Use LINQ to group a sequence by date with no gaps
- by Codesleuth
I'm trying to select a subgroup of a list where items have contiguous dates, e.g.
ID StaffID Type Title ActivityDate
-- ------- ---- ----------------- ------------
1 41 1 Doctors 07/06/2010
2 41 0 Meeting with John 08/06/2010
3 41 0 Meeting Continues 09/06/2010
4 41 0 Meeting Continues 10/06/2010
5 41 3 Annual Leave 11/06/2010
6 41 0 Meeting Continues 14/06/2010
I'm using a pivot point each time, so take the example pivot item as 3, I'd like to get the following resulting contiguous events around the pivot:
ID StaffID Type Title ActivityDate
-- ------- ---- ----------------- ------------
2 41 0 Meeting with John 08/06/2010
3 41 0 Meeting Continues 09/06/2010
4 41 0 Meeting Continues 10/06/2010
My current implementation is a laborious "walk" into the past, then into the future, to build the list:
var orderedEvents = activities.OrderBy(a => a.ActivityDate).ToArray();
// Walk into the past until a gap is found
var preceedingEvents = orderedEvents.TakeWhile(a => a.ID != activity.ID);
DateTime dayBefore;
var previousEvent = activity;
while (previousEvent != null)
{
dayBefore = previousEvent.ActivityDate.AddDays(-1).Date;
previousEvent = preceedingEvents.TakeWhile(a => a.ID != previousEvent.ID).LastOrDefault();
if (previousEvent != null)
{
if (previousEvent.ActivityDate.Date == dayBefore)
relatedActivities.Insert(0, previousEvent);
else
previousEvent = null;
}
}
// Walk into the future until a gap is found
var followingEvents = orderedEvents.SkipWhile(a => a.ID != activity.ID);
DateTime dayAfter;
var nextEvent = activity;
while (nextEvent != null)
{
dayAfter = nextEvent.ActivityDate.AddDays(1).Date;
nextEvent = followingEvents.SkipWhile(a => a.ID != nextEvent.ID).Skip(1).FirstOrDefault();
if (nextEvent != null)
{
if (nextEvent.ActivityDate.Date == dayAfter)
relatedActivities.Add(nextEvent);
else
nextEvent = null;
}
}
The list relatedActivities should then contain the contiguous events, in order.
Is there a better way (maybe using LINQ) for this?
I had an idea of using .Aggregate() but couldn't think how to get the aggregate to break out when it finds a gap in the sequence.
