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C--------------------------------------------------------------------------
subroutine ftesum(sum,complm,string)
C encode the 32 bit checksum by converting every
C 2 bits of each byte into an ASCII character (32 bit word encoded
C as 16 character string). Only ASCII letters and digits are used
C to encode the values (no ASCII punctuation characters).
C If complm=.true., then the complement of the sum will be encoded.
C This Fortran algorithm is based on the C algorithm developed by Rob
C Seaman at NOAO that was presented at the 1994 ADASS conference, to be
C published in the Astronomical Society of the Pacific Conference Series.
C
C sum d checksum value
C complm l encode the complement of the sum?
C string c output ASCII encoded check sum
C
C written by Wm Pence, HEASARC/GSFC, Sept, 1994
double precision sum,tmpsum,all32
character*16 string,tmpstr
integer offset,exclud(13),nbyte(4),ch(4),i,j,k
integer quot,remain,check,nc
logical complm
C all32 equals 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 punctutation characters to be excluded
data exclud/58,59,60,61,62,63,64,91,92,93,94,95,96/
if (complm)then
C complement the 32-bit unsigned integer equivalent (flip every bit)
tmpsum=all32-sum
else
tmpsum=sum
end if
C separate each 8-bit byte into separate 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 100 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 30 k=1,13
do 20 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
20 continue
30 continue
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 40 j=0,3
nc=4*j+i
tmpstr(nc:nc)=char(ch(j+1))
40 continue
100 continue
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)
C convert characters from ASCII codes to machine's native character
C coding sequence. (The string gets converted back to ASCII when it
C is written to the FITS file). This only affects IBM mainframe computers
C that do not use ASCII for the internal character representation.
call ftas2c(string,16)
end
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