Add experimental MeCab filter

It can be enabled by running fluentd with
"DROONGA_ENABLE_SEARCH_MECAB_FILTER=yes" environment variable.

lib/droonga/searcher.rb

@@ -641,3 +641,7 @@ def format_elapsed_time
     end
   end
 end
+
+if ENV["DROONGA_ENABLE_SEARCH_MECAB_FILTER"] == "yes"
+  require "droonga/searcher/mecab_filter"
+end

lib/droonga/searcher/mecab_filter.rb

@@ -0,0 +1,67 @@
+# -*- coding: utf-8 -*-
+#
+# Copyright (C) 2014 Droonga Project
+#
+# This library is free software; you can redistribute it and/or
+# modify it under the terms of the GNU Lesser General Public
+# License version 2.1 as published by the Free Software Foundation.
+#
+# This library is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+# Lesser General Public License for more details.
+#
+# You should have received a copy of the GNU Lesser General Public
+# License along with this library; if not, write to the Free Software
+# Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
+
+require "MeCab"
+
+module Droonga
+  class Searcher
+    class QuerySearcher
+      class MeCabTokenizer
+        def initialize
+          @mecab = MeCab::Tagger.new("-Owakati")
+        end
+
+        def tokenize(text)
+          tokens = @mecab.parse(text).force_encoding("utf-8").split(/\s+/)
+          tokens.reject do |token|
+            token.empty?
+          end
+        end
+      end
+
+      def apply_mecab_filter(condition)
+        return unless condition.is_a?(Hash)
+        return unless condition["useMeCabFilter"]
+        query = condition["query"]
+        return if query.nil?
+        match_columns = condition["matchTo"]
+        return unless match_columns.is_a?(Array)
+        return if match_columns.size != 1
+        match_column = match_columns.first
+
+        tokenizer = MeCabTokenizer.new
+
+        @records.open_cursor do |cursor|
+          count = 0
+          cursor.each do |record|
+            match_target = record[match_column]
+            body_terms = tokenizer.tokenize(match_target)
+            unless body_terms.include?(query)
+              record.delete
+            end
+          end
+        end
+      end
+
+      alias_method :original_apply_condition!, :apply_condition!
+      def apply_condition!(condition)
+        original_apply_condition!(condition)
+        apply_mecab_filter(condition)
+      end
+    end
+  end
+end