    /*
     *  call-seq:
     *     MappingFilter.new(token_stream, mapping) -> token_stream
     *
     *  Create an MappingFilter which maps strings in tokens. This is usually used
     *  to map UTF-8 characters to ASCII characters for easier searching and
     *  better search recall. The mapping is compiled into a Deterministic Finite
     *  Automata so it is super fast. This Filter can therefor be used for
     *  indexing very large datasets. Currently regular expressions are not
     *  supported. If you are really interested in the feature, please contact me
     *  at dbalmain@gmail.com.
     *
     *  token_stream:: TokenStream to be filtered
     *  mapping::      Hash of mappings to apply to tokens. The key can be a
     *                 String or an Array of Strings. The value must be a String
     *
     *  == Example
     *
     *     filt = MappingFilter.new(token_stream,
     *                              {
     *                                ['à','á','â','ã','ä','å'] => 'a',
     *                                ['è','é','ê','ë','ē','ę'] => 'e'
     *                              })
     */
    static VALUE
    frt_mapping_filter_init(VALUE self, VALUE rsub_ts, VALUE mapping)
    {
        TokenStream *ts;
        ts = frt_get_cwrapped_rts(rsub_ts);
        ts = mapping_filter_new(ts);
        rb_hash_foreach(mapping, frt_add_mappings_i, (VALUE)ts);
        mulmap_compile(((MappingFilter *)ts)->mapper);
        object_add(&(TkFilt(ts)->sub_ts), rsub_ts);

        Frt_Wrap_Struct(self, &frt_tf_mark, &frt_tf_free, ts);
        object_add(ts, self);
        return self;
    }