c   read_gadget_header.f   
c
c     read a gadget header with lenght unit Mpc
c

      integer npart(6),                          !c1(6)
     +        SF,Fb,Cool,nFiles,npartot1(6)      !c3(10)
      double precision  masses(6),aexpn,z,       !c2(8)
     +               box_dp, h,Lambda,Om         !c4(4)

      integer*8 n32bit
      parameter ( n32bit=4294967296 )
      integer*8 npartot(6)
      integer iflag_star, iflag_met, nallhigh(6)

      integer     n_o_b     ! number of bytes

      CHARACTER*120 snapshot

      write(*,*) 'which file'
      read(*,*) snapshot

      open (2, file =snapshot,
     +        form = 'unformatted', STATUS = 'OLD')

      Read (2) npart,                                !c1
     +     masses,aexpn,z,                           !c2
     +     SF,Fb, npartot1,                           !c3
     +     Cool,nFiles,box_dp,Om,Lambda,h,           !c4
     +     iflag_star, iflag_met, nallhigh

!   print header informations
      write (*,'(a,f7.3,a,f6.1,a)') 'z     =',z,'  Box=',
     +     box_dp,' h^-1 Mpc  (GADGET unit is Mpc)'
      write(*,'(a,f7.4)') 'h     = ',h
      write(*,'(a,f7.4)') 'aexpn = ',aexpn
      write(*,'(a,f6.3,a,f8.3)') 'Omega = ',OM,'  Lambda = ',Lambda
      Do i = 1,6
        masses(i) = masses(i)*1.E10
      ENDDO

      if(npartot1(2).lt.0)  then
          npartot(2) = (npartot1(3)+1)*n32bit + npartot1(2)
      else
          npartot(2) = npartot1(3)*n32bit + npartot1(2)
      endif

      write (*,'(a,e12.4,a)')   'm     =',masses(2),' h^-1 M_sun'
      write(*,*) ' '
      nn2 = INT( (FLOAT(npartot(2)))**0.333333 + 0.5 ) 
      write(*,1234) npartot(2),nn2 
 1234 FORMAT('total number of particles =',I16,' = ',I5,'^3')

      IF(nFiles .gt. 1) THEN
        write(*,*) ' '
        write(*,'(a,I6,a)') 'Output is divided into ',nFiles,' files.'
        write(*,'(a,I11,a)') 'this file contains ',npart(2),' particles'
      endif

      END