subroutine check_encode(sum32,complement,string)
C sum32 double precision checksum value
C complement (logical) whether to encode the complement of the sum (true)
C or encode the sum value itself (false).
C string c*16 output ASCII encoded checksum
double precision sum32,tmpsum,all32
logical complement
character*16 string,tmpstr
integer offset,exclud(13),nbyte(4),ch(4),i,j,k
integer quot,remain,check,nc
C all32 is equivalent to a 32 bit unsigned integer with all bits set
parameter (all32=4.294967295D+09)
C ASCII 0 is the offset value
parameter (offset=48)
C this is the list of ASCII punctuation characters to be excluded
data exclud/58,59,60,61,62,63,64,91,92,93,94,95,96/
if (complement)then
C complement the 32-bit unsigned integer equivalent (flip every bit)
tmpsum=all32-sum32
else
C just encode the sum, not its complement
tmpsum=sum32
end if
C separate each byte of the 32-bit integer into separate 8-bit integers
nbyte(1)=tmpsum/16777216.
tmpsum=tmpsum-nbyte(1)*16777216.
nbyte(2)=tmpsum/65536.
tmpsum=tmpsum-nbyte(2)*65536.
nbyte(3)=tmpsum/256.
nbyte(4)=tmpsum-nbyte(3)*256.
C encode each 8-bit integer as 4-characters
do i=1,4
quot=nbyte(i)/4+offset
remain=nbyte(i) - (nbyte(i)/4*4)
ch(1)=quot+remain
ch(2)=quot
ch(3)=quot
ch(4)=quot
C avoid ASCII punctuation characters by incrementing and
C decrementing adjacent characters thus preserving checksum value
10 check=0
do k=1,13
do j=1,4,2
if (ch(j) .eq. exclud(k) .or.
& ch(j+1) .eq. exclud(k))then
ch(j)=ch(j)+1
ch(j+1)=ch(j+1)-1
check=1
end if
end do
end do
C keep repeating, until all punctuation character are removed
if (check .ne. 0)go to 10
C convert the byte values to the equivalent ASCII characters
do j=0,3
nc=4*j+i
tmpstr(nc:nc)=char(ch(j+1))
end do
end do
C shift the characters 1 place to the right, since the FITS character
C string value starts in column 12, which is not word aligned
string(2:16)=tmpstr(1:15)
string(1:1)=tmpstr(16:16)
end
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