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C--------------------------------------------------------------------------
subroutine ftdsum(string,complm,sum)
C decode the 32 bit checksum
C If complm=.true., then the complement of the sum will be decoded.
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 sum d checksum value
C
C written by Wm Pence, HEASARC/GSFC, May, 1995
double precision sum,all32,word32,factor(4)
character*16 string,tmpstr
integer offset,i,j,k,temp,hibits
logical complm
C all32 equals a 32 bit unsigned integer with all bits set
C word32 is equal to 2**32
parameter (all32=4.294967295D+09)
parameter (word32=4.294967296D+09)
C ASCII 0 is the offset value
parameter (offset=48)
data factor/16777216.,65536.,256.,1./
sum=0
C shift the characters 1 place to the left, since the FITS character
C string value starts in column 12, which is not word aligned
tmpstr(1:15)=string(2:16)
tmpstr(16:16)=string(1:1)
C convert characters from machine's native character coding sequence
C to ASCII codes. This only affects IBM mainframe computers
C that do not use ASCII for the internal character representation.
call ftc2as(tmpstr,16)
C substract the offset from each byte and interpret each 4 character
C string as a 4-byte unsigned integer; sum the 4 integers
k=0
do 10 i=1,4
do 20 j=1,4
k=k+1
temp=ichar(tmpstr(k:k))-offset
sum=sum+temp*factor(j)
20 continue
10 continue
C fold any overflow bits beyond 32 back into the word
30 hibits=sum/word32
if (hibits .gt. 0)then
sum=sum-(hibits*word32)+hibits
go to 30
end if
if (complm)then
C complement the 32-bit unsigned integer equivalent (flip every bit)
sum=all32-sum
end if
end
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