idlastro / Miscellaneous (Non-Astronomy) Procedures: CHECKSUM32

[Source code]

NAME
CHECKSUM32
PURPOSE
To compute the 32bit checksum of an array (ones-complement arithmetic)
EXPLANATION
The 32bit checksum is adopted in the FITS Checksum convention
http://fits.gsfc.nasa.gov/registry/checksum.html
CALLING SEQUENCE
CHECKSUM32, array, checksum, [/FROM_IEEE, /NoSAVE]
INPUTS
array - any numeric idl array.  If the number of bytes in the array is 
        not a multiple of four then it is padded with zeros internally
        (the array is returned unchanged).   Convert a string array 
        (e.g. a FITS header) to bytes prior to calling CHECKSUM32.
OUTPUTS
checksum - unsigned long scalar, giving sum of array elements using 
           ones-complement arithmetic
OPTIONAL INPUT KEYWORD
/FROM_IEEE - If this keyword is set, then the input is assumed to be in
     big endian format (e.g. an untranslated FITS array).   This keyword
     only has an effect on little endian machines (e.g. Linux boxes).
/NoSAVE - if set, then the input array is not saved upon exiting.   Use 
     the /NoSave keyword to save time if the input array is not needed 
     in further computations. 
METHOD
Uses TOTAL() to sum the array into an unsigned integer variable.  The
overflow bits beyond 2^32 are then shifted back to the least significant
bits.    The summing is done in chunks of 2^31 numbers to avoid loss
f precision.    Adapted from FORTRAN code in
easarc.gsfc.nasa.gov/docs/heasarc/ofwg/docs/general/checksum/node30.html
RESTRICTIONS
(1) Not valid for object or pointer data types
EXAMPLE
Find the 32 bit checksum of the array x = findgen(35)
IDL> checksum32, x, s    ===> s =  2920022024
FUNCTION CALLED
HOST_TO_IEEE, IS_IEEE_BIG(), N_BYTES()
MODIFICATION HISTORY
Written    W. Landsman          June 2001
Work correctly on little endian machines, added /FROM_IEEE and /NoSave
           W. Landsman          November 2002
Pad with zeros when array size not a multiple of 4 W.Landsman Aug 2003
Always copy to new array, somewhat slower but more robust algorithm
    especially for Linux boxes   W. Landsman Sep. 2004 
Sep. 2004 update not implemented correctly (sigh) W. Landsman Dec 2004         
No need to byteswap 4 byte datatypes on little endian W. L. May 2009
Use /INTEGER keyword to TOTAL() function W.L. June 2009