let find key (xs: ('a * 'b) []) =
    let first,_ = xs.[0]

    match key < first with
    | true -> None
    | _ ->

    let length = Array.length xs

    let rec find key startIndex endIndex =
        match endIndex - startIndex with
        | 0
        | 1 -> Some <| xs.[startIndex]
        | _ ->

        let half = startIndex + ((endIndex - startIndex) / 2)
        let date,value = xs.[half]

        match compare key date with
        | 0 -> Some (date,value)
        | 1 -> find key half endIndex
        | _ -> find key startIndex half

    find key 0 (length - 1)