fitp_intrv_helper Function

  integer function fitp_intrv_helper(tt, x, n, periodic) result(index)
    implicit none
    integer, intent(in) :: n
    real, intent(in) :: tt
    real, dimension(n), intent(in) :: x
    logical, intent(in) :: periodic
    integer, save :: i = 1
    integer :: il, ih, upper
    if (periodic) then
       upper = n
    else
       upper = n - 1
    end if

!
! check for illegal i
!
    if (i .ge. n) i = n / 2
!
! check old interval and extremes
!
    if (tt .lt. x(i)) then
       if (tt .le. x(2)) then
          i = 1
          index = 1
          return
       else
          il = 2
          ih = i
       end if
    else if (tt .le. x(i+1)) then
       index = i
       return
    else if (tt .ge. x(upper)) then
       i = upper
       index = upper
       return
    else
       il = i+1
       ih = upper
    end if

    !
    ! binary search loop
    !
    do while (.true.)
       i = (il + ih) / 2
       if (tt < x(i)) then
          ih = i
       else if (tt > x(i + 1)) then
          il = i + 1
       else
          index = i
          exit
       end if
    end do
  end function fitp_intrv_helper