State windows are the unit of evidence.

Problem

Latest state erases the shape of the incident.

If a device is healthy now, a latest-state table cannot show that it was offline earlier, which provider saw it, how long it lasted, or whether another provider missed it. That missing duration is usually the evidence needed for debugging, audit, and quality checks.

Solution

Record predicate truth as a half-open range.

A window opens when the active predicate becomes true for a key and closes when it becomes false. Think of it as [start, end): active at the start point and no longer active at the end point. That convention lets touching windows line up without overlapping by one unit.

var pipeline = Spanfold.Spanfold
    .For<DeviceSignal>()
    .RecordWindows()
    .Window("DeviceOffline", window => window
        .Key(signal => signal.DeviceId)
        .ActiveWhen(signal => !signal.IsOnline))
    .Build();