Faster checksum implementation

[seanhess]

What is the problem this feature will solve?

Astro.py currently takes several seconds to add a datasum to large fits files. In my tests using a 1.1GB fits file, where most of the data is in the primary HDU, it took ~2.5s to perform the following:

hdul = fits.open('inversion_output/scan1807/inv_res_pre.fits')
hdul[0].add_datasum()
hdul[0].header

Describe the desired outcome

While implementing this in Haskell, I used the c code in the FITS 4.0 spec, and adding a datasum to the same HDU was about 10x faster.

Code is included here. There are small differences from the code in the FITS 4.0 spec, namely returning the resulting sum instead of passing in a pointer.

unsigned int checksum (
  unsigned char *buf,
  int length)
{

  unsigned int sum32 = 0;
  unsigned int hi, lo, hicarry, locarry, i;

  // Accumulate the sum of the high-order 16 bits and the */
  // low-order 16 bits of each 32-bit word, separately. */
  // The first byte in each pair is the most significant. */
  // This algorithm works on both big and little endian machines.
  hi = (sum32 >> 16);
  lo = sum32 & 0xFFFF;
  for (i=0; i < length; i+=4) {
    hi += ((buf[i] << 8) + buf[i+1]);
    lo += ((buf[i+2] << 8) + buf[i+3]);
  }

  // fold carry bits from each 16 bit sum into the other sum
  hicarry = hi >> 16;
  locarry = lo >> 16;

  while (hicarry || locarry) {
    hi = (hi & 0xFFFF) + locarry;
    lo = (lo & 0xFFFF) + hicarry;
    hicarry = hi >> 16;
    locarry = lo >> 16;
  }

  // Concatenate the high and low parts to form the full 32-bit checksum
  return (hi << 16) + lo;
}

Additional context

No response

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