program main

c*********************************************************************72
c
cc PRIME counts the prime numbers between 2 and N.
c
c  Discussion:
c
c    We divide the calculation up into P parts.
c
c    Part 0   checks 2+0,   2+P+0,   2+2*P+0, ...
c    Part 2   checks 2+1,   2+P+1,   2+2*P+1, ...
c    Part P-1 checks 2+P-1, 2+2*P-1, 2+3*P-1, ...
c
c    Right now, the outer loop executes all P parts.
c    But it would be simple to make an MPI version, in which the
c    program with identifier ID only executes part ID.
c
c    Modify this program so that it runs under MPI.
c
c  Licensing:
c
c    This code is distributed under the GNU LGPL license.
c
c  Modified:
c
c    07 September 2013
c
c  Author:
c
c    John Burkardt
c
      implicit none

      integer id
      integer n
      parameter ( n = 100000 )
      integer p
      integer total
      integer total_part

      write ( *, '(a)' ) ' '
      write ( *, '(a)' ) 'PRIME'
      write ( *, '(a)' ) '  FORTRAN77 version'

      p = 4

      total = 0
      do id = 0, p - 1
        call prime_part ( id, p, n, total_part )
        total = total + total_part
      end do

      write ( *, '(a)' ) ' '
      write ( *, '(a,i8,a,i8,a)' ) 
     &  ' From 2 to ', n, ' there are ', total, ' primes.'

      stop
      end
      subroutine prime_part ( id, p, n, total_part )

      implicit none

      integer i
      integer id
      integer j
      integer n
      integer p
      logical prime
      integer total_part

      total_part = 0

      do i = 2 + id, n, p

        prime = .true.

        do j = 2, i - 1
          if ( mod ( i, j ) .eq. 0 ) then
            prime = .false.
            go to 10
          end if
        end do

        if ( prime ) then
          total_part = total_part + 1
        end if

10      continue

      end do

      return
      end