MOC 2.0 WD Feedback

[Tom Donaldson]

Dear Pierre, et al.,

I spent a little time reviewing the MOC 2.0 working draft (https://www.ivoa.net/documents/MOC/20201112/WD-MOC-2.0-20201112.pdf)
and have some feedback.

First,  thank you to all the authors.  I found the document well written, clearly explaining both the motivation
and required details.  This functionality seems very worthwhile, and the technique well-studied and effective.
I don't have any major concerns with the content.

I could imagine some discussion about generalizing the technique further to incorporate other dimensions
such as spectral coverage.  However, I note that the space and time representations are customized based on
the specific characteristics of those values, so other dimensions may need other customizations.  Without
studying the use cases and values for at least one other dimension at the same level of detail as
space and time, generalizing the customizations seems premature.


I do have a few editorial suggestions:

Table of Contents
- starts with a "Todo list" referring to page 4, but there is no Todo list on that or other pages.
- does not include the Acknowledgements or Conformance-related definitions from page 5

Introduction
- Should the reference to be to MOC 1.1 instead of MOC 1.0?

Section 2.3, p14 
- in "more than 73000 years at 1μ resolution", "1μ" should be "1μs".

Section 3.3
- comma unnecessary after "(coded according to the TMOC convention)".  Possible alternate wording to make it a little smoother:
    "by associating each time period (coded according to the TMOC convention) with its spatial region 
     (coded according to the SMOC convention)."

Section 4.2 
- "about 1000 seconds (see Table 1)" should instead refer to Table 2 for the time resolutions.

Section 4.2.1
- awkward phrasing in, "The numbering scheme used in TMOC for specifying the time cell indices must reuse a similar hierarchical 
  principle as for the SMOC with the difference that the time line has only one dimension, and there is no need to use an HEALPix 
  mapping in addition of a progression of a factor of 2 instead of 4."  
  Suggest maybe:
  "The numbering scheme used in TMOC for specifying the time cell indices must reuse a similar hierarchical principle as for the 
  SMOC with the difference that the time line has only one dimension, so the hierarchical progression uses a factor of 2 instead of 4, 
  and there is no need to use a HEALPix mapping."

Section 4.3.1
- in Backward compatibility, "existing library" should probably be "existing libraries".

Section 4.3.2
- The EBNF does not seem to allow the final index-free order (the example ends with "8/" to indicate an order of 8.)  Maybe update to:
   moc ::= ordpix (sep+ ordpix)* [sep+ order]
   ordpix ::= order sep* pixs
   order ::= int ’/’
   pixs ::= pix (sep+ pix)*
   pix ::= int? | (int ’-’ int)
   sep ::= [ \n\r]
   int ::= [0-9]+

Section 5.1
- This sentence equates forcing the 64th bit to 1 with negating the integer:
     "To distinguish time and space indices, the time indices must have the 64th bits forced to 1 - i. e. represented
     as a negative integer."
  Should we clarify that the time indices must be the twos complement negative of the actual value?  This would prevent
  any possible confusion between that and just masking that last bit to 1 without changing the other bits.  I believe the
  FITS standard uses twos complement for signed ints, but it can't hurt to be explicit.

References
- I'm not sure whether it's important for references to include URLs, or what the best links are (DOIs, etc.), but some
  reference have a URL and some don't, so maybe some more consistency could be achieved?
- The MOC 1.0 reference describes it as Working Draft (it is REC) and links to the most recent version (1.1) instead of 1.0.


Best regards,
Tom