[fitsbits] reopening of Public Comment Period on the CHECKSUM convention

[Rob Seaman]

Hi Lucio,

> This is to announce the official reopening of a formal Public Comment Period on the incorporation of the CHECKSUM convention in the FITS Standard.

I love a chance to crack open ADASS proceedings from the left hand side of the shelf ;-)


> The proposed text consists just
> 
> - in the ADDITION of a new section now numbered 4.4.2.7 (hilighted in
>  magenta colour)  describing the two keywords.
> 
>  The section has been re-titled, and the text order re-arranged to
>  avoid repetitions and re-worded to meet the raised comments
> 
> - in the addition of an Appendix (provisionally numbered J) describing
>  a recommended algorithm.
> 
>  This is virtually unchanged, except that improper references to FITS
>  "file" now point correctly to FITS "HDU".
> 
> - the relevant bibliographic references, and appendices C and H.3 will be
>  updated accordingly at the end of the convention incorporation procedure
>  (this text is NOT shown)
> 
> The updated draft text is available at http://sax.iasf-milano.inaf.it/~lucio/FITS/Conventions/checksum-rev2.pdf

General layout is good. Delighted to see this moving forward!


> Please review the text carefully and post any comments, criticisms, or suggestions on the FITSBITS mailing list.

Just a few comments:

1) section 4.4.2.7, para. 5, omit “for analogy”

2) same section, para. 7, “on that case” should be “in that case”

3) same paragraph, immediately before this change “also be written to every other HDU in the file” to "also be written to every other HDU in the file with values appropriate to each HDU in turn” (the point being that the values differ per-HDU, of course).

4) same section, para. 3, change “Datetime when the value of this keyword record is created or updated is recommended.” to something like: “Datetime when the value of the DATASUM keyword record is created or updated is recommended. The Datetime may be omitted from the comment for some purposes, e.g., if repeating an identical workflow across multiple copies of an archive with the intent of generating identical output files. Note that if DATASUM is updated, so must the corresponding CHECKSUM keyword.”

5) same section, para. 5, change “Datetime when the value of this keyword record is created or updated is recommended.” to something like: “Datetime when the value of the CHECKSUM keyword record is created or updated is recommended. The Datetime may be omitted from the comment for some purposes, e.g., if repeating an identical workflow across multiple copies of an archive with the intent of generating identical output files. Note that any timestamp on the CHECKSUM keyword should always be no earlier than the timestamp on the corresponding DATASUM keyword. (They may differ if the HDU’s header keywords are later updated, for instance.)”

6) Suggest that the fact that the DATASUM (and any other keyword editing) must be done before the CHECKSUM be explicitly stated. Perhaps add a paragraph “0” (or renumber):

	"0. The DATASUM keyword must be updated before the CHECKSUM keyword. In general updating the two checksum keywords should be the final step of any update to either data or header records in a FITS HDU. If the checksum handling described here will not be performed, the two keywords should either be deleted or be given null values as described in section 4.4.2.7."

7) section J.1, para. currently tagged “1”, change “It is recommended that the current data and time be recorded in the comment field to document when the checksum was computed.” to "It is recommended that the current data and time be recorded in ISO-8601 format in the comment field to document when the checksum was computed. The same timestamp may be used for both DATASUM and CHECKSUM keywords if they are updated at the same time, or the CHECKSUM keyword may be later. Timestamps between these keywords in different HDUs may differ. The timestamps may be omitted from the comments for some purposes, e.g., if repeating an identical workflow across multiple copies of an archive with the intent of generating identical output files.”

8) Just a reminder to indeed include the references cited in section J.4

9) I have not reviewed the example C code. Note that this can be checked by zeroing the checksum of ASCII files of any sort, not just FITS header keywords. Just repeat the trick of replacing a string of 16 zeroes, e.g., cut-and-paste, taking care to align the string at a 4 byte boundary.


> Please accept my best wishes for the coming holidays and the new year.

Happy holidays!

Rob