Asses the complexity of adding support for two levels (or multi-level) sorting

[SebastianMC]

The issue #85 requires the two levels sorting. Investigate the complexity of adding such capability.

What syntax? Using indentation (nesting) seems tempting:

  // indentation-based multi-level sorting syntax (potential)
  target-folder: Journal
  > a-z by-metadata: date
    < a-z by-metadata: time

yet this is already used to distinguish between folder-level sorting and group-level sorting. Road closed in this direction. Also, indentation turned out to be troublesome for copy & paste of sorting spec from a web page (e.g. documentation on github) or from email. Another reason to avoid this direction.

The other (and safer) option is to involve the comma-separated list of sorting levels:

  // comma-separated notation for multi-level sorting
  target-folder: Journal
  > a-z by-metadata: date, < a-z by-metadata: time

Before implementing this capability assess the complexity, if it is not an overkill

Take into account the existing cumbersome syntax for multi-level alike sorting:

  < a-z, created

• is anyone using that ???
• remove it or keep backward-compatibility?
• related code of parser

If implementing the straight explicit multi-level sorting seems to be too complex (to be worth it), some simpler options do exist, more specific, less generic:

• extension of the with-metadata: syntax to allow sorting specification at group level
  • technically this would generate the group identifier on-the-fly (instead of the simple number assignment) and grouping would be done based on that dynamic identifier. In result, the sorting order specification would act as a second-level sorting
• generation of a compound sort-string on the fly, based on two or more metadata values, with assumption that they are numbers or dates (in number-sortable format) plus the ability to use a minus - to indicate the descending order (which is natural for negative numbers)
  • the syntax in this example could look like numeric by-metadata: -date, time

[SebastianMC]

The assessment is done, implementation of multi-level (2-3) sorting is feasible without any significant increase in complexity, will be done under #89:

• application of the multi-level sorting is simple, to be done in custom-sort.ts in a single and isolated place
• parsing of the extended syntax for multi-level sorting is a bit more complex, yet it will also simplify the code in some places
• the refactorings done so far under #88 help here, they introduced more abstractions in the sorting area via functional programming elements