#include "py27_backports.h"


// Provide implementations from python 2.7.15 as backports

int
_PySlice_Unpack(PyObject *_r,
                Py_ssize_t *start, Py_ssize_t *stop, Py_ssize_t *step)
{
    PySliceObject *r = (PySliceObject *)_r;
    /* this is harder to get right than you might think */

    assert(PY_SSIZE_T_MIN + 1 <= -PY_SSIZE_T_MAX);

    if (r->step == Py_None) {
        *step = 1;
    }
    else {
        if (!_PyEval_SliceIndex(r->step, step)) return -1;
        if (*step == 0) {
            PyErr_SetString(PyExc_ValueError,
                            "slice step cannot be zero");
            return -1;
        }
        /* Here *step might be -PY_SSIZE_T_MAX-1; in this case we replace it
         * with -PY_SSIZE_T_MAX.  This doesn't affect the semantics, and it
         * guards against later undefined behaviour resulting from code that
         * does "step = -step" as part of a slice reversal.
         */
        if (*step < -PY_SSIZE_T_MAX)
            *step = -PY_SSIZE_T_MAX;
    }

    if (r->start == Py_None) {
        *start = *step < 0 ? PY_SSIZE_T_MAX : 0;
    }
    else {
        if (!_PyEval_SliceIndex(r->start, start)) return -1;
    }

    if (r->stop == Py_None) {
        *stop = *step < 0 ? PY_SSIZE_T_MIN : PY_SSIZE_T_MAX;
    }
    else {
        if (!_PyEval_SliceIndex(r->stop, stop)) return -1;
    }

    return 0;
}

Py_ssize_t
_PySlice_AdjustIndices(Py_ssize_t length,
                       Py_ssize_t *start, Py_ssize_t *stop, Py_ssize_t step)
{
    /* this is harder to get right than you might think */

    assert(step != 0);
    assert(step >= -PY_SSIZE_T_MAX);

    if (*start < 0) {
        *start += length;
        if (*start < 0) {
            *start = (step < 0) ? -1 : 0;
        }
    }
    else if (*start >= length) {
        *start = (step < 0) ? length - 1 : length;
    }

    if (*stop < 0) {
        *stop += length;
        if (*stop < 0) {
            *stop = (step < 0) ? -1 : 0;
        }
    }
    else if (*stop >= length) {
        *stop = (step < 0) ? length - 1 : length;
    }

    if (step < 0) {
        if (*stop < *start) {
            return (*start - *stop - 1) / (-step) + 1;
        }
    }
    else {
        if (*start < *stop) {
            return (*stop - *start - 1) / step + 1;
        }
    }
    return 0;
}