    /*
     *  call-seq:
     *     top_doc.to_s(field = :id) -> string
     *
     *  Returns a string representation of the top_doc in readable format.
     */
    static VALUE
    frt_td_to_s(int argc, VALUE *argv, VALUE self)
    {
        int i;
        VALUE rhits = rb_funcall(self, id_hits, 0);
        Searcher *sea = (Searcher *)DATA_PTR(rb_funcall(self, id_searcher, 0));
        const int len = RARRAY(rhits)->len;
        char *str = ALLOC_N(char, len * 64 + 100);
        char *s = str;
        char *field = "id";
        VALUE rstr;

        if (argc) {
            field = frt_field(argv[0]);
        }

        sprintf(s, "TopDocs: total_hits = %ld, max_score = %f [\n",
                FIX2INT(rb_funcall(self, id_total_hits, 0)),
                NUM2DBL(rb_funcall(self, id_max_score, 0)));
        s += strlen(s);

        for (i = 0; i < len; i++) {
            VALUE rhit = RARRAY(rhits)->ptr[i];
            int doc_id = FIX2INT(rb_funcall(rhit, id_doc, 0));
            char *value = "";
            LazyDoc *lzd = sea->get_lazy_doc(sea, doc_id);
            LazyDocField *lzdf = h_get(lzd->field_dict, field);
            if (NULL != lzdf) {
                value = lazy_df_get_data(lzdf, 0);
            }

            sprintf(s, "\t%d \"%s\": %f\n", doc_id, value,
                    NUM2DBL(rb_funcall(rhit, id_score, 0)));
            s += strlen(s);
            lazy_doc_close(lzd);
        }

        sprintf(s, "]\n");
        rstr = rb_str_new2(str);
        free(str);
        return rstr;
    }