+
+ ##
+ # download the changeset as an osmChange document.
+ #
+ # to make it easier to revert diffs it would be better if the osmChange
+ # format were reversible, i.e: contained both old and new versions of
+ # modified elements. but it doesn't at the moment...
+ #
+ # this method cannot order the database changes fully (i.e: timestamp and
+ # version number may be too coarse) so the resulting diff may not apply
+ # to a different database. however since changesets are not atomic this
+ # behaviour cannot be guaranteed anyway and is the result of a design
+ # choice.
+ def download
+ changeset = Changeset.find(params[:id])
+
+ # get all the elements in the changeset which haven't been redacted
+ # and stick them in a big array.
+ elements = [changeset.old_nodes.unredacted,
+ changeset.old_ways.unredacted,
+ changeset.old_relations.unredacted].flatten
+
+ # sort the elements by timestamp and version number, as this is the
+ # almost sensible ordering available. this would be much nicer if
+ # global (SVN-style) versioning were used - then that would be
+ # unambiguous.
+ elements.sort! do |a, b|
+ if (a.timestamp == b.timestamp)
+ a.version <=> b.version
+ else
+ a.timestamp <=> b.timestamp
+ end
+ end
+
+ # create changeset and user caches
+ changeset_cache = {}
+ user_display_name_cache = {}
+
+ # create an osmChange document for the output
+ result = OSM::API.new.get_xml_doc
+ result.root.name = "osmChange"
+
+ # generate an output element for each operation. note: we avoid looking
+ # at the history because it is simpler - but it would be more correct to
+ # check these assertions.
+ elements.each do |elt|
+ result.root <<
+ if (elt.version == 1)
+ # first version, so it must be newly-created.
+ created = XML::Node.new "create"
+ created << elt.to_xml_node(changeset_cache, user_display_name_cache)
+ else
+ unless elt.visible
+ # if the element isn't visible then it must have been deleted
+ deleted = XML::Node.new "delete"
+ deleted << elt.to_xml_node(changeset_cache, user_display_name_cache)
+ else
+ # must be a modify
+ modified = XML::Node.new "modify"
+ modified << elt.to_xml_node(changeset_cache, user_display_name_cache)
+ end
+ end