!   D02PS_P0W_F Example Program Text
!   Mark 30.1 Release. NAG Copyright 2024.

    Module d02ps_p0w_fe_mod

!     D02PS_P0W_F Example Program Module:
!            Parameters and User-defined Routines

!     .. Use Statements ..
      Use iso_c_binding, Only: c_ptr
      Use nag_library, Only: nag_wp
!     .. Implicit None Statement ..
      Implicit None
!     .. Accessibility Statements ..
      Private
      Public                           :: f
!     .. Parameters ..
      Integer, Parameter, Public       :: liwsav = 130, n = 2, nin = 5,        &
                                          nout = 6, npts = 16, nwant = 1
      Integer, Parameter, Public       :: lrwsav = 350 + 32*n
      Integer, Parameter, Public       :: lwcomm = n + 5*nwant
    Contains

      Subroutine f(ad_handle,t,n,y,yp,iuser,ruser)
!       .. Implicit None Statement ..
        Implicit None
!       .. Scalar Arguments ..
        Type (c_ptr), Intent (Inout)   :: ad_handle
        Real (Kind=nag_wp), Intent (In) :: t
        Integer, Intent (In)           :: n
!       .. Array Arguments ..
        Real (Kind=nag_wp), Intent (Inout) :: ruser(*)
        Real (Kind=nag_wp), Intent (In) :: y(n)
        Real (Kind=nag_wp), Intent (Out) :: yp(n)
        Integer, Intent (Inout)        :: iuser(*)
!       .. Executable Statements ..
        yp(1) = y(2)
        yp(2) = -y(1)
        Return
      End Subroutine f
    End Module d02ps_p0w_fe_mod

    Program d02ps_p0w_fe

!     D02PS_P0W_F Example Main Program

!     .. Use Statements ..
      Use d02ps_p0w_fe_mod, Only: f, liwsav, lrwsav, lwcomm, n, nin, nout,     &
                                  npts, nwant
      Use iso_c_binding, Only: c_ptr
      Use nagad_library, Only: d02pf_p0w_f, d02pq_p0w_f, d02ps_p0w_f,          &
                               d02pt_p0w_f
      Use nag_library, Only: nag_wp
!     .. Implicit None Statement ..
      Implicit None
!     .. Local Scalars ..
      Type (c_ptr)                     :: ad_handle
      Real (Kind=nag_wp)               :: hnext, hstart, tend, tinc, tnow,     &
                                          tol, tstart, twant, waste
      Integer                          :: fevals, ideriv, ifail, method,       &
                                          stepcost, stepsok
!     .. Local Arrays ..
      Real (Kind=nag_wp)               :: ruser(1), thresh(n), y(n), yinit(n), &
                                          ynow(n), ypnow(n), ypwant(nwant),    &
                                          ywant(nwant)
      Real (Kind=nag_wp), Allocatable  :: rwsav(:), wcomm(:)
      Integer                          :: iuser(1)
      Integer, Allocatable             :: iwsav(:)
!     .. Intrinsic Procedures ..
      Intrinsic                        :: real
!     .. Executable Statements ..
      Write (nout,*) 'D02PS_P0W_F Example Program Results'

      Allocate (iwsav(liwsav),rwsav(lrwsav),wcomm(lwcomm))

!     Set initial conditions and input

!     Skip heading in data file
      Read (nin,*)
      Read (nin,*) method
      Read (nin,*) tstart, tend
      Read (nin,*) yinit(1:n)
      Read (nin,*) hstart
      Read (nin,*) thresh(1:n)

!     Set control for output

      tol = 1.0E-4_nag_wp

      y(1:n) = yinit(1:n)
      tinc = (tend-tstart)/real(npts,kind=nag_wp)

!     ifail: behaviour on error exit
!            =0 for hard exit, =1 for quiet-soft, =-1 for noisy-soft
      ifail = 0
      Call d02pq_p0w_f(ad_handle,n,tstart,tend,y,tol,thresh,method,hstart,     &
        iwsav,rwsav,ifail)

      Write (nout,99999) tol
      Write (nout,99998)
      Write (nout,99997) tstart, yinit(1:n)

      twant = tstart + tinc
      tnow = tstart
loop: Do While (tnow<tend)

!       Integrate one step to tnow
        ifail = 0
        Call d02pf_p0w_f(ad_handle,f,n,tnow,ynow,ypnow,iuser,ruser,iwsav,      &
          rwsav,ifail)

!       Interpolate at any required points in previous step
interp: Do While (twant<=tnow)

          ideriv = 2
          ifail = 0
          Call d02ps_p0w_f(ad_handle,n,twant,ideriv,nwant,ywant,ypwant,f,      &
            wcomm,lwcomm,iuser,ruser,iwsav,rwsav,ifail)

          Write (nout,99997) twant, ywant(1), ypwant(1)

!         Set next required solution point
          twant = twant + tinc
        End Do interp
      End Do loop

      ifail = 0
      Call d02pt_p0w_f(ad_handle,fevals,stepcost,waste,stepsok,hnext,iwsav,    &
        rwsav,ifail)
      Write (nout,99996) fevals

99999 Format (/,' Calculation with TOL = ',1P,E8.1)
99998 Format (/,'    t         y1        y1''',/)
99997 Format (1X,F6.3,2(3X,F7.3))
99996 Format (/,' Cost of the integration in evaluations of F is',I6)


    End Program d02ps_p0w_fe