design for a xapian search plugin

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@@ -12,6 +12,75 @@ search so it understands what words are most important in a search. (So
 does Lucene..) Another nice thing is it supports "more documents like this
 one" kind of search. --[[Joey]]
 
+## xapian
+
+I've invesitgated xapian briefly. I think a custom xapian indexer and use
+of the omega for cgi searches could work well for ikiwiki. --[[Joey]]
+
+### indexer
+
+A custom indexer is needed because omindex isn't good enough for ikiwiki's
+needs for incremental rendering. (And because, since ikiwiki has page info
+in memory, it's silly to write it to disk and have omindex read it back.)
+
+The indexer would run as a ikiwiki hook. It needs to be passed the page
+name, and the content. Which hook to use is an open question.
+Possibilities:
+
+* `filter` - Since this runs before preprocess, only the actual text
+  written on the page would be indexed. Not text generated by directives,
+  pulled in by inlining, etc. There's something to be said for that. And
+  something to be said against it. It would also get markdown formatted
+  content, mostly, though it would still need to strip html.
+* `sanitize` - Would get the htmlized content, so would need to strip html.
+  Preprocessor directive output would be indexed.
+* `format` - Would get the entire html page, including the page template.
+  Probably not a good choice as indexing the same template for each page
+  is unnecessary.
+
+Currently, a filter hook seems the best option.
+
+The hook would remove any html from the content, and index it.
+It would need to add the same document data that omindex would, as well as
+adding the same special terms (see
+http://xapian.org/docs/omega/overview.html "Boolean terms").
+
+(Note that the U term is a bit tricky because I'll have to replicate
+ominxes's hash_string() to hash terms > 240 chars.)
+
+The indexer (and deleter) will need a way to figure out the ids in xapian
+of the documents to delete. One way is storing the id of each page in the
+ikiwiki index.
+
+The other way would be adding a special term to the xapian db that can be
+used with replace_document_by_term/delete_document_by_term. omindex uses
+U<url> as a term, and I guess I could just use that, and then map page
+names to urls when deleting a page  ... only real problem being the
+hashing; a collision would be bad.
+
+At the moment, storing xapian ids in the ikiwiki index file seems like the
+best approach.
+
+The hook should try to avoid re-indexing pages that have not changed since
+they were last indexed. One problem is that, if a page with an inline is
+built, every inlined item will get each hook run. And so a naive hook would
+index each of those items, even though none of them have necessarily
+changed. Date stamps are one possibility. Another would be to avoid having
+the hook not do any indexing when `%preprocessing` is set (Ikiwiki.pm would
+need to expose that variable.)
+
+#### cgi
+
+The cgi hook would exec omega to handle the searching, much as is done
+with estseek in the current search plugin.
+
+It would first set `OMEGA_CONFIG_FILE=.ikiwiki/omega.conf` ; that omega.conf
+would set `database_dir=.ikiwiki/xapian` and probably also set a custom
+`template_dir`, which would have modified templates branded for ikiwiki. So
+the actual xapian db would be in `.ikiwiki/xapian/default/`.
+
+## lucene
+
 >> I've done a bit of prototyping on this. The current hip search library is [Lucene](http://lucene.apache.org/java/docs/). There's a Perl port called [Plucene](http://search.cpan.org/~tmtm/Plucene-1.25/). Given that it's already packaged, as `libplucene-perl`, I assumed it would be a good starting point. I've written a **very rough** patch against `IkiWiki/Plugin/search.pm` to handle the indexing side (there's no facility to view the results yet, although I have a command-line interface working). That's below, and should apply to SVN trunk.
 
 >> Of course, there are problems. ;-)