Time difference since previous record in time series aggregation - mongodb

Question

I have a collection of events made by different actors. I now need to calculate the amount of events per actor that occurred x amount of time since the last time an event occurred.
A more concrete example: a collection of login events made by different actors. Every login event that happened less than 8 hours since the previous login should be ignored. So let's say I log in at 2pm and again at 11pm, the count we want would be 2. If I would log in at 2pm and again at 5pm, that would have to count for 1.
I don't really see a viable solution to this problem using the aggregation framework. A possible solution would be to calculate (and save) the time between te previous event for each record. But I feel there should be a better solution.
Can anyone point me in the right direction? Didn't really find any usecases that are similar to this one.
If my question isn't clear, let me know!
Edit:
An example:
Simplified events:
[{
_id: 1,
actor: X,
date: ISODate("2018-09-20T18:00:00.000Z")
},
{
_id: 2,
actor: X,
date: ISODate("2018-09-21T05:00:00.000Z") // 11 hours since previous
},
{
_id: 3,
actor: X,
date: ISODate("2018-09-21T07:00:00.000Z") // 2 hours since previous
},
{
_id: 4,
actor: Y,
date: ISODate("2018-09-21T06:00:00.000Z")
},
{
_id: 5,
actor: Y,
date: ISODate("2018-09-21T09:00:00.000Z") // 3 hours since previous
}]
Expected output:
[{
_id: X,
count: 2 // 3 events, but one is less than 8 hours since previous
},
{
_id: Y,
count: 1 // 2 events, but one is less than 8 hours since previous
}]

Answer 1

You can compare values from different documents by grouping them into array and iterating over it. In your case $reduce is probably the simplest way:
db.collection.aggregate([
// ensure order
{ $sort: { date: 1 } },
// get all dates per actor
{ $group: { _id: "$actor", dates: { $push: "$date" } } },
{ $addFields: {
// iterate dates
events: { $reduce: {
input: "$dates",
initialValue: { last: null, count: 1 },
// increment counter if difference is > than 8 * 60 * 60 * 1000 millis
in: { last: "$$this", count: { $add: [
"$$value.count",
{ $cond: [
{$gt: [ { $subtract: [ "$$this", "$$value.last" ] }, 28800000 ] },
1,
0
] }
] } }
} }
} },
{ $project: { count: "$events.count" } }
])
It's gonna be slow on large dataset. In this case you may need to pre-aggregate counters at insert time.