c     examples/int_pt2pt
c
      include "mpif.h"
      integer my_rank,nproc,ierr
      double precision value,sum
c     
c     Initialize MPI and determine individual processor and
c     the size of the world
c     
      call init(nproc,my_rank)
      if(my_rank.eq.0) then
         call master(nproc)
      else
         call slave(my_rank,value)
      endif
      ihost=0
      call MPI_Reduce(value,sum,1,MPI_DOUBLE_PRECISION,MPI_SUM,
     1     ihost,MPI_COMM_WORLD,ierr)
      if(my_rank.eq.0) then
         write(*,*) 'area=',sum
      endif
      call MPI_Finalize(ierr)
      stop
      end
c
c
      subroutine init(nproc,my_rank)
      include "mpif.h"
c
c     initialize the MPI
c
      integer my_rank,nproc
      character*30 nodename
      integer nchar
      call MPI_init(ierr)
      call MPI_Comm_rank(MPI_COMM_WORLD,my_rank,ierr)
      call MPI_Comm_size(MPI_COMM_WORLD,nproc,ierr)
      call MPI_Get_processor_name(nodename,nchar,ierr)
      write(*,*) 'my_rank=',my_rank,' processor=',nodename
      return
      end
c
c
      subroutine slave(my_rank,value)
      include "mpif.h"
c     
c     1. the slave process receives 'a' and 'b' from master
c     2. the slave sends `my_rank' and 'value' to master (proc 0)
c     
      integer my_rank,ierr,itag1
      integer status(MPI_STATUS_SIZE),ipos
      double precision a,b,value,trap
      parameter (BUFSIZE=100)
      character buf(100)
      idest=0
      itag1=10
      call MPI_Recv(buf,BUFSIZE,MPI_PACKED,idest,itag1,
     1     MPI_COMM_WORLD,status,ierr)
      ipos=0
      call MPI_Unpack(buf,BUFSIZE,ipos,a,1,MPI_DOUBLE_PRECISION,
     1     MPI_COMM_WORLD,ierr)
      call MPI_Unpack(buf,BUFSIZE,ipos,b,1,MPI_DOUBLE_PRECISION,
     1     MPI_COMM_WORLD,ierr)
      write(*,*) 'my_rank=',my_rank,'  a=',a,'  b=',b
      value=trap(a,b)
      return
      end
c
c      
      subroutine master(nproc)
      include "mpif.h"
      integer i,nproc,itag1,ierr
      double precision xl,a(10),b(10),dx
      parameter (BUFSIZE=100)
      character buf(100)
      n1=nproc-1
      xl=1.0
      dx=xl/dfloat(n1)
      do i=1,n1
         a(i)=dx*(i-1)
         b(i)=a(i)+dx
      enddo
      itag1=10
      do i=1,n1
         idest=i
         ipos=0
         call MPI_Pack(a(i),1,MPI_DOUBLE_PRECISION,buf,BUFSIZE,ipos,
     *        MPI_COMM_WORLD,ierr)
         call MPI_Pack(b(i),1,MPI_DOUBLE_PRECISION,buf,BUFSIZE,ipos,
     *        MPI_COMM_WORLD,ierr)
         call MPI_Send(buf,ipos,MPI_PACKED,idest,itag1,
     *        MPI_COMM_WORLD,ierr)
      enddo
      return
      end
c
      double precision function trap(a,b)
      double precision a,b,calf,x,y
      integer n
      n=100
      dx=(b-a)/dfloat(n)
      trap=calf(a)
      do i=2,n
         x=a+dx*(i-1)
         y=calf(x)
         trap=trap+2.*y
      enddo
      trap=0.5*(trap+calf(b))*dx
      return
      end
c
c
      double precision function calf(x)
      double precision x
      calf=x*x
      return
      end