memoryobject.c

/*
 * Memoryview object implementation
 * --------------------------------
 *
 *   This implementation is a complete rewrite contributed by Stefan Krah in
 *   Python 3.3.  Substantial credit goes to Antoine Pitrou (who had already
 *   fortified and rewritten the previous implementation) and Nick Coghlan
 *   (who came up with the idea of the ManagedBuffer) for analyzing the complex
 *   ownership rules.
 *
 */

#include "Python.h"
#include "pycore_abstract.h"      // _PyIndex_Check()
#include "pycore_object.h"        // _PyObject_GC_UNTRACK()
#include "pycore_strhex.h"        // _Py_strhex_with_sep()
#include <stddef.h>               // offsetof()

/*[clinic input]
class memoryview "PyMemoryViewObject *" "&PyMemoryView_Type"
[clinic start generated code]*/
/*[clinic end generated code: output=da39a3ee5e6b4b0d input=e2e49d2192835219]*/

#include "clinic/memoryobject.c.h"

/****************************************************************************/
/*                           ManagedBuffer Object                           */
/****************************************************************************/

/*
   ManagedBuffer Object:
   ---------------------

     The purpose of this object is to facilitate the handling of chained
     memoryviews that have the same underlying exporting object. PEP-3118
     allows the underlying object to change while a view is exported. This
     could lead to unexpected results when constructing a new memoryview
     from an existing memoryview.

     Rather than repeatedly redirecting buffer requests to the original base
     object, all chained memoryviews use a single buffer snapshot. This
     snapshot is generated by the constructor _PyManagedBuffer_FromObject().

   Ownership rules:
   ----------------

     The master buffer inside a managed buffer is filled in by the original
     base object. shape, strides, suboffsets and format are read-only for
     all consumers.

     A memoryview's buffer is a private copy of the exporter's buffer. shape,
     strides and suboffsets belong to the memoryview and are thus writable.

     If a memoryview itself exports several buffers via memory_getbuf(), all
     buffer copies share shape, strides and suboffsets. In this case, the
     arrays are NOT writable.

   Reference count assumptions:
   ----------------------------

     The 'obj' member of a Py_buffer must either be NULL or refer to the
     exporting base object. In the Python codebase, all getbufferprocs
     return a new reference to view.obj (example: bytes_buffer_getbuffer()).

     PyBuffer_Release() decrements view.obj (if non-NULL), so the
     releasebufferprocs must NOT decrement view.obj.
*/


static inline _PyManagedBufferObject *
mbuf_alloc(void)
{
    _PyManagedBufferObject *mbuf;

    mbuf = (_PyManagedBufferObject *)
        PyObject_GC_New(_PyManagedBufferObject, &_PyManagedBuffer_Type);
    if (mbuf == NULL)
        return NULL;
    mbuf->flags = 0;
    mbuf->exports = 0;
    mbuf->master.obj = NULL;
    _PyObject_GC_TRACK(mbuf);

    return mbuf;
}

static PyObject *
_PyManagedBuffer_FromObject(PyObject *base)
{
    _PyManagedBufferObject *mbuf;

    mbuf = mbuf_alloc();
    if (mbuf == NULL)
        return NULL;

    if (PyObject_GetBuffer(base, &mbuf->master, PyBUF_FULL_RO) < 0) {
        mbuf->master.obj = NULL;
        Py_DECREF(mbuf);
        return NULL;
    }

    return (PyObject *)mbuf;
}

static void
mbuf_release(_PyManagedBufferObject *self)
{
    if (self->flags&_Py_MANAGED_BUFFER_RELEASED)
        return;

    /* NOTE: at this point self->exports can still be > 0 if this function
       is called from mbuf_clear() to break up a reference cycle. */
    self->flags |= _Py_MANAGED_BUFFER_RELEASED;

    /* PyBuffer_Release() decrements master->obj and sets it to NULL. */
    _PyObject_GC_UNTRACK(self);
    PyBuffer_Release(&self->master);
}

static void
mbuf_dealloc(_PyManagedBufferObject *self)
{
    assert(self->exports == 0);
    mbuf_release(self);
    if (self->flags&_Py_MANAGED_BUFFER_FREE_FORMAT)
        PyMem_Free(self->master.format);
    PyObject_GC_Del(self);
}

static int
mbuf_traverse(_PyManagedBufferObject *self, visitproc visit, void *arg)
{
    Py_VISIT(self->master.obj);
    return 0;
}

static int
mbuf_clear(_PyManagedBufferObject *self)
{
    assert(self->exports >= 0);
    mbuf_release(self);
    return 0;
}

PyTypeObject _PyManagedBuffer_Type = {
    PyVarObject_HEAD_INIT(&PyType_Type, 0)
    "managedbuffer",
    sizeof(_PyManagedBufferObject),
    0,
    (destructor)mbuf_dealloc,                /* tp_dealloc */
    0,                                       /* tp_vectorcall_offset */
    0,                                       /* tp_getattr */
    0,                                       /* tp_setattr */
    0,                                       /* tp_as_async */
    0,                                       /* tp_repr */
    0,                                       /* tp_as_number */
    0,                                       /* tp_as_sequence */
    0,                                       /* tp_as_mapping */
    0,                                       /* tp_hash */
    0,                                       /* tp_call */
    0,                                       /* tp_str */
    PyObject_GenericGetAttr,                 /* tp_getattro */
    0,                                       /* tp_setattro */
    0,                                       /* tp_as_buffer */
    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC, /* tp_flags */
    0,                                       /* tp_doc */
    (traverseproc)mbuf_traverse,             /* tp_traverse */
    (inquiry)mbuf_clear                      /* tp_clear */
};


/****************************************************************************/
/*                             MemoryView Object                            */
/****************************************************************************/

/* In the process of breaking reference cycles mbuf_release() can be
   called before memory_release(). */
#define BASE_INACCESSIBLE(mv) \
    (((PyMemoryViewObject *)mv)->flags&_Py_MEMORYVIEW_RELEASED || \
     ((PyMemoryViewObject *)mv)->mbuf->flags&_Py_MANAGED_BUFFER_RELEASED)

#define CHECK_RELEASED(mv) \
    if (BASE_INACCESSIBLE(mv)) {                                  \
        PyErr_SetString(PyExc_ValueError,                         \
            "operation forbidden on released memoryview object"); \
        return NULL;                                              \
    }

#define CHECK_RELEASED_INT(mv) \
    if (BASE_INACCESSIBLE(mv)) {                                  \
        PyErr_SetString(PyExc_ValueError,                         \
            "operation forbidden on released memoryview object"); \
        return -1;                                                \
    }

/* See gh-92888. These macros signal that we need to check the memoryview
   again due to possible read after frees. */
#define CHECK_RELEASED_AGAIN(mv) CHECK_RELEASED(mv)
#define CHECK_RELEASED_INT_AGAIN(mv) CHECK_RELEASED_INT(mv)

#define CHECK_LIST_OR_TUPLE(v) \
    if (!PyList_Check(v) && !PyTuple_Check(v)) { \
        PyErr_SetString(PyExc_TypeError,         \
            #v " must be a list or a tuple");    \
        return NULL;                             \
    }

#define VIEW_ADDR(mv) (&((PyMemoryViewObject *)mv)->view)

/* Check for the presence of suboffsets in the first dimension. */
#define HAVE_PTR(suboffsets, dim) (suboffsets && suboffsets[dim] >= 0)
/* Adjust ptr if suboffsets are present. */
#define ADJUST_PTR(ptr, suboffsets, dim) \
    (HAVE_PTR(suboffsets, dim) ? *((char**)ptr) + suboffsets[dim] : ptr)

/* Memoryview buffer properties */
#define MV_C_CONTIGUOUS(flags) (flags&(_Py_MEMORYVIEW_SCALAR|_Py_MEMORYVIEW_C))
#define MV_F_CONTIGUOUS(flags) \
    (flags&(_Py_MEMORYVIEW_SCALAR|_Py_MEMORYVIEW_FORTRAN))
#define MV_ANY_CONTIGUOUS(flags) \
    (flags&(_Py_MEMORYVIEW_SCALAR|_Py_MEMORYVIEW_C|_Py_MEMORYVIEW_FORTRAN))

/* Fast contiguity test. Caller must ensure suboffsets==NULL and ndim==1. */
#define MV_CONTIGUOUS_NDIM1(view) \
    ((view)->shape[0] == 1 || (view)->strides[0] == (view)->itemsize)

/* getbuffer() requests */
#define REQ_INDIRECT(flags) ((flags&PyBUF_INDIRECT) == PyBUF_INDIRECT)
#define REQ_C_CONTIGUOUS(flags) ((flags&PyBUF_C_CONTIGUOUS) == PyBUF_C_CONTIGUOUS)
#define REQ_F_CONTIGUOUS(flags) ((flags&PyBUF_F_CONTIGUOUS) == PyBUF_F_CONTIGUOUS)
#define REQ_ANY_CONTIGUOUS(flags) ((flags&PyBUF_ANY_CONTIGUOUS) == PyBUF_ANY_CONTIGUOUS)
#define REQ_STRIDES(flags) ((flags&PyBUF_STRIDES) == PyBUF_STRIDES)
#define REQ_SHAPE(flags) ((flags&PyBUF_ND) == PyBUF_ND)
#define REQ_WRITABLE(flags) (flags&PyBUF_WRITABLE)
#define REQ_FORMAT(flags) (flags&PyBUF_FORMAT)


/**************************************************************************/
/*                       Copy memoryview buffers                          */
/**************************************************************************/

/* The functions in this section take a source and a destination buffer
   with the same logical structure: format, itemsize, ndim and shape
   are identical, with ndim > 0.

   NOTE: All buffers are assumed to have PyBUF_FULL information, which
   is the case for memoryviews! */


/* Assumptions: ndim >= 1. The macro tests for a corner case that should
   perhaps be explicitly forbidden in the PEP. */
#define HAVE_SUBOFFSETS_IN_LAST_DIM(view) \
    (view->suboffsets && view->suboffsets[dest->ndim-1] >= 0)

static inline int
last_dim_is_contiguous(const Py_buffer *dest, const Py_buffer *src)
{
    assert(dest->ndim > 0 && src->ndim > 0);
    return (!HAVE_SUBOFFSETS_IN_LAST_DIM(dest) &&
            !HAVE_SUBOFFSETS_IN_LAST_DIM(src) &&
            dest->strides[dest->ndim-1] == dest->itemsize &&
            src->strides[src->ndim-1] == src->itemsize);
}

/* This is not a general function for determining format equivalence.
   It is used in copy_single() and copy_buffer() to weed out non-matching
   formats. Skipping the '@' character is specifically used in slice
   assignments, where the lvalue is already known to have a single character
   format. This is a performance hack that could be rewritten (if properly
   benchmarked). */
static inline int
equiv_format(const Py_buffer *dest, const Py_buffer *src)
{
    const char *dfmt, *sfmt;

    assert(dest->format && src->format);
    dfmt = dest->format[0] == '@' ? dest->format+1 : dest->format;
    sfmt = src->format[0] == '@' ? src->format+1 : src->format;

    if (strcmp(dfmt, sfmt) != 0 ||
        dest->itemsize != src->itemsize) {
        return 0;
    }

    return 1;
}

/* Two shapes are equivalent if they are either equal or identical up
   to a zero element at the same position. For example, in NumPy arrays
   the shapes [1, 0, 5] and [1, 0, 7] are equivalent. */
static inline int
equiv_shape(const Py_buffer *dest, const Py_buffer *src)
{
    int i;

    if (dest->ndim != src->ndim)
        return 0;

    for (i = 0; i < dest->ndim; i++) {
        if (dest->shape[i] != src->shape[i])
            return 0;
        if (dest->shape[i] == 0)
            break;
    }

    return 1;
}

/* Check that the logical structure of the destination and source buffers
   is identical. */
static int
equiv_structure(const Py_buffer *dest, const Py_buffer *src)
{
    if (!equiv_format(dest, src) ||
        !equiv_shape(dest, src)) {
        PyErr_SetString(PyExc_ValueError,
            "memoryview assignment: lvalue and rvalue have different "
            "structures");
        return 0;
    }

    return 1;
}

/* Base case for recursive multi-dimensional copying. Contiguous arrays are
   copied with very little overhead. Assumptions: ndim == 1, mem == NULL or
   sizeof(mem) == shape[0] * itemsize. */
static void
copy_base(const Py_ssize_t *shape, Py_ssize_t itemsize,
          char *dptr, const Py_ssize_t *dstrides, const Py_ssize_t *dsuboffsets,
          char *sptr, const Py_ssize_t *sstrides, const Py_ssize_t *ssuboffsets,
          char *mem)
{
    if (mem == NULL) { /* contiguous */
        Py_ssize_t size = shape[0] * itemsize;
        if (dptr + size < sptr || sptr + size < dptr)
            memcpy(dptr, sptr, size); /* no overlapping */
        else
            memmove(dptr, sptr, size);
    }
    else {
        char *p;
        Py_ssize_t i;
        for (i=0, p=mem; i < shape[0]; p+=itemsize, sptr+=sstrides[0], i++) {
            char *xsptr = ADJUST_PTR(sptr, ssuboffsets, 0);
            memcpy(p, xsptr, itemsize);
        }
        for (i=0, p=mem; i < shape[0]; p+=itemsize, dptr+=dstrides[0], i++) {
            char *xdptr = ADJUST_PTR(dptr, dsuboffsets, 0);
            memcpy(xdptr, p, itemsize);
        }
    }

}

/* Recursively copy a source buffer to a destination buffer. The two buffers
   have the same ndim, shape and itemsize. */
static void
copy_rec(const Py_ssize_t *shape, Py_ssize_t ndim, Py_ssize_t itemsize,
         char *dptr, const Py_ssize_t *dstrides, const Py_ssize_t *dsuboffsets,
         char *sptr, const Py_ssize_t *sstrides, const Py_ssize_t *ssuboffsets,
         char *mem)
{
    Py_ssize_t i;

    assert(ndim >= 1);

    if (ndim == 1) {
        copy_base(shape, itemsize,
                  dptr, dstrides, dsuboffsets,
                  sptr, sstrides, ssuboffsets,
                  mem);
        return;
    }

    for (i = 0; i < shape[0]; dptr+=dstrides[0], sptr+=sstrides[0], i++) {
        char *xdptr = ADJUST_PTR(dptr, dsuboffsets, 0);
        char *xsptr = ADJUST_PTR(sptr, ssuboffsets, 0);

        copy_rec(shape+1, ndim-1, itemsize,
                 xdptr, dstrides+1, dsuboffsets ? dsuboffsets+1 : NULL,
                 xsptr, sstrides+1, ssuboffsets ? ssuboffsets+1 : NULL,
                 mem);
    }
}

/* Faster copying of one-dimensional arrays. */
static int
copy_single(PyMemoryViewObject *self, const Py_buffer *dest, const Py_buffer *src)
{
    CHECK_RELEASED_INT_AGAIN(self);
    char *mem = NULL;

    assert(dest->ndim == 1);

    if (!equiv_structure(dest, src))
        return -1;

    if (!last_dim_is_contiguous(dest, src)) {
        mem = PyMem_Malloc(dest->shape[0] * dest->itemsize);
        if (mem == NULL) {
            PyErr_NoMemory();
            return -1;
        }
    }

    copy_base(dest->shape, dest->itemsize,
              dest->buf, dest->strides, dest->suboffsets,
              src->buf, src->strides, src->suboffsets,
              mem);

    if (mem)
        PyMem_Free(mem);

    return 0;
}

/* Recursively copy src to dest. Both buffers must have the same basic
   structure. Copying is atomic, the function never fails with a partial
   copy. */
static int
copy_buffer(const Py_buffer *dest, const Py_buffer *src)
{
    char *mem = NULL;

    assert(dest->ndim > 0);

    if (!equiv_structure(dest, src))
        return -1;

    if (!last_dim_is_contiguous(dest, src)) {
        mem = PyMem_Malloc(dest->shape[dest->ndim-1] * dest->itemsize);
        if (mem == NULL) {
            PyErr_NoMemory();
            return -1;
        }
    }

    copy_rec(dest->shape, dest->ndim, dest->itemsize,
             dest->buf, dest->strides, dest->suboffsets,
             src->buf, src->strides, src->suboffsets,
             mem);

    if (mem)
        PyMem_Free(mem);

    return 0;
}

/* Initialize strides for a C-contiguous array. */
static inline void
init_strides_from_shape(Py_buffer *view)
{
    Py_ssize_t i;

    assert(view->ndim > 0);

    view->strides[view->ndim-1] = view->itemsize;
    for (i = view->ndim-2; i >= 0; i--)
        view->strides[i] = view->strides[i+1] * view->shape[i+1];
}

/* Initialize strides for a Fortran-contiguous array. */
static inline void
init_fortran_strides_from_shape(Py_buffer *view)
{
    Py_ssize_t i;

    assert(view->ndim > 0);

    view->strides[0] = view->itemsize;
    for (i = 1; i < view->ndim; i++)
        view->strides[i] = view->strides[i-1] * view->shape[i-1];
}

/* Copy src to a contiguous representation. order is one of 'C', 'F' (Fortran)
   or 'A' (Any). Assumptions: src has PyBUF_FULL information, src->ndim >= 1,
   len(mem) == src->len. */
static int
buffer_to_contiguous(char *mem, const Py_buffer *src, char order)
{
    Py_buffer dest;
    Py_ssize_t *strides;
    int ret;

    assert(src->ndim >= 1);
    assert(src->shape != NULL);
    assert(src->strides != NULL);

    strides = PyMem_Malloc(src->ndim * (sizeof *src->strides));
    if (strides == NULL) {
        PyErr_NoMemory();
        return -1;
    }

    /* initialize dest */
    dest = *src;
    dest.buf = mem;
    /* shape is constant and shared: the logical representation of the
       array is unaltered. */

    /* The physical representation determined by strides (and possibly
       suboffsets) may change. */
    dest.strides = strides;
    if (order == 'C' || order == 'A') {
        init_strides_from_shape(&dest);
    }
    else {
        init_fortran_strides_from_shape(&dest);
    }

    dest.suboffsets = NULL;

    ret = copy_buffer(&dest, src);

    PyMem_Free(strides);
    return ret;
}


/****************************************************************************/
/*                               Constructors                               */
/****************************************************************************/

/* Initialize values that are shared with the managed buffer. */
static inline void
init_shared_values(Py_buffer *dest, const Py_buffer *src)
{
    dest->obj = src->obj;
    dest->buf = src->buf;
    dest->len = src->len;
    dest->itemsize = src->itemsize;
    dest->readonly = src->readonly;
    dest->format = src->format ? src->format : "B";
    dest->internal = src->internal;
}

/* Copy shape and strides. Reconstruct missing values. */
static void
init_shape_strides(Py_buffer *dest, const Py_buffer *src)
{
    Py_ssize_t i;

    if (src->ndim == 0) {
        dest->shape = NULL;
        dest->strides = NULL;
        return;
    }
    if (src->ndim == 1) {
        dest->shape[0] = src->shape ? src->shape[0] : src->len / src->itemsize;
        dest->strides[0] = src->strides ? src->strides[0] : src->itemsize;
        return;
    }

    for (i = 0; i < src->ndim; i++)
        dest->shape[i] = src->shape[i];
    if (src->strides) {
        for (i = 0; i < src->ndim; i++)
            dest->strides[i] = src->strides[i];
    }
    else {
        init_strides_from_shape(dest);
    }
}

static inline void
init_suboffsets(Py_buffer *dest, const Py_buffer *src)
{
    Py_ssize_t i;

    if (src->suboffsets == NULL) {
        dest->suboffsets = NULL;
        return;
    }
    for (i = 0; i < src->ndim; i++)
        dest->suboffsets[i] = src->suboffsets[i];
}

/* len = product(shape) * itemsize */
static inline void
init_len(Py_buffer *view)
{
    Py_ssize_t i, len;

    len = 1;
    for (i = 0; i < view->ndim; i++)
        len *= view->shape[i];
    len *= view->itemsize;

    view->len = len;
}

/* Initialize memoryview buffer properties. */
static void
init_flags(PyMemoryViewObject *mv)
{
    const Py_buffer *view = &mv->view;
    int flags = 0;

    switch (view->ndim) {
    case 0:
        flags |= (_Py_MEMORYVIEW_SCALAR|_Py_MEMORYVIEW_C|
                  _Py_MEMORYVIEW_FORTRAN);
        break;
    case 1:
        if (MV_CONTIGUOUS_NDIM1(view))
            flags |= (_Py_MEMORYVIEW_C|_Py_MEMORYVIEW_FORTRAN);
        break;
    default:
        if (PyBuffer_IsContiguous(view, 'C'))
            flags |= _Py_MEMORYVIEW_C;
        if (PyBuffer_IsContiguous(view, 'F'))
            flags |= _Py_MEMORYVIEW_FORTRAN;
        break;
    }

    if (view->suboffsets) {
        flags |= _Py_MEMORYVIEW_PIL;
        flags &= ~(_Py_MEMORYVIEW_C|_Py_MEMORYVIEW_FORTRAN);
    }

    mv->flags = flags;
}

/* Allocate a new memoryview and perform basic initialization. New memoryviews
   are exclusively created through the mbuf_add functions. */
static inline PyMemoryViewObject *
memory_alloc(int ndim)
{
    PyMemoryViewObject *mv;

    mv = (PyMemoryViewObject *)
        PyObject_GC_NewVar(PyMemoryViewObject, &PyMemoryView_Type, 3*ndim);
    if (mv == NULL)
        return NULL;

    mv->mbuf = NULL;
    mv->hash = -1;
    mv->flags = 0;
    mv->exports = 0;
    mv->view.ndim = ndim;
    mv->view.shape = mv->ob_array;
    mv->view.strides = mv->ob_array + ndim;
    mv->view.suboffsets = mv->ob_array + 2 * ndim;
    mv->weakreflist = NULL;

    _PyObject_GC_TRACK(mv);
    return mv;
}

/*
   Return a new memoryview that is registered with mbuf. If src is NULL,
   use mbuf->master as the underlying buffer. Otherwise, use src.

   The new memoryview has full buffer information: shape and strides
   are always present, suboffsets as needed. Arrays are copied to
   the memoryview's ob_array field.
 */
static PyObject *
mbuf_add_view(_PyManagedBufferObject *mbuf, const Py_buffer *src)
{
    PyMemoryViewObject *mv;
    Py_buffer *dest;

    if (src == NULL)
        src = &mbuf->master;

    if (src->ndim > PyBUF_MAX_NDIM) {
        PyErr_SetString(PyExc_ValueError,
            "memoryview: number of dimensions must not exceed "
            Py_STRINGIFY(PyBUF_MAX_NDIM));
        return NULL;
    }

    mv = memory_alloc(src->ndim);
    if (mv == NULL)
        return NULL;

    dest = &mv->view;
    init_shared_values(dest, src);
    init_shape_strides(dest, src);
    init_suboffsets(dest, src);
    init_flags(mv);

    mv->mbuf = mbuf;
    Py_INCREF(mbuf);
    mbuf->exports++;

    return (PyObject *)mv;
}

/* Register an incomplete view: shape, strides, suboffsets and flags still
   need to be initialized. Use 'ndim' instead of src->ndim to determine the
   size of the memoryview's ob_array.

   Assumption: ndim <= PyBUF_MAX_NDIM. */
static PyObject *
mbuf_add_incomplete_view(_PyManagedBufferObject *mbuf, const Py_buffer *src,
                         int ndim)
{
    PyMemoryViewObject *mv;
    Py_buffer *dest;

    if (src == NULL)
        src = &mbuf->master;

    assert(ndim <= PyBUF_MAX_NDIM);

    mv = memory_alloc(ndim);
    if (mv == NULL)
        return NULL;

    dest = &mv->view;
    init_shared_values(dest, src);

    mv->mbuf = mbuf;
    Py_INCREF(mbuf);
    mbuf->exports++;

    return (PyObject *)mv;
}

/* Expose a raw memory area as a view of contiguous bytes. flags can be
   PyBUF_READ or PyBUF_WRITE. view->format is set to "B" (unsigned bytes).
   The memoryview has complete buffer information. */
PyObject *
PyMemoryView_FromMemory(char *mem, Py_ssize_t size, int flags)
{
    _PyManagedBufferObject *mbuf;
    PyObject *mv;
    int readonly;

    assert(mem != NULL);
    assert(flags == PyBUF_READ || flags == PyBUF_WRITE);

    mbuf = mbuf_alloc();
    if (mbuf == NULL)
        return NULL;

    readonly = (flags == PyBUF_WRITE) ? 0 : 1;
    (void)PyBuffer_FillInfo(&mbuf->master, NULL, mem, size, readonly,
                            PyBUF_FULL_RO);

    mv = mbuf_add_view(mbuf, NULL);
    Py_DECREF(mbuf);

    return mv;
}

/* Create a memoryview from a given Py_buffer. For simple byte views,
   PyMemoryView_FromMemory() should be used instead.
   This function is the only entry point that can create a master buffer
   without full information. Because of this fact init_shape_strides()
   must be able to reconstruct missing values.  */
PyObject *
PyMemoryView_FromBuffer(const Py_buffer *info)
{
    _PyManagedBufferObject *mbuf;
    PyObject *mv;

    if (info->buf == NULL) {
        PyErr_SetString(PyExc_ValueError,
            "PyMemoryView_FromBuffer(): info->buf must not be NULL");
        return NULL;
    }

    mbuf = mbuf_alloc();
    if (mbuf == NULL)
        return NULL;

    /* info->obj is either NULL or a borrowed reference. This reference
       should not be decremented in PyBuffer_Release(). */
    mbuf->master = *info;
    mbuf->master.obj = NULL;

    mv = mbuf_add_view(mbuf, NULL);
    Py_DECREF(mbuf);

    return mv;
}

/* Create a memoryview from an object that implements the buffer protocol.
   If the object is a memoryview, the new memoryview must be registered
   with the same managed buffer. Otherwise, a new managed buffer is created. */
PyObject *
PyMemoryView_FromObject(PyObject *v)
{
    _PyManagedBufferObject *mbuf;

    if (PyMemoryView_Check(v)) {
        PyMemoryViewObject *mv = (PyMemoryViewObject *)v;
        CHECK_RELEASED(mv);
        return mbuf_add_view(mv->mbuf, &mv->view);
    }
    else if (PyObject_CheckBuffer(v)) {
        PyObject *ret;
        mbuf = (_PyManagedBufferObject *)_PyManagedBuffer_FromObject(v);
        if (mbuf == NULL)
            return NULL;
        ret = mbuf_add_view(mbuf, NULL);
        Py_DECREF(mbuf);
        return ret;
    }

    PyErr_Format(PyExc_TypeError,
        "memoryview: a bytes-like object is required, not '%.200s'",
        Py_TYPE(v)->tp_name);
    return NULL;
}

/* Copy the format string from a base object that might vanish. */
static int
mbuf_copy_format(_PyManagedBufferObject *mbuf, const char *fmt)
{
    if (fmt != NULL) {
        char *cp = PyMem_Malloc(strlen(fmt)+1);
        if (cp == NULL) {
            PyErr_NoMemory();
            return -1;
        }
        mbuf->master.format = strcpy(cp, fmt);
        mbuf->flags |= _Py_MANAGED_BUFFER_FREE_FORMAT;
    }

    return 0;
}

/*
   Return a memoryview that is based on a contiguous copy of src.
   Assumptions: src has PyBUF_FULL_RO information, src->ndim > 0.

   Ownership rules:
     1) As usual, the returned memoryview has a private copy
        of src->shape, src->strides and src->suboffsets.
     2) src->format is copied to the master buffer and released
        in mbuf_dealloc(). The releasebufferproc of the bytes
        object is NULL, so it does not matter that mbuf_release()
        passes the altered format pointer to PyBuffer_Release().
*/
static PyObject *
memory_from_contiguous_copy(const Py_buffer *src, char order)
{
    _PyManagedBufferObject *mbuf;
    PyMemoryViewObject *mv;
    PyObject *bytes;
    Py_buffer *dest;
    int i;

    assert(src->ndim > 0);
    assert(src->shape != NULL);

    bytes = PyBytes_FromStringAndSize(NULL, src->len);
    if (bytes == NULL)
        return NULL;

    mbuf = (_PyManagedBufferObject *)_PyManagedBuffer_FromObject(bytes);
    Py_DECREF(bytes);
    if (mbuf == NULL)
        return NULL;

    if (mbuf_copy_format(mbuf, src->format) < 0) {
        Py_DECREF(mbuf);
        return NULL;
    }

    mv = (PyMemoryViewObject *)mbuf_add_incomplete_view(mbuf, NULL, src->ndim);
    Py_DECREF(mbuf);
    if (mv == NULL)
        return NULL;

    dest = &mv->view;

    /* shared values are initialized correctly except for itemsize */
    dest->itemsize = src->itemsize;

    /* shape and strides */
    for (i = 0; i < src->ndim; i++) {
        dest->shape[i] = src->shape[i];
    }
    if (order == 'C' || order == 'A') {
        init_strides_from_shape(dest);
    }
    else {
        init_fortran_strides_from_shape(dest);
    }
    /* suboffsets */
    dest->suboffsets = NULL;

    /* flags */
    init_flags(mv);

    if (copy_buffer(dest, src) < 0) {
        Py_DECREF(mv);
        return NULL;
    }

    return (PyObject *)mv;
}

/*
   Return a new memoryview object based on a contiguous exporter with
   buffertype={PyBUF_READ, PyBUF_WRITE} and order={'C', 'F'ortran, or 'A'ny}.
   The logical structure of the input and output buffers is the same
   (i.e. tolist(input) == tolist(output)), but the physical layout in
   memory can be explicitly chosen.

   As usual, if buffertype=PyBUF_WRITE, the exporter's buffer must be writable,
   otherwise it may be writable or read-only.

   If the exporter is already contiguous with the desired target order,
   the memoryview will be directly based on the exporter.

   Otherwise, if the buffertype is PyBUF_READ, the memoryview will be
   based on a new bytes object. If order={'C', 'A'ny}, use 'C' order,
   'F'ortran order otherwise.
*/
PyObject *
PyMemoryView_GetContiguous(PyObject *obj, int buffertype, char order)
{
    PyMemoryViewObject *mv;
    PyObject *ret;
    Py_buffer *view;

    assert(buffertype == PyBUF_READ || buffertype == PyBUF_WRITE);
    assert(order == 'C' || order == 'F' || order == 'A');

    mv = (PyMemoryViewObject *)PyMemoryView_FromObject(obj);
    if (mv == NULL)
        return NULL;

    view = &mv->view;
    if (buffertype == PyBUF_WRITE && view->readonly) {
        PyErr_SetString(PyExc_BufferError,
            "underlying buffer is not writable");
        Py_DECREF(mv);
        return NULL;
    }

    if (PyBuffer_IsContiguous(view, order))
        return (PyObject *)mv;

    if (buffertype == PyBUF_WRITE) {
        PyErr_SetString(PyExc_BufferError,
            "writable contiguous buffer requested "
            "for a non-contiguous object.");
        Py_DECREF(mv);
        return NULL;
    }

    ret = memory_from_contiguous_copy(view, order);
    Py_DECREF(mv);
    return ret;
}


/*[clinic input]
@classmethod
memoryview.__new__

    object: object

Create a new memoryview object which references the given object.
[clinic start generated code]*/

static PyObject *
memoryview_impl(PyTypeObject *type, PyObject *object)
/*[clinic end generated code: output=7de78e184ed66db8 input=f04429eb0bdf8c6e]*/
{
    return PyMemoryView_FromObject(object);
}


/****************************************************************************/
/*                         Previously in abstract.c                         */
/****************************************************************************/

typedef struct {
    Py_buffer view;
    Py_ssize_t array[1];
} Py_buffer_full;

int
PyBuffer_ToContiguous(void *buf, const Py_buffer *src, Py_ssize_t len, char order)
{
    Py_buffer_full *fb = NULL;
    int ret;

    assert(order == 'C' || order == 'F' || order == 'A');

    if (len != src->len) {
        PyErr_SetString(PyExc_ValueError,
            "PyBuffer_ToContiguous: len != view->len");
        return -1;
    }

    if (PyBuffer_IsContiguous(src, order)) {
        memcpy((char *)buf, src->buf, len);
        return 0;
    }

    /* buffer_to_contiguous() assumes PyBUF_FULL */
    fb = PyMem_Malloc(sizeof *fb + 3 * src->ndim * (sizeof *fb->array));
    if (fb == NULL) {
        PyErr_NoMemory();
        return -1;
    }
    fb->view.ndim = src->ndim;
    fb->view.shape = fb->array;
    fb->view.strides = fb->array + src->ndim;
    fb->view.suboffsets = fb->array + 2 * src->ndim;

    init_shared_values(&fb->view, src);
    init_shape_strides(&fb->view, src);
    init_suboffsets(&fb->view, src);

    src = &fb->view;

    ret = buffer_to_contiguous(buf, src, order);
    PyMem_Free(fb);
    return ret;
}


/****************************************************************************/
/*                           Release/GC management                          */
/****************************************************************************/

/* Inform the managed buffer that this particular memoryview will not access
   the underlying buffer again. If no other memoryviews are registered with
   the managed buffer, the underlying buffer is released instantly and
   marked as inaccessible for both the memoryview and the managed buffer.

   This function fails if the memoryview itself has exported buffers. */
static int
_memory_release(PyMemoryViewObject *self)
{
    if (self->flags & _Py_MEMORYVIEW_RELEASED)
        return 0;

    if (self->exports == 0) {
        self->flags |= _Py_MEMORYVIEW_RELEASED;
        assert(self->mbuf->exports > 0);
        if (--self->mbuf->exports == 0)
            mbuf_release(self->mbuf);
        return 0;
    }
    if (self->exports > 0) {
        PyErr_Format(PyExc_BufferError,
            "memoryview has %zd exported buffer%s", self->exports,
            self->exports==1 ? "" : "s");
        return -1;
    }

    PyErr_SetString(PyExc_SystemError,
                    "_memory_release(): negative export count");
    return -1;
}

/*[clinic input]
memoryview.release

Release the underlying buffer exposed by the memoryview object.
[clinic start generated code]*/

static PyObject *
memoryview_release_impl(PyMemoryViewObject *self)
/*[clinic end generated code: output=d0b7e3ba95b7fcb9 input=bc71d1d51f4a52f0]*/
{
    if (_memory_release(self) < 0)
        return NULL;
    Py_RETURN_NONE;
}

static void
memory_dealloc(PyMemoryViewObject *self)
{
    assert(self->exports == 0);
    _PyObject_GC_UNTRACK(self);
    (void)_memory_release(self);
    Py_CLEAR(self->mbuf);
    if (self->weakreflist != NULL)
        PyObject_ClearWeakRefs((PyObject *) self);
    PyObject_GC_Del(self);
}

static int
memory_traverse(PyMemoryViewObject *self, visitproc visit, void *arg)
{
    Py_VISIT(self->mbuf);
    return 0;
}

static int
memory_clear(PyMemoryViewObject *self)
{
    (void)_memory_release(self);
    Py_CLEAR(self->mbuf);
    return 0;
}

static PyObject *
memory_enter(PyObject *self, PyObject *args)
{
    CHECK_RELEASED(self);
    Py_INCREF(self);
    return self;
}

static PyObject *
memory_exit(PyObject *self, PyObject *args)
{
    return memoryview_release_impl((PyMemoryViewObject *)self);
}


/****************************************************************************/
/*                         Casting format and shape                         */
/****************************************************************************/

#define IS_BYTE_FORMAT(f) (f == 'b' || f == 'B' || f == 'c')

static inline Py_ssize_t
get_native_fmtchar(char *result, const char *fmt)
{
    Py_ssize_t size = -1;

    if (fmt[0] == '@') fmt++;

    switch (fmt[0]) {
    case 'c': case 'b': case 'B': size = sizeof(char); break;
    case 'h': case 'H': size = sizeof(short); break;
    case 'i': case 'I': size = sizeof(int); break;
    case 'l': case 'L': size = sizeof(long); break;
    case 'q': case 'Q': size = sizeof(long long); break;
    case 'n': case 'N': size = sizeof(Py_ssize_t); break;
    case 'f': size = sizeof(float); break;
    case 'd': size = sizeof(double); break;
    case 'e': size = sizeof(float) / 2; break;
    case '?': size = sizeof(_Bool); break;
    case 'P': size = sizeof(void *); break;
    }

    if (size > 0 && fmt[1] == '\0') {
        *result = fmt[0];
        return size;
    }

    return -1;
}

static inline const char *
get_native_fmtstr(const char *fmt)
{
    int at = 0;

    if (fmt[0] == '@') {
        at = 1;
        fmt++;
    }
    if (fmt[0] == '\0' || fmt[1] != '\0') {
        return NULL;
    }

#define RETURN(s) do { return at ? "@" s : s; } while (0)

    switch (fmt[0]) {
    case 'c': RETURN("c");
    case 'b': RETURN("b");
    case 'B': RETURN("B");
    case 'h': RETURN("h");
    case 'H': RETURN("H");
    case 'i': RETURN("i");
    case 'I': RETURN("I");
    case 'l': RETURN("l");
    case 'L': RETURN("L");
    case 'q': RETURN("q");
    case 'Q': RETURN("Q");
    case 'n': RETURN("n");
    case 'N': RETURN("N");
    case 'f': RETURN("f");
    case 'd': RETURN("d");
    case 'e': RETURN("e");
    case '?': RETURN("?");
    case 'P': RETURN("P");
    }

    return NULL;
}


/* Cast a memoryview's data type to 'format'. The input array must be
   C-contiguous. At least one of input-format, output-format must have
   byte size. The output array is 1-D, with the same byte length as the
   input array. Thus, view->len must be a multiple of the new itemsize. */
static int
cast_to_1D(PyMemoryViewObject *mv, PyObject *format)
{
    Py_buffer *view = &mv->view;
    PyObject *asciifmt;
    char srcchar, destchar;
    Py_ssize_t itemsize;
    int ret = -1;

    assert(view->ndim >= 1);
    assert(Py_SIZE(mv) == 3*view->ndim);
    assert(view->shape == mv->ob_array);
    assert(view->strides == mv->ob_array + view->ndim);
    assert(view->suboffsets == mv->ob_array + 2*view->ndim);

    asciifmt = PyUnicode_AsASCIIString(format);
    if (asciifmt == NULL)
        return ret;

    itemsize = get_native_fmtchar(&destchar, PyBytes_AS_STRING(asciifmt));
    if (itemsize < 0) {
        PyErr_SetString(PyExc_ValueError,
            "memoryview: destination format must be a native single "
            "character format prefixed with an optional '@'");
        goto out;
    }

    if ((get_native_fmtchar(&srcchar, view->format) < 0 ||
         !IS_BYTE_FORMAT(srcchar)) && !IS_BYTE_FORMAT(destchar)) {
        PyErr_SetString(PyExc_TypeError,
            "memoryview: cannot cast between two non-byte formats");
        goto out;
    }
    if (view->len % itemsize) {
        PyErr_SetString(PyExc_TypeError,
            "memoryview: length is not a multiple of itemsize");
        goto out;
    }

    view->format = (char *)get_native_fmtstr(PyBytes_AS_STRING(asciifmt));
    if (view->format == NULL) {
        /* NOT_REACHED: get_native_fmtchar() already validates the format. */
        PyErr_SetString(PyExc_RuntimeError,
            "memoryview: internal error");
        goto out;
    }
    view->itemsize = itemsize;

    view->ndim = 1;
    view->shape[0] = view->len / view->itemsize;
    view->strides[0] = view->itemsize;
    view->suboffsets = NULL;

    init_flags(mv);

    ret = 0;

out:
    Py_DECREF(asciifmt);
    return ret;
}

/* The memoryview must have space for 3*len(seq) elements. */
static Py_ssize_t
copy_shape(Py_ssize_t *shape, const PyObject *seq, Py_ssize_t ndim,
           Py_ssize_t itemsize)
{
    Py_ssize_t x, i;
    Py_ssize_t len = itemsize;

    for (i = 0; i < ndim; i++) {
        PyObject *tmp = PySequence_Fast_GET_ITEM(seq, i);
        if (!PyLong_Check(tmp)) {
            PyErr_SetString(PyExc_TypeError,
                "memoryview.cast(): elements of shape must be integers");
            return -1;
        }
        x = PyLong_AsSsize_t(tmp);
        if (x == -1 && PyErr_Occurred()) {
            return -1;
        }
        if (x <= 0) {
            /* In general elements of shape may be 0, but not for casting. */
            PyErr_Format(PyExc_ValueError,
                "memoryview.cast(): elements of shape must be integers > 0");
            return -1;
        }
        if (x > PY_SSIZE_T_MAX / len) {
            PyErr_Format(PyExc_ValueError,
                "memoryview.cast(): product(shape) > SSIZE_MAX");
            return -1;
        }
        len *= x;
        shape[i] = x;
    }

    return len;
}

/* Cast a 1-D array to a new shape. The result array will be C-contiguous.
   If the result array does not have exactly the same byte length as the
   input array, raise ValueError. */
static int
cast_to_ND(PyMemoryViewObject *mv, const PyObject *shape, int ndim)
{
    Py_buffer *view = &mv->view;
    Py_ssize_t len;

    assert(view->ndim == 1); /* ndim from cast_to_1D() */
    assert(Py_SIZE(mv) == 3*(ndim==0?1:ndim)); /* ndim of result array */
    assert(view->shape == mv->ob_array);
    assert(view->strides == mv->ob_array + (ndim==0?1:ndim));
    assert(view->suboffsets == NULL);

    view->ndim = ndim;
    if (view->ndim == 0) {
        view->shape = NULL;
        view->strides = NULL;
        len = view->itemsize;
    }
    else {
        len = copy_shape(view->shape, shape, ndim, view->itemsize);
        if (len < 0)
            return -1;
        init_strides_from_shape(view);
    }

    if (view->len != len) {
        PyErr_SetString(PyExc_TypeError,
            "memoryview: product(shape) * itemsize != buffer size");
        return -1;
    }

    init_flags(mv);

    return 0;
}

static int
zero_in_shape(PyMemoryViewObject *mv)
{
    Py_buffer *view = &mv->view;
    Py_ssize_t i;

    for (i = 0; i < view->ndim; i++)
        if (view->shape[i] == 0)
            return 1;

    return 0;
}

/*
   Cast a copy of 'self' to a different view. The input view must
   be C-contiguous. The function always casts the input view to a
   1-D output according to 'format'. At least one of input-format,
   output-format must have byte size.

   If 'shape' is given, the 1-D view from the previous step will
   be cast to a C-contiguous view with new shape and strides.

   All casts must result in views that will have the exact byte
   size of the original input. Otherwise, an error is raised.
*/
/*[clinic input]
memoryview.cast

    format: unicode
    shape: object = NULL

Cast a memoryview to a new format or shape.
[clinic start generated code]*/

static PyObject *
memoryview_cast_impl(PyMemoryViewObject *self, PyObject *format,
                     PyObject *shape)
/*[clinic end generated code: output=bae520b3a389cbab input=138936cc9041b1a3]*/
{
    PyMemoryViewObject *mv = NULL;
    Py_ssize_t ndim = 1;

    CHECK_RELEASED(self);

    if (!MV_C_CONTIGUOUS(self->flags)) {
        PyErr_SetString(PyExc_TypeError,
            "memoryview: casts are restricted to C-contiguous views");
        return NULL;
    }
    if ((shape || self->view.ndim != 1) && zero_in_shape(self)) {
        PyErr_SetString(PyExc_TypeError,
            "memoryview: cannot cast view with zeros in shape or strides");
        return NULL;
    }
    if (shape) {
        CHECK_LIST_OR_TUPLE(shape)
        ndim = PySequence_Fast_GET_SIZE(shape);
        if (ndim > PyBUF_MAX_NDIM) {
            PyErr_SetString(PyExc_ValueError,
                "memoryview: number of dimensions must not exceed "
                Py_STRINGIFY(PyBUF_MAX_NDIM));
            return NULL;
        }
        if (self->view.ndim != 1 && ndim != 1) {
            PyErr_SetString(PyExc_TypeError,
                "memoryview: cast must be 1D -> ND or ND -> 1D");
            return NULL;
        }
    }

    mv = (PyMemoryViewObject *)
        mbuf_add_incomplete_view(self->mbuf, &self->view, ndim==0 ? 1 : (int)ndim);
    if (mv == NULL)
        return NULL;

    if (cast_to_1D(mv, format) < 0)
        goto error;
    if (shape && cast_to_ND(mv, shape, (int)ndim) < 0)
        goto error;

    return (PyObject *)mv;

error:
    Py_DECREF(mv);
    return NULL;
}

/*[clinic input]
memoryview.toreadonly

Return a readonly version of the memoryview.
[clinic start generated code]*/

static PyObject *
memoryview_toreadonly_impl(PyMemoryViewObject *self)
/*[clinic end generated code: output=2c7e056f04c99e62 input=dc06d20f19ba236f]*/
{
    CHECK_RELEASED(self);
    /* Even if self is already readonly, we still need to create a new
     * object for .release() to work correctly.
     */
    self = (PyMemoryViewObject *) mbuf_add_view(self->mbuf, &self->view);
    if (self != NULL) {
        self->view.readonly = 1;
    };
    return (PyObject *) self;
}


/**************************************************************************/
/*                               getbuffer                                */
/**************************************************************************/

static int
memory_getbuf(PyMemoryViewObject *self, Py_buffer *view, int flags)
{
    Py_buffer *base = &self->view;
    int baseflags = self->flags;

    CHECK_RELEASED_INT(self);

    /* start with complete information */
    *view = *base;
    view->obj = NULL;

    if (REQ_WRITABLE(flags) && base->readonly) {
        PyErr_SetString(PyExc_BufferError,
            "memoryview: underlying buffer is not writable");
        return -1;
    }
    if (!REQ_FORMAT(flags)) {
        /* NULL indicates that the buffer's data type has been cast to 'B'.
           view->itemsize is the _previous_ itemsize. If shape is present,
           the equality product(shape) * itemsize = len still holds at this
           point. The equality calcsize(format) = itemsize does _not_ hold
           from here on! */
        view->format = NULL;
    }

    if (REQ_C_CONTIGUOUS(flags) && !MV_C_CONTIGUOUS(baseflags)) {
        PyErr_SetString(PyExc_BufferError,
            "memoryview: underlying buffer is not C-contiguous");
        return -1;
    }
    if (REQ_F_CONTIGUOUS(flags) && !MV_F_CONTIGUOUS(baseflags)) {
        PyErr_SetString(PyExc_BufferError,
            "memoryview: underlying buffer is not Fortran contiguous");
        return -1;
    }
    if (REQ_ANY_CONTIGUOUS(flags) && !MV_ANY_CONTIGUOUS(baseflags)) {
        PyErr_SetString(PyExc_BufferError,
            "memoryview: underlying buffer is not contiguous");
        return -1;
    }
    if (!REQ_INDIRECT(flags) && (baseflags & _Py_MEMORYVIEW_PIL)) {
        PyErr_SetString(PyExc_BufferError,
            "memoryview: underlying buffer requires suboffsets");
        return -1;
    }
    if (!REQ_STRIDES(flags)) {
        if (!MV_C_CONTIGUOUS(baseflags)) {
            PyErr_SetString(PyExc_BufferError,
                "memoryview: underlying buffer is not C-contiguous");
            return -1;
        }
        view->strides = NULL;
    }
    if (!REQ_SHAPE(flags)) {
        /* PyBUF_SIMPLE or PyBUF_WRITABLE: at this point buf is C-contiguous,
           so base->buf = ndbuf->data. */
        if (view->format != NULL) {
            /* PyBUF_SIMPLE|PyBUF_FORMAT and PyBUF_WRITABLE|PyBUF_FORMAT do
               not make sense. */
            PyErr_Format(PyExc_BufferError,
                "memoryview: cannot cast to unsigned bytes if the format flag "
                "is present");
            return -1;
        }
        /* product(shape) * itemsize = len and calcsize(format) = itemsize
           do _not_ hold from here on! */
        view->ndim = 1;
        view->shape = NULL;
    }


    view->obj = (PyObject *)self;
    Py_INCREF(view->obj);
    self->exports++;

    return 0;
}

static void
memory_releasebuf(PyMemoryViewObject *self, Py_buffer *view)
{
    self->exports--;
    return;
    /* PyBuffer_Release() decrements view->obj after this function returns. */
}

/* Buffer methods */
static PyBufferProcs memory_as_buffer = {
    (getbufferproc)memory_getbuf,         /* bf_getbuffer */
    (releasebufferproc)memory_releasebuf, /* bf_releasebuffer */
};


/****************************************************************************/
/*           Optimized pack/unpack for all native format specifiers         */
/****************************************************************************/

/*
  Fix exceptions:
     1) Include format string in the error message.
     2) OverflowError -> ValueError.
     3) The error message from PyNumber_Index() is not ideal.
*/
static int
type_error_int(const char *fmt)
{
    PyErr_Format(PyExc_TypeError,
        "memoryview: invalid type for format '%s'", fmt);
    return -1;
}

static int
value_error_int(const char *fmt)
{
    PyErr_Format(PyExc_ValueError,
        "memoryview: invalid value for format '%s'", fmt);
    return -1;
}

static int
fix_error_int(const char *fmt)
{
    assert(PyErr_Occurred());
    if (PyErr_ExceptionMatches(PyExc_TypeError)) {
        PyErr_Clear();
        return type_error_int(fmt);
    }
    else if (PyErr_ExceptionMatches(PyExc_OverflowError) ||
             PyErr_ExceptionMatches(PyExc_ValueError)) {
        PyErr_Clear();
        return value_error_int(fmt);
    }

    return -1;
}

/* Accept integer objects or objects with an __index__() method. */
static long
pylong_as_ld(PyObject *item)
{
    PyObject *tmp;
    long ld;

    tmp = _PyNumber_Index(item);
    if (tmp == NULL)
        return -1;

    ld = PyLong_AsLong(tmp);
    Py_DECREF(tmp);
    return ld;
}

static unsigned long
pylong_as_lu(PyObject *item)
{
    PyObject *tmp;
    unsigned long lu;

    tmp = _PyNumber_Index(item);
    if (tmp == NULL)
        return (unsigned long)-1;

    lu = PyLong_AsUnsignedLong(tmp);
    Py_DECREF(tmp);
    return lu;
}

static long long
pylong_as_lld(PyObject *item)
{
    PyObject *tmp;
    long long lld;

    tmp = _PyNumber_Index(item);
    if (tmp == NULL)
        return -1;

    lld = PyLong_AsLongLong(tmp);
    Py_DECREF(tmp);
    return lld;
}

static unsigned long long
pylong_as_llu(PyObject *item)
{
    PyObject *tmp;
    unsigned long long llu;

    tmp = _PyNumber_Index(item);
    if (tmp == NULL)
        return (unsigned long long)-1;

    llu = PyLong_AsUnsignedLongLong(tmp);
    Py_DECREF(tmp);
    return llu;
}

static Py_ssize_t
pylong_as_zd(PyObject *item)
{
    PyObject *tmp;
    Py_ssize_t zd;

    tmp = _PyNumber_Index(item);
    if (tmp == NULL)
        return -1;

    zd = PyLong_AsSsize_t(tmp);
    Py_DECREF(tmp);
    return zd;
}

static size_t
pylong_as_zu(PyObject *item)
{
    PyObject *tmp;
    size_t zu;

    tmp = _PyNumber_Index(item);
    if (tmp == NULL)
        return (size_t)-1;

    zu = PyLong_AsSize_t(tmp);
    Py_DECREF(tmp);
    return zu;
}

/* Timings with the ndarray from _testbuffer.c indicate that using the
   struct module is around 15x slower than the two functions below. */

#define UNPACK_SINGLE(dest, ptr, type) \
    do {                                   \
        type x;                            \
        memcpy((char *)&x, ptr, sizeof x); \
        dest = x;                          \
    } while (0)

/* Unpack a single item. 'fmt' can be any native format character in struct
   module syntax. This function is very sensitive to small changes. With this
   layout gcc automatically generates a fast jump table. */
static inline PyObject *
unpack_single(PyMemoryViewObject *self, const char *ptr, const char *fmt)
{
    unsigned long long llu;
    unsigned long lu;
    size_t zu;
    long long lld;
    long ld;
    Py_ssize_t zd;
    double d;
    unsigned char uc;
    void *p;

    CHECK_RELEASED_AGAIN(self);

#if PY_LITTLE_ENDIAN
    int endian = 1;
#else
    int endian = 0;
#endif

    switch (fmt[0]) {

    /* signed integers and fast path for 'B' */
    case 'B': uc = *((const unsigned char *)ptr); goto convert_uc;
    case 'b': ld =   *((const signed char *)ptr); goto convert_ld;
    case 'h': UNPACK_SINGLE(ld, ptr, short); goto convert_ld;
    case 'i': UNPACK_SINGLE(ld, ptr, int); goto convert_ld;
    case 'l': UNPACK_SINGLE(ld, ptr, long); goto convert_ld;

    /* boolean */
    case '?': UNPACK_SINGLE(ld, ptr, _Bool); goto convert_bool;

    /* unsigned integers */
    case 'H': UNPACK_SINGLE(lu, ptr, unsigned short); goto convert_lu;
    case 'I': UNPACK_SINGLE(lu, ptr, unsigned int); goto convert_lu;
    case 'L': UNPACK_SINGLE(lu, ptr, unsigned long); goto convert_lu;

    /* native 64-bit */
    case 'q': UNPACK_SINGLE(lld, ptr, long long); goto convert_lld;
    case 'Q': UNPACK_SINGLE(llu, ptr, unsigned long long); goto convert_llu;

    /* ssize_t and size_t */
    case 'n': UNPACK_SINGLE(zd, ptr, Py_ssize_t); goto convert_zd;
    case 'N': UNPACK_SINGLE(zu, ptr, size_t); goto convert_zu;

    /* floats */
    case 'f': UNPACK_SINGLE(d, ptr, float); goto convert_double;
    case 'd': UNPACK_SINGLE(d, ptr, double); goto convert_double;
    case 'e': d = PyFloat_Unpack2(ptr, endian); goto convert_double;

    /* bytes object */
    case 'c': goto convert_bytes;

    /* pointer */
    case 'P': UNPACK_SINGLE(p, ptr, void *); goto convert_pointer;

    /* default */
    default: goto err_format;
    }

convert_uc:
    /* PyLong_FromUnsignedLong() is slower */
    return PyLong_FromLong(uc);
convert_ld:
    return PyLong_FromLong(ld);
convert_lu:
    return PyLong_FromUnsignedLong(lu);
convert_lld:
    return PyLong_FromLongLong(lld);
convert_llu:
    return PyLong_FromUnsignedLongLong(llu);
convert_zd:
    return PyLong_FromSsize_t(zd);
convert_zu:
    return PyLong_FromSize_t(zu);
convert_double:
    return PyFloat_FromDouble(d);
convert_bool:
    return PyBool_FromLong(ld);
convert_bytes:
    return PyBytes_FromStringAndSize(ptr, 1);
convert_pointer:
    return PyLong_FromVoidPtr(p);
err_format:
    PyErr_Format(PyExc_NotImplementedError,
        "memoryview: format %s not supported", fmt);
    return NULL;
}

#define PACK_SINGLE(ptr, src, type) \
    do {                                     \
        type x;                              \
        x = (type)src;                       \
        memcpy(ptr, (char *)&x, sizeof x);   \
    } while (0)

/* Pack a single item. 'fmt' can be any native format character in
   struct module syntax. */
static int
pack_single(PyMemoryViewObject *self, char *ptr, PyObject *item, const char *fmt)
{
    unsigned long long llu;
    unsigned long lu;
    size_t zu;
    long long lld;
    long ld;
    Py_ssize_t zd;
    double d;
    void *p;

#if PY_LITTLE_ENDIAN
    int endian = 1;
#else
    int endian = 0;
#endif
    switch (fmt[0]) {
    /* signed integers */
    case 'b': case 'h': case 'i': case 'l':
        ld = pylong_as_ld(item);
        if (ld == -1 && PyErr_Occurred())
            goto err_occurred;
        CHECK_RELEASED_INT_AGAIN(self);
        switch (fmt[0]) {
        case 'b':
            if (ld < SCHAR_MIN || ld > SCHAR_MAX) goto err_range;
            *((signed char *)ptr) = (signed char)ld; break;
        case 'h':
            if (ld < SHRT_MIN || ld > SHRT_MAX) goto err_range;
            PACK_SINGLE(ptr, ld, short); break;
        case 'i':
            if (ld < INT_MIN || ld > INT_MAX) goto err_range;
            PACK_SINGLE(ptr, ld, int); break;
        default: /* 'l' */
            PACK_SINGLE(ptr, ld, long); break;
        }
        break;

    /* unsigned integers */
    case 'B': case 'H': case 'I': case 'L':
        lu = pylong_as_lu(item);
        if (lu == (unsigned long)-1 && PyErr_Occurred())
            goto err_occurred;
        CHECK_RELEASED_INT_AGAIN(self);
        switch (fmt[0]) {
        case 'B':
            if (lu > UCHAR_MAX) goto err_range;
            *((unsigned char *)ptr) = (unsigned char)lu; break;
        case 'H':
            if (lu > USHRT_MAX) goto err_range;
            PACK_SINGLE(ptr, lu, unsigned short); break;
        case 'I':
            if (lu > UINT_MAX) goto err_range;
            PACK_SINGLE(ptr, lu, unsigned int); break;
        default: /* 'L' */
            PACK_SINGLE(ptr, lu, unsigned long); break;
        }
        break;

    /* native 64-bit */
    case 'q':
        lld = pylong_as_lld(item);
        if (lld == -1 && PyErr_Occurred())
            goto err_occurred;
        CHECK_RELEASED_INT_AGAIN(self);
        PACK_SINGLE(ptr, lld, long long);
        break;
    case 'Q':
        llu = pylong_as_llu(item);
        if (llu == (unsigned long long)-1 && PyErr_Occurred())
            goto err_occurred;
        CHECK_RELEASED_INT_AGAIN(self);
        PACK_SINGLE(ptr, llu, unsigned long long);
        break;

    /* ssize_t and size_t */
    case 'n':
        zd = pylong_as_zd(item);
        if (zd == -1 && PyErr_Occurred())
            goto err_occurred;
        CHECK_RELEASED_INT_AGAIN(self);
        PACK_SINGLE(ptr, zd, Py_ssize_t);
        break;
    case 'N':
        zu = pylong_as_zu(item);
        if (zu == (size_t)-1 && PyErr_Occurred())
            goto err_occurred;
        CHECK_RELEASED_INT_AGAIN(self);
        PACK_SINGLE(ptr, zu, size_t);
        break;

    /* floats */
    case 'f': case 'd': case 'e':
        d = PyFloat_AsDouble(item);
        if (d == -1.0 && PyErr_Occurred())
            goto err_occurred;
        CHECK_RELEASED_INT_AGAIN(self);
        if (fmt[0] == 'f') {
            PACK_SINGLE(ptr, d, float);
        }
        else if (fmt[0] == 'd') {
            PACK_SINGLE(ptr, d, double);
        }
        else {
            if (PyFloat_Pack2(d, ptr, endian) < 0) {
                goto err_occurred;
            }
        }
        break;

    /* bool */
    case '?':
        ld = PyObject_IsTrue(item);
        if (ld < 0)
            return -1; /* preserve original error */
        CHECK_RELEASED_INT_AGAIN(self);
        PACK_SINGLE(ptr, ld, _Bool);
        break;

    /* bytes object */
    case 'c':
        if (!PyBytes_Check(item))
            return type_error_int(fmt);
        if (PyBytes_GET_SIZE(item) != 1)
            return value_error_int(fmt);
        *ptr = PyBytes_AS_STRING(item)[0];
        break;

    /* pointer */
    case 'P':
        p = PyLong_AsVoidPtr(item);
        if (p == NULL && PyErr_Occurred())
            goto err_occurred;
        CHECK_RELEASED_INT_AGAIN(self);
        PACK_SINGLE(ptr, p, void *);
        break;

    /* default */
    default: goto err_format;
    }

    return 0;

err_occurred:
    return fix_error_int(fmt);
err_range:
    return value_error_int(fmt);
err_format:
    PyErr_Format(PyExc_NotImplementedError,
        "memoryview: format %s not supported", fmt);
    return -1;
}


/****************************************************************************/
/*                       unpack using the struct module                     */
/****************************************************************************/

/* For reasonable performance it is necessary to cache all objects required
   for unpacking. An unpacker can handle the format passed to unpack_from().
   Invariant: All pointer fields of the struct should either be NULL or valid
   pointers. */
struct unpacker {
    PyObject *unpack_from; /* Struct.unpack_from(format) */
    PyObject *mview;       /* cached memoryview */
    char *item;            /* buffer for mview */
    Py_ssize_t itemsize;   /* len(item) */
};

static struct unpacker *
unpacker_new(void)
{
    struct unpacker *x = PyMem_Malloc(sizeof *x);

    if (x == NULL) {
        PyErr_NoMemory();
        return NULL;
    }

    x->unpack_from = NULL;
    x->mview = NULL;
    x->item = NULL;
    x->itemsize = 0;

    return x;
}

static void
unpacker_free(struct unpacker *x)
{
    if (x) {
        Py_XDECREF(x->unpack_from);
        Py_XDECREF(x->mview);
        PyMem_Free(x->item);
        PyMem_Free(x);
    }
}

/* Return a new unpacker for the given format. */
static struct unpacker *
struct_get_unpacker(const char *fmt, Py_ssize_t itemsize)
{
    PyObject *Struct = NULL;    /* XXX cache it in globals? */
    PyObject *structobj = NULL;
    PyObject *format = NULL;
    struct unpacker *x = NULL;

    Struct = _PyImport_GetModuleAttrString("struct", "Struct");
    if (Struct == NULL)
        return NULL;

    x = unpacker_new();
    if (x == NULL)
        goto error;

    format = PyBytes_FromString(fmt);
    if (format == NULL)
        goto error;

    structobj = PyObject_CallOneArg(Struct, format);
    if (structobj == NULL)
        goto error;

    x->unpack_from = PyObject_GetAttrString(structobj, "unpack_from");
    if (x->unpack_from == NULL)
        goto error;

    x->item = PyMem_Malloc(itemsize);
    if (x->item == NULL) {
        PyErr_NoMemory();
        goto error;
    }
    x->itemsize = itemsize;

    x->mview = PyMemoryView_FromMemory(x->item, itemsize, PyBUF_WRITE);
    if (x->mview == NULL)
        goto error;


out:
    Py_XDECREF(Struct);
    Py_XDECREF(format);
    Py_XDECREF(structobj);
    return x;

error:
    unpacker_free(x);
    x = NULL;
    goto out;
}

/* unpack a single item */
static PyObject *
struct_unpack_single(const char *ptr, struct unpacker *x)
{
    PyObject *v;

    memcpy(x->item, ptr, x->itemsize);
    v = PyObject_CallOneArg(x->unpack_from, x->mview);
    if (v == NULL)
        return NULL;

    if (PyTuple_GET_SIZE(v) == 1) {
        PyObject *tmp = PyTuple_GET_ITEM(v, 0);
        Py_INCREF(tmp);
        Py_DECREF(v);
        return tmp;
    }

    return v;
}


/****************************************************************************/
/*                              Representations                             */
/****************************************************************************/

/* allow explicit form of native format */
static inline const char *
adjust_fmt(const Py_buffer *view)
{
    const char *fmt;

    fmt = (view->format[0] == '@') ? view->format+1 : view->format;
    if (fmt[0] && fmt[1] == '\0')
        return fmt;

    PyErr_Format(PyExc_NotImplementedError,
        "memoryview: unsupported format %s", view->format);
    return NULL;
}

/* Base case for multi-dimensional unpacking. Assumption: ndim == 1. */
static PyObject *
tolist_base(PyMemoryViewObject *self, const char *ptr, const Py_ssize_t *shape,
            const Py_ssize_t *strides, const Py_ssize_t *suboffsets,
            const char *fmt)
{
    PyObject *lst, *item;
    Py_ssize_t i;

    lst = PyList_New(shape[0]);
    if (lst == NULL)
        return NULL;

    for (i = 0; i < shape[0]; ptr+=strides[0], i++) {
        const char *xptr = ADJUST_PTR(ptr, suboffsets, 0);
        item = unpack_single(self, xptr, fmt);
        if (item == NULL) {
            Py_DECREF(lst);
            return NULL;
        }
        PyList_SET_ITEM(lst, i, item);
    }

    return lst;
}

/* Unpack a multi-dimensional array into a nested list.
   Assumption: ndim >= 1. */
static PyObject *
tolist_rec(PyMemoryViewObject *self, const char *ptr, Py_ssize_t ndim, const Py_ssize_t *shape,
           const Py_ssize_t *strides, const Py_ssize_t *suboffsets,
           const char *fmt)
{
    PyObject *lst, *item;
    Py_ssize_t i;

    assert(ndim >= 1);
    assert(shape != NULL);
    assert(strides != NULL);

    if (ndim == 1)
        return tolist_base(self, ptr, shape, strides, suboffsets, fmt);

    lst = PyList_New(shape[0]);
    if (lst == NULL)
        return NULL;

    for (i = 0; i < shape[0]; ptr+=strides[0], i++) {
        const char *xptr = ADJUST_PTR(ptr, suboffsets, 0);
        item = tolist_rec(self, xptr, ndim-1, shape+1,
                          strides+1, suboffsets ? suboffsets+1 : NULL,
                          fmt);
        if (item == NULL) {
            Py_DECREF(lst);
            return NULL;
        }
        PyList_SET_ITEM(lst, i, item);
    }

    return lst;
}

/* Return a list representation of the memoryview. Currently only buffers
   with native format strings are supported. */
/*[clinic input]
memoryview.tolist

Return the data in the buffer as a list of elements.
[clinic start generated code]*/

static PyObject *
memoryview_tolist_impl(PyMemoryViewObject *self)
/*[clinic end generated code: output=a6cda89214fd5a1b input=21e7d0c1860b211a]*/
{
    const Py_buffer *view = &self->view;
    const char *fmt;

    CHECK_RELEASED(self);

    fmt = adjust_fmt(view);
    if (fmt == NULL)
        return NULL;
    if (view->ndim == 0) {
        return unpack_single(self, view->buf, fmt);
    }
    else if (view->ndim == 1) {
        return tolist_base(self, view->buf, view->shape,
                           view->strides, view->suboffsets,
                           fmt);
    }
    else {
        return tolist_rec(self, view->buf, view->ndim, view->shape,
                          view->strides, view->suboffsets,
                          fmt);
    }
}

/*[clinic input]
memoryview.tobytes

    order: str(accept={str, NoneType}, c_default="NULL") = 'C'

Return the data in the buffer as a byte string.

Order can be {'C', 'F', 'A'}. When order is 'C' or 'F', the data of the
original array is converted to C or Fortran order. For contiguous views,
'A' returns an exact copy of the physical memory. In particular, in-memory
Fortran order is preserved. For non-contiguous views, the data is converted
to C first. order=None is the same as order='C'.
[clinic start generated code]*/

static PyObject *
memoryview_tobytes_impl(PyMemoryViewObject *self, const char *order)
/*[clinic end generated code: output=1288b62560a32a23 input=0efa3ddaeda573a8]*/
{
    Py_buffer *src = VIEW_ADDR(self);
    char ord = 'C';
    PyObject *bytes;

    CHECK_RELEASED(self);

    if (order) {
        if (strcmp(order, "F") == 0) {
            ord = 'F';
        }
        else if (strcmp(order, "A") == 0) {
            ord = 'A';
        }
        else if (strcmp(order, "C") != 0) {
            PyErr_SetString(PyExc_ValueError,
                "order must be 'C', 'F' or 'A'");
            return NULL;
        }
    }

    bytes = PyBytes_FromStringAndSize(NULL, src->len);
    if (bytes == NULL)
        return NULL;

    if (PyBuffer_ToContiguous(PyBytes_AS_STRING(bytes), src, src->len, ord) < 0) {
        Py_DECREF(bytes);
        return NULL;
    }

    return bytes;
}

/*[clinic input]
memoryview.hex

    sep: object = NULL
        An optional single character or byte to separate hex bytes.
    bytes_per_sep: int = 1
        How many bytes between separators.  Positive values count from the
        right, negative values count from the left.

Return the data in the buffer as a str of hexadecimal numbers.

Example:
>>> value = memoryview(b'\xb9\x01\xef')
>>> value.hex()
'b901ef'
>>> value.hex(':')
'b9:01:ef'
>>> value.hex(':', 2)
'b9:01ef'
>>> value.hex(':', -2)
'b901:ef'
[clinic start generated code]*/

static PyObject *
memoryview_hex_impl(PyMemoryViewObject *self, PyObject *sep,
                    int bytes_per_sep)
/*[clinic end generated code: output=430ca760f94f3ca7 input=539f6a3a5fb56946]*/
{
    Py_buffer *src = VIEW_ADDR(self);
    PyObject *bytes;
    PyObject *ret;

    CHECK_RELEASED(self);

    if (MV_C_CONTIGUOUS(self->flags)) {
        return _Py_strhex_with_sep(src->buf, src->len, sep, bytes_per_sep);
    }

    bytes = PyBytes_FromStringAndSize(NULL, src->len);
    if (bytes == NULL)
        return NULL;

    if (PyBuffer_ToContiguous(PyBytes_AS_STRING(bytes), src, src->len, 'C') < 0) {
        Py_DECREF(bytes);
        return NULL;
    }

    ret = _Py_strhex_with_sep(
            PyBytes_AS_STRING(bytes), PyBytes_GET_SIZE(bytes),
            sep, bytes_per_sep);
    Py_DECREF(bytes);

    return ret;
}

static PyObject *
memory_repr(PyMemoryViewObject *self)
{
    if (self->flags & _Py_MEMORYVIEW_RELEASED)
        return PyUnicode_FromFormat("<released memory at %p>", self);
    else
        return PyUnicode_FromFormat("<memory at %p>", self);
}


/**************************************************************************/
/*                          Indexing and slicing                          */
/**************************************************************************/

static char *
lookup_dimension(const Py_buffer *view, char *ptr, int dim, Py_ssize_t index)
{
    Py_ssize_t nitems; /* items in the given dimension */

    assert(view->shape);
    assert(view->strides);

    nitems = view->shape[dim];
    if (index < 0) {
        index += nitems;
    }
    if (index < 0 || index >= nitems) {
        PyErr_Format(PyExc_IndexError,
                     "index out of bounds on dimension %d", dim + 1);
        return NULL;
    }

    ptr += view->strides[dim] * index;

    ptr = ADJUST_PTR(ptr, view->suboffsets, dim);

    return ptr;
}

/* Get the pointer to the item at index. */
static char *
ptr_from_index(const Py_buffer *view, Py_ssize_t index)
{
    char *ptr = (char *)view->buf;
    return lookup_dimension(view, ptr, 0, index);
}

/* Get the pointer to the item at tuple. */
static char *
ptr_from_tuple(const Py_buffer *view, PyObject *tup)
{
    char *ptr = (char *)view->buf;
    Py_ssize_t dim, nindices = PyTuple_GET_SIZE(tup);

    if (nindices > view->ndim) {
        PyErr_Format(PyExc_TypeError,
                     "cannot index %zd-dimension view with %zd-element tuple",
                     view->ndim, nindices);
        return NULL;
    }

    for (dim = 0; dim < nindices; dim++) {
        Py_ssize_t index;
        index = PyNumber_AsSsize_t(PyTuple_GET_ITEM(tup, dim),
                                   PyExc_IndexError);
        if (index == -1 && PyErr_Occurred())
            return NULL;
        ptr = lookup_dimension(view, ptr, (int)dim, index);
        if (ptr == NULL)
            return NULL;
    }
    return ptr;
}

/* Return the item at index. In a one-dimensional view, this is an object
   with the type specified by view->format. Otherwise, the item is a sub-view.
   The function is used in memory_subscript() and memory_as_sequence. */
static PyObject *
memory_item(PyMemoryViewObject *self, Py_ssize_t index)
{
    Py_buffer *view = &(self->view);
    const char *fmt;

    CHECK_RELEASED(self);

    fmt = adjust_fmt(view);
    if (fmt == NULL)
        return NULL;

    if (view->ndim == 0) {
        PyErr_SetString(PyExc_TypeError, "invalid indexing of 0-dim memory");
        return NULL;
    }
    if (view->ndim == 1) {
        char *ptr = ptr_from_index(view, index);
        if (ptr == NULL)
            return NULL;
        return unpack_single(self, ptr, fmt);
    }

    PyErr_SetString(PyExc_NotImplementedError,
        "multi-dimensional sub-views are not implemented");
    return NULL;
}

/* Return the item at position *key* (a tuple of indices). */
static PyObject *
memory_item_multi(PyMemoryViewObject *self, PyObject *tup)
{
    Py_buffer *view = &(self->view);
    const char *fmt;
    Py_ssize_t nindices = PyTuple_GET_SIZE(tup);
    char *ptr;

    CHECK_RELEASED(self);

    fmt = adjust_fmt(view);
    if (fmt == NULL)
        return NULL;

    if (nindices < view->ndim) {
        PyErr_SetString(PyExc_NotImplementedError,
                        "sub-views are not implemented");
        return NULL;
    }
    ptr = ptr_from_tuple(view, tup);
    if (ptr == NULL)
        return NULL;
    return unpack_single(self, ptr, fmt);
}

static inline int
init_slice(Py_buffer *base, PyObject *key, int dim)
{
    Py_ssize_t start, stop, step, slicelength;

    if (PySlice_Unpack(key, &start, &stop, &step) < 0) {
        return -1;
    }
    slicelength = PySlice_AdjustIndices(base->shape[dim], &start, &stop, step);


    if (base->suboffsets == NULL || dim == 0) {
    adjust_buf:
        base->buf = (char *)base->buf + base->strides[dim] * start;
    }
    else {
        Py_ssize_t n = dim-1;
        while (n >= 0 && base->suboffsets[n] < 0)
            n--;
        if (n < 0)
            goto adjust_buf; /* all suboffsets are negative */
        base->suboffsets[n] = base->suboffsets[n] + base->strides[dim] * start;
    }
    base->shape[dim] = slicelength;
    base->strides[dim] = base->strides[dim] * step;

    return 0;
}

static int
is_multislice(PyObject *key)
{
    Py_ssize_t size, i;

    if (!PyTuple_Check(key))
        return 0;
    size = PyTuple_GET_SIZE(key);
    if (size == 0)
        return 0;

    for (i = 0; i < size; i++) {
        PyObject *x = PyTuple_GET_ITEM(key, i);
        if (!PySlice_Check(x))
            return 0;
    }
    return 1;
}

static Py_ssize_t
is_multiindex(PyObject *key)
{
    Py_ssize_t size, i;

    if (!PyTuple_Check(key))
        return 0;
    size = PyTuple_GET_SIZE(key);
    for (i = 0; i < size; i++) {
        PyObject *x = PyTuple_GET_ITEM(key, i);
        if (!_PyIndex_Check(x)) {
            return 0;
        }
    }
    return 1;
}

/* mv[obj] returns an object holding the data for one element if obj
   fully indexes the memoryview or another memoryview object if it
   does not.

   0-d memoryview objects can be referenced using mv[...] or mv[()]
   but not with anything else. */
static PyObject *
memory_subscript(PyMemoryViewObject *self, PyObject *key)
{
    Py_buffer *view;
    view = &(self->view);

    CHECK_RELEASED(self);

    if (view->ndim == 0) {
        if (PyTuple_Check(key) && PyTuple_GET_SIZE(key) == 0) {
            const char *fmt = adjust_fmt(view);
            if (fmt == NULL)
                return NULL;
            return unpack_single(self, view->buf, fmt);
        }
        else if (key == Py_Ellipsis) {
            Py_INCREF(self);
            return (PyObject *)self;
        }
        else {
            PyErr_SetString(PyExc_TypeError,
                "invalid indexing of 0-dim memory");
            return NULL;
        }
    }

    if (_PyIndex_Check(key)) {
        Py_ssize_t index;
        index = PyNumber_AsSsize_t(key, PyExc_IndexError);
        if (index == -1 && PyErr_Occurred())
            return NULL;
        return memory_item(self, index);
    }
    else if (PySlice_Check(key)) {
        PyMemoryViewObject *sliced;

        sliced = (PyMemoryViewObject *)mbuf_add_view(self->mbuf, view);
        if (sliced == NULL)
            return NULL;

        if (init_slice(&sliced->view, key, 0) < 0) {
            Py_DECREF(sliced);
            return NULL;
        }
        init_len(&sliced->view);
        init_flags(sliced);

        return (PyObject *)sliced;
    }
    else if (is_multiindex(key)) {
        return memory_item_multi(self, key);
    }
    else if (is_multislice(key)) {
        PyErr_SetString(PyExc_NotImplementedError,
            "multi-dimensional slicing is not implemented");
        return NULL;
    }

    PyErr_SetString(PyExc_TypeError, "memoryview: invalid slice key");
    return NULL;
}

static int
memory_ass_sub(PyMemoryViewObject *self, PyObject *key, PyObject *value)
{
    Py_buffer *view = &(self->view);
    Py_buffer src;
    const char *fmt;
    char *ptr;

    CHECK_RELEASED_INT(self);

    fmt = adjust_fmt(view);
    if (fmt == NULL)
        return -1;

    if (view->readonly) {
        PyErr_SetString(PyExc_TypeError, "cannot modify read-only memory");
        return -1;
    }
    if (value == NULL) {
        PyErr_SetString(PyExc_TypeError, "cannot delete memory");
        return -1;
    }
    if (view->ndim == 0) {
        if (key == Py_Ellipsis ||
            (PyTuple_Check(key) && PyTuple_GET_SIZE(key)==0)) {
            ptr = (char *)view->buf;
            return pack_single(self, ptr, value, fmt);
        }
        else {
            PyErr_SetString(PyExc_TypeError,
                "invalid indexing of 0-dim memory");
            return -1;
        }
    }

    if (_PyIndex_Check(key)) {
        Py_ssize_t index;
        if (1 < view->ndim) {
            PyErr_SetString(PyExc_NotImplementedError,
                            "sub-views are not implemented");
            return -1;
        }
        index = PyNumber_AsSsize_t(key, PyExc_IndexError);
        if (index == -1 && PyErr_Occurred())
            return -1;
        ptr = ptr_from_index(view, index);
        if (ptr == NULL)
            return -1;
        return pack_single(self, ptr, value, fmt);
    }
    /* one-dimensional: fast path */
    if (PySlice_Check(key) && view->ndim == 1) {
        Py_buffer dest; /* sliced view */
        Py_ssize_t arrays[3];
        int ret = -1;

        /* rvalue must be an exporter */
        if (PyObject_GetBuffer(value, &src, PyBUF_FULL_RO) < 0)
            return ret;

        dest = *view;
        dest.shape = &arrays[0]; dest.shape[0] = view->shape[0];
        dest.strides = &arrays[1]; dest.strides[0] = view->strides[0];
        if (view->suboffsets) {
            dest.suboffsets = &arrays[2]; dest.suboffsets[0] = view->suboffsets[0];
        }

        if (init_slice(&dest, key, 0) < 0)
            goto end_block;
        dest.len = dest.shape[0] * dest.itemsize;

        ret = copy_single(self, &dest, &src);

    end_block:
        PyBuffer_Release(&src);
        return ret;
    }
    if (is_multiindex(key)) {
        char *ptr;
        if (PyTuple_GET_SIZE(key) < view->ndim) {
            PyErr_SetString(PyExc_NotImplementedError,
                            "sub-views are not implemented");
            return -1;
        }
        ptr = ptr_from_tuple(view, key);
        if (ptr == NULL)
            return -1;
        return pack_single(self, ptr, value, fmt);
    }
    if (PySlice_Check(key) || is_multislice(key)) {
        /* Call memory_subscript() to produce a sliced lvalue, then copy
           rvalue into lvalue. This is already implemented in _testbuffer.c. */
        PyErr_SetString(PyExc_NotImplementedError,
            "memoryview slice assignments are currently restricted "
            "to ndim = 1");
        return -1;
    }

    PyErr_SetString(PyExc_TypeError, "memoryview: invalid slice key");
    return -1;
}

static Py_ssize_t
memory_length(PyMemoryViewObject *self)
{
    CHECK_RELEASED_INT(self);
    return self->view.ndim == 0 ? 1 : self->view.shape[0];
}

/* As mapping */
static PyMappingMethods memory_as_mapping = {
    (lenfunc)memory_length,               /* mp_length */
    (binaryfunc)memory_subscript,         /* mp_subscript */
    (objobjargproc)memory_ass_sub,        /* mp_ass_subscript */
};

/* As sequence */
static PySequenceMethods memory_as_sequence = {
        (lenfunc)memory_length,           /* sq_length */
        0,                                /* sq_concat */
        0,                                /* sq_repeat */
        (ssizeargfunc)memory_item,        /* sq_item */
};


/**************************************************************************/
/*                             Comparisons                                */
/**************************************************************************/

#define MV_COMPARE_EX -1       /* exception */
#define MV_COMPARE_NOT_IMPL -2 /* not implemented */

/* Translate a StructError to "not equal". Preserve other exceptions. */
static int
fix_struct_error_int(void)
{
    assert(PyErr_Occurred());
    /* XXX Cannot get at StructError directly? */
    if (PyErr_ExceptionMatches(PyExc_ImportError) ||
        PyErr_ExceptionMatches(PyExc_MemoryError)) {
        return MV_COMPARE_EX;
    }
    /* StructError: invalid or unknown format -> not equal */
    PyErr_Clear();
    return 0;
}

/* Unpack and compare single items of p and q using the struct module. */
static int
struct_unpack_cmp(const char *p, const char *q,
                  struct unpacker *unpack_p, struct unpacker *unpack_q)
{
    PyObject *v, *w;
    int ret;

    /* At this point any exception from the struct module should not be
       StructError, since both formats have been accepted already. */
    v = struct_unpack_single(p, unpack_p);
    if (v == NULL)
        return MV_COMPARE_EX;

    w = struct_unpack_single(q, unpack_q);
    if (w == NULL) {
        Py_DECREF(v);
        return MV_COMPARE_EX;
    }

    /* MV_COMPARE_EX == -1: exceptions are preserved */
    ret = PyObject_RichCompareBool(v, w, Py_EQ);
    Py_DECREF(v);
    Py_DECREF(w);

    return ret;
}

/* Unpack and compare single items of p and q. If both p and q have the same
   single element native format, the comparison uses a fast path (gcc creates
   a jump table and converts memcpy into simple assignments on x86/x64).

   Otherwise, the comparison is delegated to the struct module, which is
   30-60x slower. */
#define CMP_SINGLE(p, q, type) \
    do {                                 \
        type x;                          \
        type y;                          \
        memcpy((char *)&x, p, sizeof x); \
        memcpy((char *)&y, q, sizeof y); \
        equal = (x == y);                \
    } while (0)

static inline int
unpack_cmp(const char *p, const char *q, char fmt,
           struct unpacker *unpack_p, struct unpacker *unpack_q)
{
    int equal;

    switch (fmt) {

    /* signed integers and fast path for 'B' */
    case 'B': return *((const unsigned char *)p) == *((const unsigned char *)q);
    case 'b': return *((const signed char *)p) == *((const signed char *)q);
    case 'h': CMP_SINGLE(p, q, short); return equal;
    case 'i': CMP_SINGLE(p, q, int); return equal;
    case 'l': CMP_SINGLE(p, q, long); return equal;

    /* boolean */
    case '?': CMP_SINGLE(p, q, _Bool); return equal;

    /* unsigned integers */
    case 'H': CMP_SINGLE(p, q, unsigned short); return equal;
    case 'I': CMP_SINGLE(p, q, unsigned int); return equal;
    case 'L': CMP_SINGLE(p, q, unsigned long); return equal;

    /* native 64-bit */
    case 'q': CMP_SINGLE(p, q, long long); return equal;
    case 'Q': CMP_SINGLE(p, q, unsigned long long); return equal;

    /* ssize_t and size_t */
    case 'n': CMP_SINGLE(p, q, Py_ssize_t); return equal;
    case 'N': CMP_SINGLE(p, q, size_t); return equal;

    /* floats */
    /* XXX DBL_EPSILON? */
    case 'f': CMP_SINGLE(p, q, float); return equal;
    case 'd': CMP_SINGLE(p, q, double); return equal;
    case 'e': {
#if PY_LITTLE_ENDIAN
        int endian = 1;
#else
        int endian = 0;
#endif
        /* Note: PyFloat_Unpack2 should never fail */
        double u = PyFloat_Unpack2(p, endian);
        double v = PyFloat_Unpack2(q, endian);
        return (u == v);
    }

    /* bytes object */
    case 'c': return *p == *q;

    /* pointer */
    case 'P': CMP_SINGLE(p, q, void *); return equal;

    /* use the struct module */
    case '_':
        assert(unpack_p);
        assert(unpack_q);
        return struct_unpack_cmp(p, q, unpack_p, unpack_q);
    }

    /* NOT REACHED */
    PyErr_SetString(PyExc_RuntimeError,
        "memoryview: internal error in richcompare");
    return MV_COMPARE_EX;
}

/* Base case for recursive array comparisons. Assumption: ndim == 1. */
static int
cmp_base(const char *p, const char *q, const Py_ssize_t *shape,
         const Py_ssize_t *pstrides, const Py_ssize_t *psuboffsets,
         const Py_ssize_t *qstrides, const Py_ssize_t *qsuboffsets,
         char fmt, struct unpacker *unpack_p, struct unpacker *unpack_q)
{
    Py_ssize_t i;
    int equal;

    for (i = 0; i < shape[0]; p+=pstrides[0], q+=qstrides[0], i++) {
        const char *xp = ADJUST_PTR(p, psuboffsets, 0);
        const char *xq = ADJUST_PTR(q, qsuboffsets, 0);
        equal = unpack_cmp(xp, xq, fmt, unpack_p, unpack_q);
        if (equal <= 0)
            return equal;
    }

    return 1;
}

/* Recursively compare two multi-dimensional arrays that have the same
   logical structure. Assumption: ndim >= 1. */
static int
cmp_rec(const char *p, const char *q,
        Py_ssize_t ndim, const Py_ssize_t *shape,
        const Py_ssize_t *pstrides, const Py_ssize_t *psuboffsets,
        const Py_ssize_t *qstrides, const Py_ssize_t *qsuboffsets,
        char fmt, struct unpacker *unpack_p, struct unpacker *unpack_q)
{
    Py_ssize_t i;
    int equal;

    assert(ndim >= 1);
    assert(shape != NULL);
    assert(pstrides != NULL);
    assert(qstrides != NULL);

    if (ndim == 1) {
        return cmp_base(p, q, shape,
                        pstrides, psuboffsets,
                        qstrides, qsuboffsets,
                        fmt, unpack_p, unpack_q);
    }

    for (i = 0; i < shape[0]; p+=pstrides[0], q+=qstrides[0], i++) {
        const char *xp = ADJUST_PTR(p, psuboffsets, 0);
        const char *xq = ADJUST_PTR(q, qsuboffsets, 0);
        equal = cmp_rec(xp, xq, ndim-1, shape+1,
                        pstrides+1, psuboffsets ? psuboffsets+1 : NULL,
                        qstrides+1, qsuboffsets ? qsuboffsets+1 : NULL,
                        fmt, unpack_p, unpack_q);
        if (equal <= 0)
            return equal;
    }

    return 1;
}

static PyObject *
memory_richcompare(PyObject *v, PyObject *w, int op)
{
    PyObject *res;
    Py_buffer wbuf, *vv;
    Py_buffer *ww = NULL;
    struct unpacker *unpack_v = NULL;
    struct unpacker *unpack_w = NULL;
    char vfmt, wfmt;
    int equal = MV_COMPARE_NOT_IMPL;

    if (op != Py_EQ && op != Py_NE)
        goto result; /* Py_NotImplemented */

    assert(PyMemoryView_Check(v));
    if (BASE_INACCESSIBLE(v)) {
        equal = (v == w);
        goto result;
    }
    vv = VIEW_ADDR(v);

    if (PyMemoryView_Check(w)) {
        if (BASE_INACCESSIBLE(w)) {
            equal = (v == w);
            goto result;
        }
        ww = VIEW_ADDR(w);
    }
    else {
        if (PyObject_GetBuffer(w, &wbuf, PyBUF_FULL_RO) < 0) {
            PyErr_Clear();
            goto result; /* Py_NotImplemented */
        }
        ww = &wbuf;
    }

    if (!equiv_shape(vv, ww)) {
        PyErr_Clear();
        equal = 0;
        goto result;
    }

    /* Use fast unpacking for identical primitive C type formats. */
    if (get_native_fmtchar(&vfmt, vv->format) < 0)
        vfmt = '_';
    if (get_native_fmtchar(&wfmt, ww->format) < 0)
        wfmt = '_';
    if (vfmt == '_' || wfmt == '_' || vfmt != wfmt) {
        /* Use struct module unpacking. NOTE: Even for equal format strings,
           memcmp() cannot be used for item comparison since it would give
           incorrect results in the case of NaNs or uninitialized padding
           bytes. */
        vfmt = '_';
        unpack_v = struct_get_unpacker(vv->format, vv->itemsize);
        if (unpack_v == NULL) {
            equal = fix_struct_error_int();
            goto result;
        }
        unpack_w = struct_get_unpacker(ww->format, ww->itemsize);
        if (unpack_w == NULL) {
            equal = fix_struct_error_int();
            goto result;
        }
    }

    if (vv->ndim == 0) {
        equal = unpack_cmp(vv->buf, ww->buf,
                           vfmt, unpack_v, unpack_w);
    }
    else if (vv->ndim == 1) {
        equal = cmp_base(vv->buf, ww->buf, vv->shape,
                         vv->strides, vv->suboffsets,
                         ww->strides, ww->suboffsets,
                         vfmt, unpack_v, unpack_w);
    }
    else {
        equal = cmp_rec(vv->buf, ww->buf, vv->ndim, vv->shape,
                        vv->strides, vv->suboffsets,
                        ww->strides, ww->suboffsets,
                        vfmt, unpack_v, unpack_w);
    }

result:
    if (equal < 0) {
        if (equal == MV_COMPARE_NOT_IMPL)
            res = Py_NotImplemented;
        else /* exception */
            res = NULL;
    }
    else if ((equal && op == Py_EQ) || (!equal && op == Py_NE))
        res = Py_True;
    else
        res = Py_False;

    if (ww == &wbuf)
        PyBuffer_Release(ww);

    unpacker_free(unpack_v);
    unpacker_free(unpack_w);

    Py_XINCREF(res);
    return res;
}

/**************************************************************************/
/*                                Hash                                    */
/**************************************************************************/

static Py_hash_t
memory_hash(PyMemoryViewObject *self)
{
    if (self->hash == -1) {
        Py_buffer *view = &self->view;
        char *mem = view->buf;
        Py_ssize_t ret;
        char fmt;

        CHECK_RELEASED_INT(self);

        if (!view->readonly) {
            PyErr_SetString(PyExc_ValueError,
                "cannot hash writable memoryview object");
            return -1;
        }
        ret = get_native_fmtchar(&fmt, view->format);
        if (ret < 0 || !IS_BYTE_FORMAT(fmt)) {
            PyErr_SetString(PyExc_ValueError,
                "memoryview: hashing is restricted to formats 'B', 'b' or 'c'");
            return -1;
        }
        if (view->obj != NULL && PyObject_Hash(view->obj) == -1) {
            /* Keep the original error message */
            return -1;
        }

        if (!MV_C_CONTIGUOUS(self->flags)) {
            mem = PyMem_Malloc(view->len);
            if (mem == NULL) {
                PyErr_NoMemory();
                return -1;
            }
            if (buffer_to_contiguous(mem, view, 'C') < 0) {
                PyMem_Free(mem);
                return -1;
            }
        }

        /* Can't fail */
        self->hash = _Py_HashBytes(mem, view->len);

        if (mem != view->buf)
            PyMem_Free(mem);
    }

    return self->hash;
}


/**************************************************************************/
/*                                 getters                                */
/**************************************************************************/

static PyObject *
_IntTupleFromSsizet(int len, Py_ssize_t *vals)
{
    int i;
    PyObject *o;
    PyObject *intTuple;

    if (vals == NULL)
        return PyTuple_New(0);

    intTuple = PyTuple_New(len);
    if (!intTuple)
        return NULL;
    for (i=0; i<len; i++) {
        o = PyLong_FromSsize_t(vals[i]);
        if (!o) {
            Py_DECREF(intTuple);
            return NULL;
        }
        PyTuple_SET_ITEM(intTuple, i, o);
    }
    return intTuple;
}

static PyObject *
memory_obj_get(PyMemoryViewObject *self, void *Py_UNUSED(ignored))
{
    Py_buffer *view = &self->view;

    CHECK_RELEASED(self);
    if (view->obj == NULL) {
        Py_RETURN_NONE;
    }
    Py_INCREF(view->obj);
    return view->obj;
}

static PyObject *
memory_nbytes_get(PyMemoryViewObject *self, void *Py_UNUSED(ignored))
{
    CHECK_RELEASED(self);
    return PyLong_FromSsize_t(self->view.len);
}

static PyObject *
memory_format_get(PyMemoryViewObject *self, void *Py_UNUSED(ignored))
{
    CHECK_RELEASED(self);
    return PyUnicode_FromString(self->view.format);
}

static PyObject *
memory_itemsize_get(PyMemoryViewObject *self, void *Py_UNUSED(ignored))
{
    CHECK_RELEASED(self);
    return PyLong_FromSsize_t(self->view.itemsize);
}

static PyObject *
memory_shape_get(PyMemoryViewObject *self, void *Py_UNUSED(ignored))
{
    CHECK_RELEASED(self);
    return _IntTupleFromSsizet(self->view.ndim, self->view.shape);
}

static PyObject *
memory_strides_get(PyMemoryViewObject *self, void *Py_UNUSED(ignored))
{
    CHECK_RELEASED(self);
    return _IntTupleFromSsizet(self->view.ndim, self->view.strides);
}

static PyObject *
memory_suboffsets_get(PyMemoryViewObject *self, void *Py_UNUSED(ignored))
{
    CHECK_RELEASED(self);
    return _IntTupleFromSsizet(self->view.ndim, self->view.suboffsets);
}

static PyObject *
memory_readonly_get(PyMemoryViewObject *self, void *Py_UNUSED(ignored))
{
    CHECK_RELEASED(self);
    return PyBool_FromLong(self->view.readonly);
}

static PyObject *
memory_ndim_get(PyMemoryViewObject *self, void *Py_UNUSED(ignored))
{
    CHECK_RELEASED(self);
    return PyLong_FromLong(self->view.ndim);
}

static PyObject *
memory_c_contiguous(PyMemoryViewObject *self, PyObject *dummy)
{
    CHECK_RELEASED(self);
    return PyBool_FromLong(MV_C_CONTIGUOUS(self->flags));
}

static PyObject *
memory_f_contiguous(PyMemoryViewObject *self, PyObject *dummy)
{
    CHECK_RELEASED(self);
    return PyBool_FromLong(MV_F_CONTIGUOUS(self->flags));
}

static PyObject *
memory_contiguous(PyMemoryViewObject *self, PyObject *dummy)
{
    CHECK_RELEASED(self);
    return PyBool_FromLong(MV_ANY_CONTIGUOUS(self->flags));
}

PyDoc_STRVAR(memory_obj_doc,
             "The underlying object of the memoryview.");
PyDoc_STRVAR(memory_nbytes_doc,
             "The amount of space in bytes that the array would use in\n"
             " a contiguous representation.");
PyDoc_STRVAR(memory_readonly_doc,
             "A bool indicating whether the memory is read only.");
PyDoc_STRVAR(memory_itemsize_doc,
             "The size in bytes of each element of the memoryview.");
PyDoc_STRVAR(memory_format_doc,
             "A string containing the format (in struct module style)\n"
             " for each element in the view.");
PyDoc_STRVAR(memory_ndim_doc,
             "An integer indicating how many dimensions of a multi-dimensional\n"
             " array the memory represents.");
PyDoc_STRVAR(memory_shape_doc,
             "A tuple of ndim integers giving the shape of the memory\n"
             " as an N-dimensional array.");
PyDoc_STRVAR(memory_strides_doc,
             "A tuple of ndim integers giving the size in bytes to access\n"
             " each element for each dimension of the array.");
PyDoc_STRVAR(memory_suboffsets_doc,
             "A tuple of integers used internally for PIL-style arrays.");
PyDoc_STRVAR(memory_c_contiguous_doc,
             "A bool indicating whether the memory is C contiguous.");
PyDoc_STRVAR(memory_f_contiguous_doc,
             "A bool indicating whether the memory is Fortran contiguous.");
PyDoc_STRVAR(memory_contiguous_doc,
             "A bool indicating whether the memory is contiguous.");


static PyGetSetDef memory_getsetlist[] = {
    {"obj",             (getter)memory_obj_get,        NULL, memory_obj_doc},
    {"nbytes",          (getter)memory_nbytes_get,     NULL, memory_nbytes_doc},
    {"readonly",        (getter)memory_readonly_get,   NULL, memory_readonly_doc},
    {"itemsize",        (getter)memory_itemsize_get,   NULL, memory_itemsize_doc},
    {"format",          (getter)memory_format_get,     NULL, memory_format_doc},
    {"ndim",            (getter)memory_ndim_get,       NULL, memory_ndim_doc},
    {"shape",           (getter)memory_shape_get,      NULL, memory_shape_doc},
    {"strides",         (getter)memory_strides_get,    NULL, memory_strides_doc},
    {"suboffsets",      (getter)memory_suboffsets_get, NULL, memory_suboffsets_doc},
    {"c_contiguous",    (getter)memory_c_contiguous,   NULL, memory_c_contiguous_doc},
    {"f_contiguous",    (getter)memory_f_contiguous,   NULL, memory_f_contiguous_doc},
    {"contiguous",      (getter)memory_contiguous,     NULL, memory_contiguous_doc},
    {NULL, NULL, NULL, NULL},
};


static PyMethodDef memory_methods[] = {
    MEMORYVIEW_RELEASE_METHODDEF
    MEMORYVIEW_TOBYTES_METHODDEF
    MEMORYVIEW_HEX_METHODDEF
    MEMORYVIEW_TOLIST_METHODDEF
    MEMORYVIEW_CAST_METHODDEF
    MEMORYVIEW_TOREADONLY_METHODDEF
    {"__enter__",   memory_enter, METH_NOARGS, NULL},
    {"__exit__",    memory_exit, METH_VARARGS, NULL},
    {NULL,          NULL}
};

/**************************************************************************/
/*                          Memoryview Iterator                           */
/**************************************************************************/

PyTypeObject _PyMemoryIter_Type;

typedef struct {
    PyObject_HEAD
    Py_ssize_t it_index;
    PyMemoryViewObject *it_seq; // Set to NULL when iterator is exhausted
    Py_ssize_t it_length;
    const char *it_fmt;
} memoryiterobject;

static void
memoryiter_dealloc(memoryiterobject *it)
{
    _PyObject_GC_UNTRACK(it);
    Py_XDECREF(it->it_seq);
    PyObject_GC_Del(it);
}

static int
memoryiter_traverse(memoryiterobject *it, visitproc visit, void *arg)
{
    Py_VISIT(it->it_seq);
    return 0;
}

static PyObject *
memoryiter_next(memoryiterobject *it)
{
    PyMemoryViewObject *seq;
    seq = it->it_seq;
    if (seq == NULL) {
        return NULL;
    }

    if (it->it_index < it->it_length) {
        CHECK_RELEASED(seq);
        Py_buffer *view = &(seq->view);
        char *ptr = (char *)seq->view.buf;

        ptr += view->strides[0] * it->it_index++;
        ptr = ADJUST_PTR(ptr, view->suboffsets, 0);
        if (ptr == NULL) {
            return NULL;
        }
        return unpack_single(seq, ptr, it->it_fmt);
    }

    it->it_seq = NULL;
    Py_DECREF(seq);
    return NULL;
}

static PyObject *
memory_iter(PyObject *seq)
{
    if (!PyMemoryView_Check(seq)) {
        PyErr_BadInternalCall();
        return NULL;
    }
    PyMemoryViewObject *obj = (PyMemoryViewObject *)seq;
    int ndims = obj->view.ndim;
    if (ndims == 0) {
        PyErr_SetString(PyExc_TypeError, "invalid indexing of 0-dim memory");
        return NULL;
    }
    if (ndims != 1) {
        PyErr_SetString(PyExc_NotImplementedError,
            "multi-dimensional sub-views are not implemented");
        return NULL;
    }

    const char *fmt = adjust_fmt(&obj->view);
    if (fmt == NULL) {
        return NULL;
    }

    memoryiterobject *it;
    it = PyObject_GC_New(memoryiterobject, &_PyMemoryIter_Type);
    if (it == NULL) {
        return NULL;
    }
    it->it_fmt = fmt;
    it->it_length = memory_length(obj);
    it->it_index = 0;
    Py_INCREF(seq);
    it->it_seq = obj;
    _PyObject_GC_TRACK(it);
    return (PyObject *)it;
}

PyTypeObject _PyMemoryIter_Type = {
    PyVarObject_HEAD_INIT(&PyType_Type, 0)
    .tp_name = "memory_iterator",
    .tp_basicsize = sizeof(memoryiterobject),
    // methods
    .tp_dealloc = (destructor)memoryiter_dealloc,
    .tp_getattro = PyObject_GenericGetAttr,
    .tp_flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC,
    .tp_traverse = (traverseproc)memoryiter_traverse,
    .tp_iter = PyObject_SelfIter,
    .tp_iternext = (iternextfunc)memoryiter_next,
};

PyTypeObject PyMemoryView_Type = {
    PyVarObject_HEAD_INIT(&PyType_Type, 0)
    "memoryview",                             /* tp_name */
    offsetof(PyMemoryViewObject, ob_array),   /* tp_basicsize */
    sizeof(Py_ssize_t),                       /* tp_itemsize */
    (destructor)memory_dealloc,               /* tp_dealloc */
    0,                                        /* tp_vectorcall_offset */
    0,                                        /* tp_getattr */
    0,                                        /* tp_setattr */
    0,                                        /* tp_as_async */
    (reprfunc)memory_repr,                    /* tp_repr */
    0,                                        /* tp_as_number */
    &memory_as_sequence,                      /* tp_as_sequence */
    &memory_as_mapping,                       /* tp_as_mapping */
    (hashfunc)memory_hash,                    /* tp_hash */
    0,                                        /* tp_call */
    0,                                        /* tp_str */
    PyObject_GenericGetAttr,                  /* tp_getattro */
    0,                                        /* tp_setattro */
    &memory_as_buffer,                        /* tp_as_buffer */
    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC |
       Py_TPFLAGS_SEQUENCE,                   /* tp_flags */
    memoryview__doc__,                        /* tp_doc */
    (traverseproc)memory_traverse,            /* tp_traverse */
    (inquiry)memory_clear,                    /* tp_clear */
    memory_richcompare,                       /* tp_richcompare */
    offsetof(PyMemoryViewObject, weakreflist),/* tp_weaklistoffset */
    memory_iter,                              /* tp_iter */
    0,                                        /* tp_iternext */
    memory_methods,                           /* tp_methods */
    0,                                        /* tp_members */
    memory_getsetlist,                        /* tp_getset */
    0,                                        /* tp_base */
    0,                                        /* tp_dict */
    0,                                        /* tp_descr_get */
    0,                                        /* tp_descr_set */
    0,                                        /* tp_dictoffset */
    0,                                        /* tp_init */
    0,                                        /* tp_alloc */
    memoryview,                               /* tp_new */
};