open System

​

let getHashSet k (lstr:string) =

    let strs = Collections.Generic.HashSet<string>()

    for i in 0..lstr.Length - k do

        strs.Add lstr.[i..i + k - 1] |> ignore

    strs

​

let getCommons k lstrs =

    let strss = lstrs |> Seq.map (getHashSet k) |> Seq.sortBy (fun strs -> strs.Count)

    let rest =  strss |> Seq.skip 1 |> Seq.toArray

    [|  for s in Seq.head strss do

            if rest |> Array.forall (fun strs -> strs.Contains s) then yield s

    |]

​

let random = System.Random System.DateTime.Now.Millisecond

let generateString n =

    [|  for i in 1..n do

            yield random.Next 20 |> (+) 65 |> System.Convert.ToByte

    |] |> System.Text.Encoding.ASCII.GetString

​

​

[ for i in 1..3 do yield generateString 10000 ]

|> getCommons 4

|> fun l -> printfn "found %d\n %A" l.Length l