String#bytesplice

static VALUE
rb_str_bytesplice(int argc, VALUE *argv, VALUE str)
{
    long beg, end, len, slen;
    VALUE val;
    rb_encoding *enc;
    int cr;

    rb_check_arity(argc, 2, 3);
    if (argc == 2) {
        if (!rb_range_beg_len(argv[0], &beg, &len, RSTRING_LEN(str), 2)) {
            rb_raise(rb_eTypeError, "wrong argument type %s (expected Range)",
                     rb_builtin_class_name(argv[0]));
        }
        val = argv[1];
    }
    else {
        beg = NUM2LONG(argv[0]);
        len = NUM2LONG(argv[1]);
        val = argv[2];
    }
    if (len < 0) rb_raise(rb_eIndexError, "negative length %ld", len);
    slen = RSTRING_LEN(str);
    if ((slen < beg) || ((beg < 0) && (beg + slen < 0))) {
        rb_raise(rb_eIndexError, "index %ld out of string", beg);
    }
    if (beg < 0) {
        beg += slen;
    }
    assert(beg >= 0);
    assert(beg <= slen);
    if (len > slen - beg) {
        len = slen - beg;
    }
    end = beg + len;
    if (!str_check_byte_pos(str, beg)) {
        rb_raise(rb_eIndexError,
                 "offset %ld does not land on character boundary", beg);
    }
    if (!str_check_byte_pos(str, end)) {
        rb_raise(rb_eIndexError,
                 "offset %ld does not land on character boundary", end);
    }
    StringValue(val);
    enc = rb_enc_check(str, val);
    str_modify_keep_cr(str);
    rb_str_splice_0(str, beg, len, val);
    rb_enc_associate(str, enc);
    cr = ENC_CODERANGE_AND(ENC_CODERANGE(str), ENC_CODERANGE(val));
    if (cr != ENC_CODERANGE_BROKEN)
        ENC_CODERANGE_SET(str, cr);
    return val;
}