subroutine pack28(c13,n28)

! Pack a special token, a 24-bit hash code, or a valid base call into a 28-bit
! integer.

  parameter (NTOKENS=4874084,N24=16777216)
  integer nc(6)
  character*13 c13
  character*6 callsign
  character*1 c
  character*4 c4
  character*37 a1
  character*36 a2
  character*10 a3
  character*27 a4
  data a1/' 0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ'/
  data a2/'0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ'/
  data a3/'0123456789'/
  data a4/' ABCDEFGHIJKLMNOPQRSTUVWXYZ'/
  data nc/37,36,19,27,27,27/

  n28=-1
! Check for special tokens first
  if(c13(1:3).eq.'DE ') then
     n28=0
     go to 900
  endif
  
  if(c13(1:4).eq.'QRZ ') then
     n28=1
     go to 900
  endif

  if(c13(1:3).eq.'CQ ') then
     n28=2
     go to 900
  endif

  if(c13(1:3).eq.'CQ_') then
     n=len(trim(c13))
     if(n.ge.4 .and. n.le.7) then
        nlet=0
        nnum=0
        do i=4,n
           c=c13(i:i)
           if(c.ge.'A' .and. c.le.'Z') nlet=nlet+1
           if(c.ge.'0' .and. c.le.'9') nnum=nnum+1
        enddo
        if(nnum.eq.3 .and. nlet.eq.0) then
           read(c13(4:3+nnum),*) nqsy
           n28=3+nqsy
           go to 900
        endif
        if(nlet.ge.1 .and. nlet.le.4 .and. nnum.eq.0) then
           c4=c13(4:n)//'   '
           c4=adjustr(c4)
           m=0
           do i=1,4
              j=0
              c=c4(i:i)
              if(c.ge.'A' .and. c.le.'Z') j=ichar(c)-ichar('A')+1
              m=27*m + j
           enddo
           n28=3+1000+m
           go to 900
        endif
     endif
  endif
  
  callsign=c13(1:6)  
! Work-around for Swaziland prefix:
  if(c13(1:4).eq.'3DA0') callsign='3D0'//c13(5:7)
! Work-around for Guinea prefixes:
  if(c13(1:2).eq.'3X' .and. c13(3:3).ge.'A' .and.          &
       c13(3:3).le.'Z') callsign='Q'//c13(3:6)

!  if(callsign(1:3).eq.'CQ ') then
!     n28=1
!     if(callsign(4:4).ge.'0' .and. callsign(4:4).le.'9' .and.        &
!          callsign(5:5).ge.'0' .and. callsign(5:5).le.'9' .and.      &
!          callsign(6:6).ge.'0' .and. callsign(6:6).le.'9') then
!        read(callsign(4:6),*) nfreq
!        n28=3 + nfreq
!     endif
!     return
!  else if(callsign(1:4).eq.'QRZ ') then
!     n28=2
!     return
!  else if(callsign(1:3).eq.'DE ') then
!     n28=267796945
!     return
!  endif

! We have a standard callsign
  n=len(trim(callsign))
  callsign=adjustr(callsign)

  i1=36*10*27*27*27*(index(a1,callsign(1:1))-1)
  i2=10*27*27*27*(index(a2,callsign(2:2))-1)
  i3=27*27*27*(index(a3,callsign(3:3))-1)
  i4=27*27*(index(a4,callsign(4:4))-1)
  i5=27*(index(a4,callsign(5:5))-1)
  i6=index(a4,callsign(6:6))-1
  n28=i1+i2+i3+i4+i5+i6
  
!  n28=index(a1,callsign(1:1))-1
!  n28=n28*nc(2) + index(a2,callsign(2:2)) - 1
!  n28=n28*nc(3) + index(a3,callsign(3:3)) - 1
!  n28=n28*nc(4) + index(a4,callsign(4:4)) - 1
!  n28=n28*nc(5) + index(a4,callsign(5:5)) - 1
!  n28=n28*nc(6) + index(a4,callsign(6:6)) - 1
  n28=n28 + NTOKENS + N24

900 return
end subroutine pack28