a theory of pagespec influence lists, for Will's perusal

doc/todo/dependency_types.mdwn

@@ -188,7 +188,8 @@ before and it is present now.  Should this cause a re-build of any page that has
 > Yes, a presence dep will trigger when a page is added, or removed. 
 
 > Your example is valid.. but it's also not handled right by normal,
-> (content) dependencies, for the same reasons. --[[Joey]]
+> (content) dependencies, for the same reasons. Still, I think I've
+> addressed it with the pagespec influence stuff below. --[[Joey]]
 
 I think that is another version of the problem you encountered with meta-data.
 
@@ -229,16 +230,7 @@ sigh.
 
 > I have also been thinking about some sort of analysis pass over pagespecs
 > to determine what metadata, pages, etc they depend on. It is indeed
-> tricky to do. Even if it's just limited to returning a list of pages
-> as you suggest.
->
-> Consider: For a `*` glob, it has to return a list of all pages
-> in the wiki. Which is expensive. And what if the pagespec is
-> something like `* and backlink(index)`? Without analyising the
-> boolean relationship between terms, the returned list 
-> will have many more items in it than it should. Or do we not make
-> globs return their matches? (If so we have to deal with those
-> with one of the other methods disucssed.) --[[Joey]] 
+> tricky to do. More thoughts on influence lists a bit below. --[[Joey]] 
 
 ---- 
 
@@ -291,26 +283,13 @@ changed pages.
 
 ----
 
-What if there were a function that added a dependency, and at the same time
-returned a list of pages matching the pagespec? Plugins that use this would
-be exactly the ones, like inline and map, for which this is a problem, and
-which already do a match pass over all pages.
-
-Adding explicit dependencies during this pass would thus be nearly free.
-Not 100% free since it would add explicit deps for things that are not
-shown on an inline that limits its display to the first sorted N items.
-I suppose we could reach 100% free by making the function also handle
-sorting and limiting, though that could be overkill.
-
-----
-
 Found a further complication in presence dependencies. Map now uses
 presence dependencies when adding its explicit dependencies on pages. But
 this defeats the purpose of the explicit dependencies! Because, now,
 when B is changed to not match a pagespec, the A's presence dep does
 not fire.
 
-I didn't think things through when switching it to use presense
+I didn't think things through when switching it to use presence
 dependencies there. But, if I change it to use full dependencies, then all
 the work that was done to allow map to use presence dependencies for its
 main pagespec is for naught. The map will once again have to update
@@ -320,3 +299,115 @@ This points toward the conclusion that explicit dependencies, however they
 are added, are not the right solution at all. Some other approach, such as
 maintaining the list of pages that match a dependency, and noticing when it
 changes, is needed.
+
+----
+
+### pagespec influence lists
+
+I'm using this term for the concept of a list of pages whose modification
+can indirectly influence what pages a pagespec matches.
+
+#### Examples
+
+* The pagespec "created_before(foo)" has an influence list that contains foo.
+  The removal or (re)creation of foo changes what pages match it.
+
+* The pagespec "foo" has an empty influence list. This is because a
+  modification/creation/removal of foo directly changes what the pagespec
+  matches.
+
+* The pagespec "*" has an empty influence list, for the same reason.
+  Avoiding including every page in the wiki into its influence list is
+  very important!
+
+* The pagespec "title(foo)" has an influence list that contains every page
+  that currently matches it. A change to any matching page can change its
+  title. Why is that considered an indirect influence? Well, the pagespec
+  might be used in a presence dependency, and so its title changing
+  would not directly affect the dependency.
+
+* The pagespec "backlink(index)" has an influence list
+  that contains index (because a change to index changes the backlinks).
+
+* The pagespec "link(done)" has an influence list that
+  contains every page that it matches. A change to any matching page can
+  remove a link and make it not match any more, and so the list is needed
+  due to the removal problem.
+
+#### Low-level Calculation
+
+One way to calculate a pagespec's influence would be to
+expand the SuccessReason and FailReason objects used and returned
+by `pagespec_match`. Make the objects be created with an
+influence list included, and when the objects are ANDed or ORed
+together, combine the influence lists.
+
+That would have the benefit of allowing just using the existing `match_*`
+functions, with minor changes to a few of them to gather influence info.
+
+But does it work? Let's try some examples:
+
+Consider "bugs/* and link(done) and backlink(index)".
+
+Its influence list contains index, and it contains all pages that the whole
+pagespec matches. It should, ideally, not contain all pages that link
+to done. There are a lot of such pages, and only a subset influence this
+pagespec.
+
+When matching this pagespec against a page, the `link` will put the page
+on the list. The `backlink` will put index on the list, and they will be
+anded together and combined. If we combine the influences from each
+successful match, we get the right result.
+
+Now consider "bugs/* and link(done) and !backlink(index)".
+
+It influence list is the same as the previous one, even though a term has
+been negated. Because a change to index still influences it, though in a
+different way.
+
+If negation of a SuccessReason preserves the influence list, the right
+influence list will be calculated.
+
+Consider "bugs/* and (link(done) or backlink(index))"
+and      "bugs/* and (backlink(index) or link(done))'
+
+Its clear that the influence lists for these are identical. And they
+contain index, plus all matching pages.
+
+When matching the first against page P, the `link` will put P on the list.
+The OR needs to be a non-short-circuiting type. (In perl, `or`, not `||` --
+so, `pagespec_translate` will need to be changed to not use `||`.)
+Given that, the `backlink` will always be evalulated, and will put index
+onto the influence list. If we combine the influences from each
+successful match, we get the right result.
+
+#### High-level Calculation and Storage
+
+Calculating the full influence list for a pagespec requires trying to match
+it against every page in the wiki. 
+
+I'd like to avoid doing such expensive matching redundantly. So add a
+`pagespec_match_all`, which returns a list of all pages in the whole
+wiki that match the pagespec, and also adds the pagespec as a dependency,
+and while it's at it, calculates and stores the influence list.
+
+It could have an optional sort parameter, and limit parameter, to control
+how many items to return and the sort order. So when inline wants to
+display the 10 newest, only the influence lists for those ten are added.
+
+If `pagespec_match_depends` can be used by all plugins, then great,
+influences are automatically calculated, no extra work needs to be done.
+
+If not, and some plugins still need to use `pagespec_match_list` or
+`pagespec_match`, and `add_depends`, then I guess that `add_depends` can do
+a slightly more expensive influence calculation.
+
+Bonus: If `add_depends` is doing an influence calculation, then I can remove
+the nasty hack it currently uses to decide if a given pagespec is safe to use
+with an existence or links dependency.
+
+Where to store the influence list? Well, it appears that we can just add
+(content) dependencies for each item on the list, to the page's
+regular list of simple dependencies. So, the data stored ends up looking
+just like what is stored today by the explicit dependency hacks. Except,
+it's calculated more smartly, and is added automatically.