drpack.f90

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function drpack(x, x0, b, t, amp) result(f_drpack)
!-------------------------------------------------------------------
! derivative of function rpack
!------------------------------------------------------------------

  use itm_types
  
  implicit none
  
  real(r8) :: x, x0, b, t, amp
  real(r8) :: f_drpack
  
  real(r8) :: dx, p4
  
  dx = abs(x - x0)
  if (dx < b) then
    p4 = 2._r8 / b - 2._r8 * dx / b**2
  else
    dx = dx - b
    p4 = -6._r8 * dx / ((t - b)**2) + 6._r8 * dx**2 / (t - b)**3
  endif
  f_drpack = amp * p4
  if (x < x0) f_drpack = -f_drpack
  if (abs(x - x0) > t) f_drpack = 0._r8
  
end function drpack