Iterate over the cards of an HDU header.

[chrsoo]

Hi,

I would like to iterate over all the cards in the header of an HDU for use in my own tool developed in Rust. From what I can tell the current rust-fitsio API assumes prior knowledge of all header keyword names, something I would like to avoid.

I did not find this in the list of features documented in issue #15.

Iterating over the cards is possible by using fits_get_hdrspace for finding out the total number of cards and then retrieve them by number through fits_read_record or fits_read_keyn, as indicated in section 5.4.1 of the cfitsio documentation.

I have taken a first stab at this, but before creating a PR that respects the contributing guidelines, it would be great if you could provide some initial feedback, specfically:

• Does support for iterating over the header cards in an HDU seem like a reasonable addition to the API?
• In my first attempt I introduced a Card struct to conveniently hold the card data. Given that cards in the Rust fitsio API are implements as key/value and key / (value, comment) it may be preferable to use tuples, what would you suggest?
• Assuming Cards are a "go", a better idea could be to read the entire card record into one array (using ffgrec) and use slices to reference the right section in the buffer. (Being a Rust neophyte, I thought the approach of using individual buffers (and ffgkyn) was easier...)

Best regards,

Chris

[simonrw]

Hi @chrsoo , thanks for making this suggestion.

I would like to iterate over all the cards in the header of an HDU for use in my own tool developed in Rust. From what I can tell the current rust-fitsio API assumes prior knowledge of all header keyword names, something I would like to avoid.

That is a great suggestion, thank you!

I did not find this in the list of features documented in issue #15.

That issue has not been updated regularly, so is likely out of date, however looking through the documentation I agree that this feature is not supported yet.

Iterating over the cards is possible by using fits_get_hdrspace for finding out the total number of cards and then retrieve them by number through fits_read_record or fits_read_keyn, as indicated in section 5.4.1 of the cfitsio documentation.

That's a good idea, thank you.

I have taken a first stab at this, but before creating a PR that respects the contributing guidelines, it would be great if you could provide some initial feedback, specfically:

Does support for iterating over the header cards in an HDU seem like a reasonable addition to the API?
In my first attempt I introduced a Card struct to conveniently hold the card data. Given that cards in the Rust fitsio API are implements as key/value and key / (value, comment) it may be preferable to use tuples, what would you suggest?

I think a struct is definitely preferable in general.

Assuming Cards are a "go", a better idea could be to read the entire card record into one array (using ffgrec) and use slices to reference the right section in the buffer. (Being a Rust neophyte, I thought the approach of using individual buffers (and ffgkyn) was easier...)

I think there are two considerations:

1. how do we read/buffer all of the records, and
2. how do we read/buffer each individual record.

I can see a potential for using something like fits_hdr2str to optimise 1., however that is a future concern.

Regarding the suggestions for 2.: I would have a preference for using fits_read_keyn / ffgkyn initially to ensure the correct behaviour, as using fits_read_record / ffgrec is a performance optimisation we don't need right now.

I am happy to continue further discussions in the PR, but in general I am on board with this idea, thank you!