open System

let withTimeWindow<'T> (fnTime:'T-> DateTime) (window:TimeSpan) (xs:'T seq) =
    let xs' = xs |> Seq.sortBy fnTime
    let tFrom = xs' |> Seq.head |> fnTime
    let tTo = tFrom.AddSeconds(window.TotalSeconds)
    let ixs = xs'.GetEnumerator()

    let rec nxt(f:DateTime,t) prevX = 
        seq {
            let x = 
                prevX 
                |> Option.map(fun _ -> prevX)
                |> Option.defaultWith(fun () -> if ixs.MoveNext() then Some(ixs.Current) else None)
            match x with
            | Some x -> 
                let tx = fnTime x
                if tx < t then
                    yield ((f,t),x)
                    yield! nxt (f,t) None
                else
                    let f = f.AddSeconds(window.TotalSeconds)
                    let t = t.AddSeconds(window.TotalSeconds)
                    yield! nxt (f,t) (Some x)
            | None -> ixs.Dispose()
        }
        
    nxt (tFrom,tTo) None 

(*
let xs = seq{for i in 0 .. 100 -> i } |> Seq.scan (fun ((s:DateTime),_) t -> s.AddMinutes(1.0),t) (DateTime.Now,0)
let wxs = xs |> withTimeWindow fst (TimeSpan.FromMinutes 5.)
let wsg = wxs |> Seq.groupBy fst |> Seq.map (fun (interval,xs)->interval,Seq.length xs) |> Seq.toArray
> 
val wsg : ((DateTime * DateTime) * int) [] =
  [|((8/26/2020 4:36:01 PM, 8/26/2020 4:41:01 PM), 5);
    ((8/26/2020 4:41:01 PM, 8/26/2020 4:46:01 PM), 5);
    ((8/26/2020 4:46:01 PM, 8/26/2020 4:51:01 PM), 5);
    ((8/26/2020 4:51:01 PM, 8/26/2020 4:56:01 PM), 5);
    ((8/26/2020 4:56:01 PM, 8/26/2020 5:01:01 PM), 5);
    ((8/26/2020 5:01:01 PM, 8/26/2020 5:06:01 PM), 5);
    ((8/26/2020 5:06:01 PM, 8/26/2020 5:11:01 PM), 5);
    ((8/26/2020 5:11:01 PM, 8/26/2020 5:16:01 PM), 5);
    ((8/26/2020 5:16:01 PM, 8/26/2020 5:21:01 PM), 5);
    ((8/26/2020 5:21:01 PM, 8/26/2020 5:26:01 PM), 5);
    ((8/26/2020 5:26:01 PM, 8/26/2020 5:31:01 PM), 5);
    ((8/26/2020 5:31:01 PM, 8/26/2020 5:36:01 PM), 5);
    ((8/26/2020 5:36:01 PM, 8/26/2020 5:41:01 PM), 5);
    ((8/26/2020 5:41:01 PM, 8/26/2020 5:46:01 PM), 5);
    ((8/26/2020 5:46:01 PM, 8/26/2020 5:51:01 PM), 5);
    ((8/26/2020 5:51:01 PM, 8/26/2020 5:56:01 PM), 5);
    ((8/26/2020 5:56:01 PM, 8/26/2020 6:01:01 PM), 5);
    ((8/26/2020 6:01:01 PM, 8/26/2020 6:06:01 PM), 5);
    ((8/26/2020 6:06:01 PM, 8/26/2020 6:11:01 PM), 5);
    ((8/26/2020 6:11:01 PM, 8/26/2020 6:16:01 PM), 5);
    ((8/26/2020 6:16:01 PM, 8/26/2020 6:21:01 PM), 2)|]
*)