The available sorting methods are currently hard-coded in IkiWiki.pm, making it difficult to add any extra sorting mechanisms. I've prepared a branch which adds 'sort' as a hook type and uses it to implement a new meta_title sort type.

Someone could use this hook to make [[!inline sort=title]] prefer the meta title over the page name, but for compatibility, I'm not going to (I do wonder whether it would be worth making sort=name an alias for the current sort=title, and changing the meaning of sort=title in 4.0, though).

What compatability concerns, exactly, are there that prevent making that change now? --Joey

[sort-hooks branch now withdrawn in favour of sort-package --s]

I briefly tried to turn all the current sort types into hook functions, and have some of them pre-registered, but decided that probably wasn't a good idea. That earlier version of the branch is also available for comparison:

[also withdrawn in favour of sort-package --s]

I wonder if IkiWiki would benefit from the concept of a "sortspec", like a PageSpec but dedicated to sorting lists of pages rather than defining lists of pages? Rather than defining a sort-hook, define a SortSpec class, and enable people to add their own sort methods as functions defined inside that class, similarly to the way they can add their own pagespec definitions. --KathrynAndersen

Available in a git repository branch.
Branch: smcv/ready/sort-package
Author: Simon McVittie
I'd be inclined to think that's overkill, but it wasn't very hard to implement, and in a way is more elegant. I set it up so sort mechanisms share the IkiWiki::PageSpec package, but with a cmp_ prefix. Gitweb: http://git.pseudorandom.co.uk/smcv/ikiwiki.git?a=shortlog;h=refs/heads/sort-package

I agree it seems more elegant, so I have focused on it.

I don't know about reusing IkiWiki::PageSpec for this. --Joey

Fair enough, IkiWiki::SortSpec::cmp_foo would be just as easy, or IkiWiki::Sorting::cmp_foo if you don't like introducing "sort spec" in the API. I took a cue from sorting being a subpage of pagespec, and decided that yes, sorting is a bit like a pagespec :-) Which name would you prefer? --s

SortSpec --Joey

Done. --s

I would be inclined to drop the check_ stuff. --Joey

It basically exists to support title_natural, to avoid firing up the whole import mechanism on every cmp (although I suppose that could just be a call to a memoized helper function). It also lets sort specs that must have a parameter, like field, fail early (again, not so valuable).

AFAIK, use foo has very low overhead when the module is already loaded. There could be some evalation overhead in eval q{use foo}, if so it would be worth addressing across the whole codebase. --Joey

check_cmp_foo now dropped. --s

The former function could be achieved at a small compatibility cost by putting title_natural in a new sortnatural plugin (that fails to load if you don't have title_natural), if you'd prefer - that's what would have happened if title_natural was written after this code had been merged, I suspect. Would you prefer this? --s

Yes! (Assuming it does not make sense to support natural order sort of other keys than the title, at least..) --Joey

Done. I added some NEWS.Debian for it, too. --s

Wouldn't it make sense to have meta(title) instead of meta_title? --Joey

Yes, you're right. I added parameters to support field, and didn't think about making meta use them too. However, title does need a special case to make it default to the basename instead of the empty string.

Another special case for title is to use titlesort first (the name titlesort is derived from Ogg/FLAC tags, which can have titlesort and artistsort). I could easily extend that to other metas, though; in fact, for e.g. book lists it would be nice for field(bookauthor) to behave similarly, so you can display "Douglas Adams" but sort by "Adams, Douglas".

meta_title is also meant to be a prototype of how sort=title could behave in 4.0 or something - sorting by page name (which usually sorts in approximately the same place as the meta-title, but occasionally not), while displaying meta-titles, does look quite odd. --s

Agreed. --Joey

I've implemented meta(title). meta(author) also has the sortas special case; meta(updated) and meta(date) should also work how you'd expect them to (but they're earliest-first, unlike age). --s

As I read the regexp in cmpspec_translate, the "command" is required to have params. They should be optional, to match the documentation and because most sort methods do not need parameters. --Joey

No, $2 is either \w+\([^\)]*\) or [^\s]+ (with the latter causing an error later if it doesn't also match \w+). This branch doesn't add any parameterized sort methods, in fact, although I did provide one on field's discussion page. --s

I wonder if it would make sense to add some combining keywords, so a sortspec reads like sort="age then ascending title" In a way, this reduces the amount of syntax that needs to be learned. I like the "then" (and it could allow other operations than simple combination, if any others make sense). Not so sure about the "ascending", which could be "reverse" instead, but "descending age" and "ascending age" both seem useful to be able to explicitly specify. --Joey

Perhaps. I do like the simplicity of KathrynAndersen's syntax from report (which I copied verbatim, except for turning sort-by-field into a parameterized spec).

If we're getting into English-like (or at least SQL-like) queries, it might make sense to change the signature of the hook function so it's a function to return a key, e.g. sub key_age { return -%pagemtime{$_[0]) }. Then we could sort like this:

field(artistsort) or field(artist) or constant(Various Artists) then meta(titlesort) or meta(title) or title

with "or" binding more closely than "then". Does this seem valuable? I think the implementation would be somewhat more difficult. and it's probably getting too complicated to be worthwhile, though? (The keys that actually benefit from this could just have smarter cmp functions, I think.)

If the hooks return keys rather than cmp results, then we could even have "lowercase" as an adjective used like "ascending"... maybe. However, there are two types of adjective here: "lowercase" really applies to the keys, whereas "ascending" applies to the "cmp" result. Again, I think this is getting too complex, and could just be solved with smarter cmp functions.

I agree. (Also, I think returning keys may make it harder to write smarter cmp functions.) --Joey

Unfortunately, sort="ascending mtime" actually sorts by descending timestamp (butsort=age is fine, because age could be defined as now minus ctime). sort=freshness isn't right either, because "sort by freshness" seems as though it ought to mean freshest first, but "sort by ascending freshness" means put the least fresh first. If we have ascending and descending keywords which are optional, I don't think we really want different sort types to have different default directions - it seems clearer to have ascending always be a no-op, and descending always negate.

I think you've convinced me that ascending/descending impose too much semantics on it, so "-" is better. --Joey

I've kept the semantics from report as-is, then: e.g. sort="age -title". --s

Perhaps we could borrow from meta updated and use update_age? updateage would perhaps be a more normal IkiWiki style - but that makes me think that updateage is a quantity analagous to tonnage or voltage, with more or less recently updated pages being said to have more or less updateage. I don't know whether that's good or bad :-)

I'm sure there's a much better word, but I can't see it. Do you have a better idea? --s

[Regarding the meta title=foo sort=bar special case]

I feel it sould be clearer to call that "sortas", since "sort=" is used to specify a sort method in other directives. --Joey

Done. --smcv

speed

I notice the implementation does not use the magic $a and $b globals. That nasty perl optimisation is still worthwhile:

perl -e 'use warnings; use strict; use Benchmark; sub a { $a <=> $b } sub b ($$) { $_[0] <=> $_[1] }; my @list=reverse(1..9999); timethese(10000, {a => sub {my @f=sort a @list}, b => sub {my @f=sort b  @list}, c => => sub {my @f=sort { b($a,$b) } @list}})'
Benchmark: timing 10000 iterations of a, b, c...
         a: 80 wallclock secs (76.74 usr +  0.05 sys = 76.79 CPU) @ 130.23/s (n=10000)
         b: 112 wallclock secs (106.14 usr +  0.20 sys = 106.34 CPU) @ 94.04/s (n=10000)
         c: 330 wallclock secs (320.25 usr +  0.17 sys = 320.42 CPU) @ 31.21/s (n=10000)

Unfortunatly, I think that c is closest to the new implementation. --Joey

Unfortunately, $a isn't always $main::a - it's $Package::a where Package is the call site of the sort call. This was a showstopper when sort was a hook implemented in many packages, but now that it's a SortSpec, I may be able to fix this by putting a sort wrapper in the SortSpec namespace, so it's like this:

sub sort ($@)
{
    my $cmp = shift;
    return sort $cmp @_;
}

which would mean that the comparison used $IkiWiki::SortSpec::a. --s

I've now done this. On a wiki with many albums (a full rebuild takes half an hour!), I tested a refresh after touch tags/*.mdwn (my tag pages contain inlines of the form tagged(foo) sorted by date, so they exercise sorting). I also tried removing sorting from pagespec_match_list altogether, as an upper bound for how fast we can possibly make it.

  • master at branch point: 63.72user 0.29system
  • master at branch point: 63.91user 0.37system
  • my branch, with @_: 65.28user 0.29system
  • my branch, with @_: 65.21user 0.28system
  • my branch, with $a: 64.09user 0.28system
  • my branch, with $a: 63.83user 0.36system
  • not sorted at all: 58.99user 0.29system
  • not sorted at all: 58.92user 0.29system

--s

I do notice that pagespec_match_list performs the sort before the filter by pagespec. Is this a deliberate design choice, or coincidence? I can see that when limit is used, this could be used to only run the pagespec match function until limit pages have been selected, but the cost is that every page in the wiki is sorted. Or, it might be useful to do the filtering first, then sort the sub-list thus produced, then finally apply the limit? --s

Yes, it was deliberate, pagespec matching can be expensive enough that needing to sort a lot of pages seems likely to be less work. (I don't remember what benchmarking was done though.) --Joey

We discussed this on IRC and Joey pointed out that this also affects dependency calculation, so I'm not going to get into this now... --s

Joey pointed out on IRC that the titlesort feature duplicates all the meta titles. I did that in order to sort by the unescaped version, but I've now changed the branch to only store that if it makes a difference. --s

Documentation from sort-package branch

advanced sort orders (conditionally added to sorting)

  • title_natural - Orders by title, but numbers in the title are treated as such, ("1 2 9 10 20" instead of "1 10 2 20 9")
  • meta(title) - Order according to the [[!meta title="foo" sortas="bar"]] or [[!meta title="foo"]] directive, or the page name if no full title was set. meta(author), meta(date), meta(updated), etc. also work.

Multiple sort orders (added to sorting)

In addition, you can combine several sort orders and/or reverse the order of sorting, with a string like age -title (which would sort by age, then by title in reverse order if two pages have the same age).

meta sortas parameter (added to meta)

[in title]

An optional sort parameter will be used preferentially when sorting by meta(title):

   [[!meta  title="The Beatles" sort="Beatles, The"]]

   [[!meta  title="David Bowie" sort="Bowie, David"]]

[in author]

An optional sortas parameter will be used preferentially when sorting by meta(author):

    [[!meta  author="Joey Hess" sortas="Hess, Joey"]]

Sorting plugins (added to write)

Similarly, it's possible to write plugins that add new functions as sorting methods. To achieve this, add a function to the IkiWiki::SortSpec package named cmp_foo, which will be used when sorting by foo or foo(...) is requested.

The names of pages to be compared are in the global variables $a and $b in the IkiWiki::SortSpec package. The function should return the same thing as Perl's cmp and <=> operators: negative if $a is less than $b, positive if $a is greater, or zero if they are considered equal. It may also raise an error using error, for instance if it needs a parameter but one isn't provided.

The function will also be passed one or more parameters. The first is undef if invoked as foo, or the parameter "bar" if invoked as foo(bar); it may also be passed additional, named parameters.