1
  2
  3
  4
  5
  6
  7
  8
  9
 10
 11
 12
 13
 14
 15
 16
 17
 18
 19
 20
 21
 22
 23
 24
 25
 26
 27
 28
 29
 30
 31
 32
 33
 34
 35
 36
 37
 38
 39
 40
 41
 42
 43
 44
 45
 46
 47
 48
 49
 50
 51
 52
 53
 54
 55
 56
 57
 58
 59
 60
 61
 62
 63
 64
 65
 66
 67
 68
 69
 70
 71
 72
 73
 74
 75
 76
 77
 78
 79
 80
 81
 82
 83
 84
 85
 86
 87
 88
 89
 90
 91
 92
 93
 94
 95
 96
 97
 98
 99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
// Copyright (c) 2017 Daniel Grunwald
//
// Permission is hereby granted, free of charge, to any person obtaining a copy of this
// software and associated documentation files (the "Software"), to deal in the Software
// without restriction, including without limitation the rights to use, copy, modify, merge,
// publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons
// to whom the Software is furnished to do so, subject to the following conditions:
//
// The above copyright notice and this permission notice shall be included in all copies or
// substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED,
// INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR
// PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE
// FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
// OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
// DEALINGS IN THE SOFTWARE.

//! `PyBuffer` implementation
use crate::err::{self, PyResult};
use crate::{exceptions, ffi, AsPyPointer, FromPyObject, PyAny, PyNativeType, Python};
use std::ffi::CStr;
use std::marker::PhantomData;
use std::os::raw;
use std::pin::Pin;
use std::{cell, mem, ptr, slice};

/// Allows access to the underlying buffer used by a python object such as `bytes`, `bytearray` or `array.array`.
// use Pin<Box> because Python expects that the Py_buffer struct has a stable memory address
#[repr(transparent)]
pub struct PyBuffer<T>(Pin<Box<ffi::Py_buffer>>, PhantomData<T>);

// PyBuffer is thread-safe: the shape of the buffer is immutable while a Py_buffer exists.
// Accessing the buffer contents is protected using the GIL.
unsafe impl<T> Send for PyBuffer<T> {}
unsafe impl<T> Sync for PyBuffer<T> {}

#[derive(Copy, Clone, Eq, PartialEq)]
pub enum ElementType {
    SignedInteger { bytes: usize },
    UnsignedInteger { bytes: usize },
    Bool,
    Float { bytes: usize },
    Unknown,
}

impl ElementType {
    pub fn from_format(format: &CStr) -> ElementType {
        match format.to_bytes() {
            [char] | [b'@', char] => native_element_type_from_type_char(*char),
            [modifier, char]
                if (*modifier == b'='
                    || *modifier == b'<'
                    || *modifier == b'>'
                    || *modifier == b'!') =>
            {
                standard_element_type_from_type_char(*char)
            }
            _ => ElementType::Unknown,
        }
    }
}

fn native_element_type_from_type_char(type_char: u8) -> ElementType {
    use self::ElementType::*;
    match type_char {
        b'c' => UnsignedInteger {
            bytes: mem::size_of::<raw::c_char>(),
        },
        b'b' => SignedInteger {
            bytes: mem::size_of::<raw::c_schar>(),
        },
        b'B' => UnsignedInteger {
            bytes: mem::size_of::<raw::c_uchar>(),
        },
        b'?' => Bool,
        b'h' => SignedInteger {
            bytes: mem::size_of::<raw::c_short>(),
        },
        b'H' => UnsignedInteger {
            bytes: mem::size_of::<raw::c_ushort>(),
        },
        b'i' => SignedInteger {
            bytes: mem::size_of::<raw::c_int>(),
        },
        b'I' => UnsignedInteger {
            bytes: mem::size_of::<raw::c_uint>(),
        },
        b'l' => SignedInteger {
            bytes: mem::size_of::<raw::c_long>(),
        },
        b'L' => UnsignedInteger {
            bytes: mem::size_of::<raw::c_ulong>(),
        },
        b'q' => SignedInteger {
            bytes: mem::size_of::<raw::c_longlong>(),
        },
        b'Q' => UnsignedInteger {
            bytes: mem::size_of::<raw::c_ulonglong>(),
        },
        b'n' => SignedInteger {
            bytes: mem::size_of::<libc::ssize_t>(),
        },
        b'N' => UnsignedInteger {
            bytes: mem::size_of::<libc::size_t>(),
        },
        b'e' => Float { bytes: 2 },
        b'f' => Float { bytes: 4 },
        b'd' => Float { bytes: 8 },
        _ => Unknown,
    }
}

fn standard_element_type_from_type_char(type_char: u8) -> ElementType {
    use self::ElementType::*;
    match type_char {
        b'c' | b'B' => UnsignedInteger { bytes: 1 },
        b'b' => SignedInteger { bytes: 1 },
        b'?' => Bool,
        b'h' => SignedInteger { bytes: 2 },
        b'H' => UnsignedInteger { bytes: 2 },
        b'i' | b'l' => SignedInteger { bytes: 4 },
        b'I' | b'L' => UnsignedInteger { bytes: 4 },
        b'q' => SignedInteger { bytes: 8 },
        b'Q' => UnsignedInteger { bytes: 8 },
        b'e' => Float { bytes: 2 },
        b'f' => Float { bytes: 4 },
        b'd' => Float { bytes: 8 },
        _ => Unknown,
    }
}

#[cfg(target_endian = "little")]
fn is_matching_endian(c: u8) -> bool {
    c == b'@' || c == b'=' || c == b'>'
}

#[cfg(target_endian = "big")]
fn is_matching_endian(c: u8) -> bool {
    c == b'@' || c == b'=' || c == b'>' || c == b'!'
}

/// Trait implemented for possible element types of `PyBuffer`.
pub unsafe trait Element: Copy {
    /// Gets whether the element specified in the format string is potentially compatible.
    /// Alignment and size are checked separately from this function.
    fn is_compatible_format(format: &CStr) -> bool;
}

fn validate(b: &ffi::Py_buffer) -> PyResult<()> {
    // shape and stride information must be provided when we use PyBUF_FULL_RO
    if b.shape.is_null() {
        return Err(exceptions::PyBufferError::new_err("Shape is Null"));
    }
    if b.strides.is_null() {
        return Err(exceptions::PyBufferError::new_err(
            "PyBuffer: Strides is Null",
        ));
    }
    Ok(())
}

impl<'source, T: Element> FromPyObject<'source> for PyBuffer<T> {
    fn extract(obj: &PyAny) -> PyResult<PyBuffer<T>> {
        Self::get(obj)
    }
}

impl<T: Element> PyBuffer<T> {
    /// Get the underlying buffer from the specified python object.
    pub fn get(obj: &PyAny) -> PyResult<PyBuffer<T>> {
        unsafe {
            let mut buf = Box::pin(ffi::Py_buffer::new());
            err::error_on_minusone(
                obj.py(),
                ffi::PyObject_GetBuffer(obj.as_ptr(), &mut *buf, ffi::PyBUF_FULL_RO),
            )?;
            validate(&buf)?;
            let buf = PyBuffer(buf, PhantomData);
            // Type Check
            if mem::size_of::<T>() == buf.item_size()
                && (buf.0.buf as usize) % mem::align_of::<T>() == 0
                && T::is_compatible_format(buf.format())
            {
                Ok(buf)
            } else {
                Err(exceptions::PyBufferError::new_err(
                    "Incompatible type as buffer",
                ))
            }
        }
    }

    /// Gets the pointer to the start of the buffer memory.
    ///
    /// Warning: the buffer memory might be mutated by other Python functions,
    /// and thus may only be accessed while the GIL is held.
    #[inline]
    pub fn buf_ptr(&self) -> *mut raw::c_void {
        self.0.buf
    }

    /// Gets a pointer to the specified item.
    ///
    /// If `indices.len() < self.dimensions()`, returns the start address of the sub-array at the specified dimension.
    pub fn get_ptr(&self, indices: &[usize]) -> *mut raw::c_void {
        let shape = &self.shape()[..indices.len()];
        for i in 0..indices.len() {
            assert!(indices[i] < shape[i]);
        }
        unsafe {
            ffi::PyBuffer_GetPointer(
                &*self.0 as *const ffi::Py_buffer as *mut ffi::Py_buffer,
                indices.as_ptr() as *mut usize as *mut ffi::Py_ssize_t,
            )
        }
    }

    /// Gets whether the underlying buffer is read-only.
    #[inline]
    pub fn readonly(&self) -> bool {
        self.0.readonly != 0
    }

    /// Gets the size of a single element, in bytes.
    /// Important exception: when requesting an unformatted buffer, item_size still has the value
    #[inline]
    pub fn item_size(&self) -> usize {
        self.0.itemsize as usize
    }

    /// Gets the total number of items.
    #[inline]
    pub fn item_count(&self) -> usize {
        (self.0.len as usize) / (self.0.itemsize as usize)
    }

    /// `item_size() * item_count()`.
    /// For contiguous arrays, this is the length of the underlying memory block.
    /// For non-contiguous arrays, it is the length that the logical structure would have if it were copied to a contiguous representation.
    #[inline]
    pub fn len_bytes(&self) -> usize {
        self.0.len as usize
    }

    /// Gets the number of dimensions.
    ///
    /// May be 0 to indicate a single scalar value.
    #[inline]
    pub fn dimensions(&self) -> usize {
        self.0.ndim as usize
    }

    /// Returns an array of length `dimensions`. `shape()[i]` is the length of the array in dimension number `i`.
    ///
    /// May return None for single-dimensional arrays or scalar values (`dimensions() <= 1`);
    /// You can call `item_count()` to get the length of the single dimension.
    ///
    /// Despite Python using an array of signed integers, the values are guaranteed to be non-negative.
    /// However, dimensions of length 0 are possible and might need special attention.
    #[inline]
    pub fn shape(&self) -> &[usize] {
        unsafe { slice::from_raw_parts(self.0.shape as *const usize, self.0.ndim as usize) }
    }

    /// Returns an array that holds, for each dimension, the number of bytes to skip to get to the next element in the dimension.
    ///
    /// Stride values can be any integer. For regular arrays, strides are usually positive,
    /// but a consumer MUST be able to handle the case `strides[n] <= 0`.
    #[inline]
    pub fn strides(&self) -> &[isize] {
        unsafe { slice::from_raw_parts(self.0.strides, self.0.ndim as usize) }
    }

    /// An array of length ndim.
    /// If `suboffsets[n] >= 0`, the values stored along the nth dimension are pointers and the suboffset value dictates how many bytes to add to each pointer after de-referencing.
    /// A suboffset value that is negative indicates that no de-referencing should occur (striding in a contiguous memory block).
    ///
    /// If all suboffsets are negative (i.e. no de-referencing is needed), then this field must be NULL (the default value).
    #[inline]
    pub fn suboffsets(&self) -> Option<&[isize]> {
        unsafe {
            if self.0.suboffsets.is_null() {
                None
            } else {
                Some(slice::from_raw_parts(
                    self.0.suboffsets,
                    self.0.ndim as usize,
                ))
            }
        }
    }

    /// A NUL terminated string in struct module style syntax describing the contents of a single item.
    #[inline]
    pub fn format(&self) -> &CStr {
        if self.0.format.is_null() {
            CStr::from_bytes_with_nul(b"B\0").unwrap()
        } else {
            unsafe { CStr::from_ptr(self.0.format) }
        }
    }

    /// Gets whether the buffer is contiguous in C-style order (last index varies fastest when visiting items in order of memory address).
    #[inline]
    pub fn is_c_contiguous(&self) -> bool {
        unsafe {
            ffi::PyBuffer_IsContiguous(
                &*self.0 as *const ffi::Py_buffer,
                b'C' as std::os::raw::c_char,
            ) != 0
        }
    }

    /// Gets whether the buffer is contiguous in Fortran-style order (first index varies fastest when visiting items in order of memory address).
    #[inline]
    pub fn is_fortran_contiguous(&self) -> bool {
        unsafe {
            ffi::PyBuffer_IsContiguous(
                &*self.0 as *const ffi::Py_buffer,
                b'F' as std::os::raw::c_char,
            ) != 0
        }
    }

    /// Gets the buffer memory as a slice.
    ///
    /// This function succeeds if:
    /// * the buffer format is compatible with `T`
    /// * alignment and size of buffer elements is matching the expectations for type `T`
    /// * the buffer is C-style contiguous
    ///
    /// The returned slice uses type `Cell<T>` because it's theoretically possible for any call into the Python runtime
    /// to modify the values in the slice.
    pub fn as_slice<'a>(&'a self, _py: Python<'a>) -> Option<&'a [ReadOnlyCell<T>]> {
        if self.is_c_contiguous() {
            unsafe {
                Some(slice::from_raw_parts(
                    self.0.buf as *mut ReadOnlyCell<T>,
                    self.item_count(),
                ))
            }
        } else {
            None
        }
    }

    /// Gets the buffer memory as a slice.
    ///
    /// This function succeeds if:
    /// * the buffer is not read-only
    /// * the buffer format is compatible with `T`
    /// * alignment and size of buffer elements is matching the expectations for type `T`
    /// * the buffer is C-style contiguous
    ///
    /// The returned slice uses type `Cell<T>` because it's theoretically possible for any call into the Python runtime
    /// to modify the values in the slice.
    pub fn as_mut_slice<'a>(&'a self, _py: Python<'a>) -> Option<&'a [cell::Cell<T>]> {
        if !self.readonly() && self.is_c_contiguous() {
            unsafe {
                Some(slice::from_raw_parts(
                    self.0.buf as *mut cell::Cell<T>,
                    self.item_count(),
                ))
            }
        } else {
            None
        }
    }

    /// Gets the buffer memory as a slice.
    ///
    /// This function succeeds if:
    /// * the buffer format is compatible with `T`
    /// * alignment and size of buffer elements is matching the expectations for type `T`
    /// * the buffer is Fortran-style contiguous
    ///
    /// The returned slice uses type `Cell<T>` because it's theoretically possible for any call into the Python runtime
    /// to modify the values in the slice.
    pub fn as_fortran_slice<'a>(&'a self, _py: Python<'a>) -> Option<&'a [ReadOnlyCell<T>]> {
        if mem::size_of::<T>() == self.item_size() && self.is_fortran_contiguous() {
            unsafe {
                Some(slice::from_raw_parts(
                    self.0.buf as *mut ReadOnlyCell<T>,
                    self.item_count(),
                ))
            }
        } else {
            None
        }
    }

    /// Gets the buffer memory as a slice.
    ///
    /// This function succeeds if:
    /// * the buffer is not read-only
    /// * the buffer format is compatible with `T`
    /// * alignment and size of buffer elements is matching the expectations for type `T`
    /// * the buffer is Fortran-style contiguous
    ///
    /// The returned slice uses type `Cell<T>` because it's theoretically possible for any call into the Python runtime
    /// to modify the values in the slice.
    pub fn as_fortran_mut_slice<'a>(&'a self, _py: Python<'a>) -> Option<&'a [cell::Cell<T>]> {
        if !self.readonly() && self.is_fortran_contiguous() {
            unsafe {
                Some(slice::from_raw_parts(
                    self.0.buf as *mut cell::Cell<T>,
                    self.item_count(),
                ))
            }
        } else {
            None
        }
    }

    /// Copies the buffer elements to the specified slice.
    /// If the buffer is multi-dimensional, the elements are written in C-style order.
    ///
    ///  * Fails if the slice does not have the correct length (`buf.item_count()`).
    ///  * Fails if the buffer format is not compatible with type `T`.
    ///
    /// To check whether the buffer format is compatible before calling this method,
    /// you can use `<T as buffer::Element>::is_compatible_format(buf.format())`.
    /// Alternatively, `match buffer::ElementType::from_format(buf.format())`.
    pub fn copy_to_slice(&self, py: Python, target: &mut [T]) -> PyResult<()> {
        self.copy_to_slice_impl(py, target, b'C')
    }

    /// Copies the buffer elements to the specified slice.
    /// If the buffer is multi-dimensional, the elements are written in Fortran-style order.
    ///
    ///  * Fails if the slice does not have the correct length (`buf.item_count()`).
    ///  * Fails if the buffer format is not compatible with type `T`.
    ///
    /// To check whether the buffer format is compatible before calling this method,
    /// you can use `<T as buffer::Element>::is_compatible_format(buf.format())`.
    /// Alternatively, `match buffer::ElementType::from_format(buf.format())`.
    pub fn copy_to_fortran_slice(&self, py: Python, target: &mut [T]) -> PyResult<()> {
        self.copy_to_slice_impl(py, target, b'F')
    }

    fn copy_to_slice_impl(&self, py: Python, target: &mut [T], fort: u8) -> PyResult<()> {
        if mem::size_of_val(target) != self.len_bytes() {
            return Err(exceptions::PyBufferError::new_err(
                "Slice length does not match buffer length.",
            ));
        }
        unsafe {
            err::error_on_minusone(
                py,
                ffi::PyBuffer_ToContiguous(
                    target.as_ptr() as *mut raw::c_void,
                    &*self.0 as *const ffi::Py_buffer as *mut ffi::Py_buffer,
                    self.0.len,
                    fort as std::os::raw::c_char,
                ),
            )
        }
    }

    /// Copies the buffer elements to a newly allocated vector.
    /// If the buffer is multi-dimensional, the elements are written in C-style order.
    ///
    /// Fails if the buffer format is not compatible with type `T`.
    pub fn to_vec(&self, py: Python) -> PyResult<Vec<T>> {
        self.to_vec_impl(py, b'C')
    }

    /// Copies the buffer elements to a newly allocated vector.
    /// If the buffer is multi-dimensional, the elements are written in Fortran-style order.
    ///
    /// Fails if the buffer format is not compatible with type `T`.
    pub fn to_fortran_vec(&self, py: Python) -> PyResult<Vec<T>> {
        self.to_vec_impl(py, b'F')
    }

    fn to_vec_impl(&self, py: Python, fort: u8) -> PyResult<Vec<T>> {
        let item_count = self.item_count();
        let mut vec: Vec<T> = Vec::with_capacity(item_count);
        unsafe {
            // Copy the buffer into the uninitialized space in the vector.
            // Due to T:Copy, we don't need to be concerned with Drop impls.
            err::error_on_minusone(
                py,
                ffi::PyBuffer_ToContiguous(
                    vec.as_mut_ptr() as *mut raw::c_void,
                    &*self.0 as *const ffi::Py_buffer as *mut ffi::Py_buffer,
                    self.0.len,
                    fort as std::os::raw::c_char,
                ),
            )?;
            // set vector length to mark the now-initialized space as usable
            vec.set_len(item_count);
        }
        Ok(vec)
    }

    /// Copies the specified slice into the buffer.
    /// If the buffer is multi-dimensional, the elements in the slice are expected to be in C-style order.
    ///
    ///  * Fails if the buffer is read-only.
    ///  * Fails if the slice does not have the correct length (`buf.item_count()`).
    ///  * Fails if the buffer format is not compatible with type `T`.
    ///
    /// To check whether the buffer format is compatible before calling this method,
    /// use `<T as buffer::Element>::is_compatible_format(buf.format())`.
    /// Alternatively, `match buffer::ElementType::from_format(buf.format())`.
    pub fn copy_from_slice(&self, py: Python, source: &[T]) -> PyResult<()> {
        self.copy_from_slice_impl(py, source, b'C')
    }

    /// Copies the specified slice into the buffer.
    /// If the buffer is multi-dimensional, the elements in the slice are expected to be in Fortran-style order.
    ///
    ///  * Fails if the buffer is read-only.
    ///  * Fails if the slice does not have the correct length (`buf.item_count()`).
    ///  * Fails if the buffer format is not compatible with type `T`.
    ///
    /// To check whether the buffer format is compatible before calling this method,
    /// use `<T as buffer::Element>::is_compatible_format(buf.format())`.
    /// Alternatively, `match buffer::ElementType::from_format(buf.format())`.
    pub fn copy_from_fortran_slice(&self, py: Python, source: &[T]) -> PyResult<()> {
        self.copy_from_slice_impl(py, source, b'F')
    }

    fn copy_from_slice_impl(&self, py: Python, source: &[T], fort: u8) -> PyResult<()> {
        if self.readonly() {
            return buffer_readonly_error();
        }
        if mem::size_of_val(source) != self.len_bytes() {
            return Err(exceptions::PyBufferError::new_err(
                "Slice length does not match buffer length.",
            ));
        }
        unsafe {
            err::error_on_minusone(
                py,
                ffi::PyBuffer_FromContiguous(
                    &*self.0 as *const ffi::Py_buffer as *mut ffi::Py_buffer,
                    source.as_ptr() as *mut raw::c_void,
                    self.0.len,
                    fort as std::os::raw::c_char,
                ),
            )
        }
    }

    pub fn release(self, _py: Python) {
        // First move self into a ManuallyDrop, so that PyBuffer::drop will
        // never be called. (It would acquire the GIL and call PyBuffer_Release
        // again.)
        let mut mdself = mem::ManuallyDrop::new(self);
        unsafe {
            // Next, make the actual PyBuffer_Release call.
            ffi::PyBuffer_Release(&mut *mdself.0);

            // Finally, drop the contained Pin<Box<_>> in place, to free the
            // Box memory.
            let inner: *mut Pin<Box<ffi::Py_buffer>> = &mut mdself.0;
            ptr::drop_in_place(inner);
        }
    }
}

#[inline(always)]
fn buffer_readonly_error() -> PyResult<()> {
    Err(exceptions::PyBufferError::new_err(
        "Cannot write to read-only buffer.",
    ))
}

impl<T> Drop for PyBuffer<T> {
    fn drop(&mut self) {
        Python::with_gil(|_| unsafe { ffi::PyBuffer_Release(&mut *self.0) });
    }
}

/// Like `std::mem::cell`, but only provides read-only access to the data.
///
/// `&ReadOnlyCell<T>` is basically a safe version of `*const T`:
///  The data cannot be modified through the reference, but other references may
///  be modifying the data.
#[repr(transparent)]
pub struct ReadOnlyCell<T: Element>(cell::UnsafeCell<T>);

impl<T: Element> ReadOnlyCell<T> {
    #[inline]
    pub fn get(&self) -> T {
        unsafe { *self.0.get() }
    }

    #[inline]
    pub fn as_ptr(&self) -> *const T {
        self.0.get()
    }
}

macro_rules! impl_element(
    ($t:ty, $f:ident) => {
        unsafe impl Element for $t {
            fn is_compatible_format(format: &CStr) -> bool {
                let slice = format.to_bytes();
                if slice.len() > 1 && !is_matching_endian(slice[0]) {
                    return false;
                }
                ElementType::from_format(format) == ElementType::$f { bytes: mem::size_of::<$t>() }
            }
        }
    }
);

impl_element!(u8, UnsignedInteger);
impl_element!(u16, UnsignedInteger);
impl_element!(u32, UnsignedInteger);
impl_element!(u64, UnsignedInteger);
impl_element!(usize, UnsignedInteger);
impl_element!(i8, SignedInteger);
impl_element!(i16, SignedInteger);
impl_element!(i32, SignedInteger);
impl_element!(i64, SignedInteger);
impl_element!(isize, SignedInteger);
impl_element!(f32, Float);
impl_element!(f64, Float);

#[cfg(test)]
mod test {
    use super::PyBuffer;
    use crate::ffi;
    use crate::Python;

    #[test]
    fn test_compatible_size() {
        // for the cast in PyBuffer::shape()
        assert_eq!(
            std::mem::size_of::<ffi::Py_ssize_t>(),
            std::mem::size_of::<usize>()
        );
    }

    #[test]
    fn test_bytes_buffer() {
        let gil = Python::acquire_gil();
        let py = gil.python();
        let bytes = py.eval("b'abcde'", None, None).unwrap();
        let buffer = PyBuffer::get(&bytes).unwrap();
        assert_eq!(buffer.dimensions(), 1);
        assert_eq!(buffer.item_count(), 5);
        assert_eq!(buffer.format().to_str().unwrap(), "B");
        assert_eq!(buffer.shape(), [5]);
        // single-dimensional buffer is always contiguous
        assert!(buffer.is_c_contiguous());
        assert!(buffer.is_fortran_contiguous());

        let slice = buffer.as_slice(py).unwrap();
        assert_eq!(slice.len(), 5);
        assert_eq!(slice[0].get(), b'a');
        assert_eq!(slice[2].get(), b'c');

        assert!(buffer.copy_to_slice(py, &mut [0u8]).is_err());
        let mut arr = [0; 5];
        buffer.copy_to_slice(py, &mut arr).unwrap();
        assert_eq!(arr, b"abcde" as &[u8]);

        assert!(buffer.copy_from_slice(py, &[0u8; 5]).is_err());
        assert_eq!(buffer.to_vec(py).unwrap(), b"abcde");
    }

    #[allow(clippy::float_cmp)] // The test wants to ensure that no precision was lost on the Python round-trip
    #[test]
    fn test_array_buffer() {
        let gil = Python::acquire_gil();
        let py = gil.python();
        let array = py
            .import("array")
            .unwrap()
            .call_method("array", ("f", (1.0, 1.5, 2.0, 2.5)), None)
            .unwrap();
        let buffer = PyBuffer::get(array).unwrap();
        assert_eq!(buffer.dimensions(), 1);
        assert_eq!(buffer.item_count(), 4);
        assert_eq!(buffer.format().to_str().unwrap(), "f");
        assert_eq!(buffer.shape(), [4]);

        let slice = buffer.as_slice(py).unwrap();
        assert_eq!(slice.len(), 4);
        assert_eq!(slice[0].get(), 1.0);
        assert_eq!(slice[3].get(), 2.5);

        let mut_slice = buffer.as_mut_slice(py).unwrap();
        assert_eq!(mut_slice.len(), 4);
        assert_eq!(mut_slice[0].get(), 1.0);
        mut_slice[3].set(2.75);
        assert_eq!(slice[3].get(), 2.75);

        buffer
            .copy_from_slice(py, &[10.0f32, 11.0, 12.0, 13.0])
            .unwrap();
        assert_eq!(slice[2].get(), 12.0);

        assert_eq!(buffer.to_vec(py).unwrap(), [10.0, 11.0, 12.0, 13.0]);
    }
}