ddrpack.f90

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function ddrpack(x, x0, b, t, amp) result(f_ddrpack)
!-------------------------------------------------------------------
! second derivative of function rpack
!------------------------------------------------------------------
!
! attention: ddrpack not continuous !

  use itm_types

  implicit none
  
  real(r8) :: x, x0, b, t, amp
  real(r8) :: f_ddrpack                
  
  real(r8) :: dx, p4
  
  dx = abs(x - x0)
  if (dx < b) then
    p4 = -2._r8 / b**2
  else
    dx = dx - b
    p4 = -6._r8 / ((t - b)**2) + 12._r8 * dx / (t - b)**3
  endif
  f_ddrpack = amp * p4
  if (x < x0) f_ddrpack = -f_ddrpack
  if (abs(x - x0) > t) f_ddrpack = 0._r8
  
end function ddrpack