string_join(PyStringObject *self, PyObject *orig)
{
    char *sep = PyString_AS_STRING(self);
    const Py_ssize_t seplen = PyString_GET_SIZE(self);
    PyObject *res = NULL;
    char *p;
    Py_ssize_t seqlen = 0;
    size_t sz = 0;
    Py_ssize_t i;
    PyObject *seq, *item;

    seq = PySequence_Fast(orig, "can only join an iterable");
    if (seq == NULL) {
        return NULL;
    }

    seqlen = PySequence_Size(seq);
    if (seqlen == 0) {
        Py_DECREF(seq);
        return PyString_FromString("");
    }
    if (seqlen == 1) {
        item = PySequence_Fast_GET_ITEM(seq, 0);
        if (PyString_CheckExact(item) || PyUnicode_CheckExact(item)) {
            Py_INCREF(item);
            Py_DECREF(seq);
            return item;
        }
    }

    /* There are at least two things to join, or else we have a subclass
     * of the builtin types in the sequence.
     * Do a pre-pass to figure out the total amount of space we'll
     * need (sz), see whether any argument is absurd, and defer to
     * the Unicode join if appropriate.
     */
    for (i = 0; i < seqlen; i++) {
        const size_t old_sz = sz;
        item = PySequence_Fast_GET_ITEM(seq, i);
        if (!PyString_Check(item)){
#ifdef Py_USING_UNICODE
            if (PyUnicode_Check(item)) {
                /* Defer to Unicode join.
                 * CAUTION:  There's no guarantee that the
                 * original sequence can be iterated over
                 * again, so we must pass seq here.
                 */
                PyObject *result;
                result = PyUnicode_Join((PyObject *)self, seq);
                Py_DECREF(seq);
                return result;
            }
#endif
            PyErr_Format(PyExc_TypeError,
                         "sequence item %zd: expected string,"
                         " %.80s found",
                         i, Py_TYPE(item)->tp_name);
            Py_DECREF(seq);
            return NULL;
        }
        sz += PyString_GET_SIZE(item);
        if (i != 0)
            sz += seplen;
        if (sz < old_sz || sz > PY_SSIZE_T_MAX) {
            PyErr_SetString(PyExc_OverflowError,
                "join() result is too long for a Python string");
            Py_DECREF(seq);
            return NULL;
        }
    }

    /* Allocate result space. */
    res = PyString_FromStringAndSize((char*)NULL, sz);
    if (res == NULL) {
        Py_DECREF(seq);
        return NULL;
    }

    /* Catenate everything. */
    p = PyString_AS_STRING(res);
    for (i = 0; i < seqlen; ++i) {
        size_t n;
        item = PySequence_Fast_GET_ITEM(seq, i);
        n = PyString_GET_SIZE(item);
        Py_MEMCPY(p, PyString_AS_STRING(item), n);
        p += n;
        if (i < seqlen - 1) {
            Py_MEMCPY(p, sep, seplen);
            p += seplen;
        }
    }

    Py_DECREF(seq);
    return res;
}
Unknown