/*
 *  call-seq:
 *     Regexp.union([pattern]*)   => new_str
 *  
 *  Return a <code>Regexp</code> object that is the union of the given
 *  <em>pattern</em>s, i.e., will match any of its parts. The <em>pattern</em>s
 *  can be Regexp objects, in which case their options will be preserved, or
 *  Strings. If no arguments are given, returns <code>/(?!)/</code>.
 *     
 *     Regexp.union                         #=> /(?!)/
 *     Regexp.union("penzance")             #=> /penzance/
 *     Regexp.union("skiing", "sledding")   #=> /skiing|sledding/
 *     Regexp.union(/dogs/, /cats/i)        #=> /(?-mix:dogs)|(?i-mx:cats)/
 */
static VALUE
rb_reg_s_union(argc, argv)
    int argc;
    VALUE *argv;
{
    if (argc == 0) {
        VALUE args[1];
        args[0] = rb_str_new2("(?!)");
        return rb_class_new_instance(1, args, rb_cRegexp);
    }
    else if (argc == 1) {
        VALUE v;
        v = rb_check_convert_type(argv[0], T_REGEXP, "Regexp", "to_regexp");
        if (!NIL_P(v))
            return v;
        else {
            VALUE args[1];
            args[0] = rb_reg_s_quote(argc, argv);
            return rb_class_new_instance(1, args, rb_cRegexp);
        }
    }
    else {
        int i, kcode = -1;
        VALUE kcode_re = Qnil;
        VALUE source = rb_str_buf_new(0);
        VALUE args[3];
        for (i = 0; i < argc; i++) {
            volatile VALUE v;
            if (0 < i)
                rb_str_buf_cat2(source, "|");
            v = rb_check_convert_type(argv[i], T_REGEXP, "Regexp", "to_regexp");
            if (!NIL_P(v)) {
                if (FL_TEST(v, KCODE_FIXED)) {
                    if (kcode == -1) {
                        kcode_re = v;
                        kcode = RBASIC(v)->flags & KCODE_MASK;
                    }
                    else if ((RBASIC(v)->flags & KCODE_MASK) != kcode) {
                        volatile VALUE str1, str2;
                        str1 = rb_inspect(kcode_re);
                        str2 = rb_inspect(v);
                        rb_raise(rb_eArgError, "mixed kcode: %s and %s",
                            RSTRING(str1)->ptr, RSTRING(str2)->ptr);
                    }
                }
                v = rb_reg_to_s(v);
            }
            else {
                args[0] = argv[i];
                v = rb_reg_s_quote(1, args);
            }
            rb_str_buf_append(source, v);
        }
        args[0] = source;
        args[1] = Qnil;
        switch (kcode) {
          case -1:
            args[2] = Qnil;
            break;
          case KCODE_NONE:
            args[2] = rb_str_new2("n");
            break;
          case KCODE_EUC:
            args[2] = rb_str_new2("e");
            break;
          case KCODE_SJIS:
            args[2] = rb_str_new2("s");
            break;
          case KCODE_UTF8:
            args[2] = rb_str_new2("u");
            break;
        }
        return rb_class_new_instance(3, args, rb_cRegexp);
    }
}