Function time_to_time_resolve_descriptors

numpy/core/src/multiarray/datetime.c:3758–3838

* Describes casting within datetimes or timedelta */

 * Describes casting within datetimes or timedelta
 */
static NPY_CASTING
time_to_time_resolve_descriptors(
        PyArrayMethodObject *NPY_UNUSED(self),
        PyArray_DTypeMeta *NPY_UNUSED(dtypes[2]),
        PyArray_Descr *given_descrs[2],
        PyArray_Descr *loop_descrs[2],
        npy_intp *view_offset)
{
    /* This is a within-dtype cast, which currently must handle byteswapping */
    Py_INCREF(given_descrs[0]);
    loop_descrs[0] = given_descrs[0];
    if (given_descrs[1] == NULL) {
        loop_descrs[1] = NPY_DT_CALL_ensure_canonical(given_descrs[0]);
    }
    else {
        Py_INCREF(given_descrs[1]);
        loop_descrs[1] = given_descrs[1];
    }

    int is_timedelta = given_descrs[0]->type_num == NPY_TIMEDELTA;

    if (given_descrs[0] == given_descrs[1]) {
        *view_offset = 0;
        return NPY_NO_CASTING;
    }

    npy_bool byteorder_may_allow_view = (
            PyDataType_ISNOTSWAPPED(loop_descrs[0])
            == PyDataType_ISNOTSWAPPED(loop_descrs[1]));

    PyArray_DatetimeMetaData *meta1, *meta2;
    meta1 = get_datetime_metadata_from_dtype(loop_descrs[0]);
    assert(meta1 != NULL);
    meta2 = get_datetime_metadata_from_dtype(loop_descrs[1]);
    assert(meta2 != NULL);

    if ((meta1->base == meta2->base && meta1->num == meta2->num) ||
            // handle some common metric prefix conversions
            // 1000 fold conversions
            ((meta2->base >= 7) && (meta1->base - meta2->base == 1)
              && ((meta1->num / meta2->num) == 1000)) ||
            // 10^6 fold conversions
            ((meta2->base >= 7) && (meta1->base - meta2->base == 2)
              && ((meta1->num / meta2->num) == 1000000)) ||
            // 10^9 fold conversions
            ((meta2->base >= 7) && (meta1->base - meta2->base == 3)
              && ((meta1->num / meta2->num) == 1000000000))) {
        if (byteorder_may_allow_view) {
            *view_offset = 0;
            return NPY_NO_CASTING;
        }
        return NPY_EQUIV_CASTING;
    }
    else if (meta1->base == NPY_FR_GENERIC) {
        if (byteorder_may_allow_view) {
            *view_offset = 0;
        }
        return NPY_SAFE_CASTING ;