif oldest.source_id > current.source_id:
+        return current
+    else:
+        assert False, "Duplicate event"