Add some documentation to the restriction parser

extractor/restriction_parser.cpp

@@ -102,6 +102,13 @@ void RestrictionParser::ReadRestrictionExceptions(lua_State *lua_state)
     }
 }
 
+/**
+ * Tries to parse an relation as turn restriction. This can fail for a number of
+ * reasons, this the return type is a mapbox::util::optional<>.
+ *
+ * Some restrictions can also be ignored: See the ```get_exceptions``` function
+ * in the corresponding profile.
+ */
 mapbox::util::optional<InputRestrictionContainer>
 RestrictionParser::TryParse(const osmium::Relation &relation) const
 {

extractor/restriction_parser.hpp

@@ -41,6 +41,24 @@ SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 struct lua_State;
 class ScriptingEnvironment;
 
+/**
+ * Parses the relations that represents turn restrictions.
+ *
+ * Currently only restrictions where the via objects is a node are supported.
+ *  from   via   to
+ * ------->(x)-------->
+ *
+ * While this class does not directly invoke any lua code _per relation_ it does
+ * load configuration values from the profile, that are saved in variables.
+ * Namely ```use_turn_restrictions``` and ```get_exceptions```.
+ *
+ * The restriction is represented by the osm id of the from way, the osm id of the
+ * to way and the osm id of the via node. This representation must be post-processed
+ * in the extractor to work with the edge-based data-model of OSRM:
+ * Since the from and to way share the via-way as node a turn will have the following form:
+ * ...----(a)-----(via)------(b)----...
+ * So it can be represented by the tripe (a, via, b).
+ */
 class RestrictionParser
 {
   public:
@@ -53,7 +71,6 @@ class RestrictionParser
     void ReadRestrictionExceptions(lua_State *lua_state);
     bool ShouldIgnoreRestriction(const std::string &except_tag_string) const;
 
-    // lua_State *lua_state;
     std::vector<std::string> restriction_exceptions;
     bool use_turn_restrictions;
 };