strided_view/

view.rs

//! Julia-like dynamic-rank strided view types.
//!
//! This module provides the canonical view types for strided operations,
//! matching Julia's StridedViews.jl data model:
//!
//! - [`StridedView`]: Immutable dynamic-rank strided view with lazy element operations
//! - [`StridedViewMut`]: Mutable dynamic-rank strided view (Identity op only)
//! - [`StridedArray`]: Owned strided multidimensional array
//!
//! A view's shape and strides are a small inline-or-heap container with
//! capacity [`crate::INLINE_RANK`]: every constructor, clone, and metadata
//! transform allocates nothing up to that rank and spills gracefully above
//! it. The view itself only borrows the element data.
//!
//! The metadata container is `tinyvec::ArrayVec`, unified across tensor4all
//! repositories (tensor4all-agent-rules#22, strided-rs#312). `ArrayVec` is
//! `Copy` for `Copy` items, so a clone is one fixed-size bitwise copy, and the
//! heap fallback for rank above [`INLINE_RANK`] lives in a separate
//! `Option<Box<SpilledMeta>>` so the inline value has no drop glue.

use std::marker::PhantomData;
use std::ops::{Index, IndexMut};

use tinyvec::ArrayVec;

use crate::element_op::{ComposableElementOp, ElementOp, ElementOpApply, Identity};
use crate::{AxisVec, Result, StrideVec, StridedError, INLINE_RANK};

/// Build an `ArrayVec` from at most [`INLINE_RANK`] elements.
///
/// The value is written with literal-count arms: a runtime-length
/// `copy_from_slice` or loop is recognized by LLVM and lowered to a `memcpy`
/// PLT call, whose overhead dominates a handful of words.
#[inline(always)]
fn arrayvec_from_slice<T: Copy + Default>(src: &[T]) -> ArrayVec<[T; INLINE_RANK]> {
    debug_assert!(src.len() <= INLINE_RANK);
    let n = src.len();
    let mut buf = [T::default(); INLINE_RANK];
    match n {
        0 => {}
        1 => buf[0] = src[0],
        2 => {
            buf[0] = src[0];
            buf[1] = src[1];
        }
        3 => {
            buf[0] = src[0];
            buf[1] = src[1];
            buf[2] = src[2];
        }
        4 => {
            buf[0] = src[0];
            buf[1] = src[1];
            buf[2] = src[2];
            buf[3] = src[3];
        }
        5 => {
            buf[0] = src[0];
            buf[1] = src[1];
            buf[2] = src[2];
            buf[3] = src[3];
            buf[4] = src[4];
        }
        6 => {
            buf[0] = src[0];
            buf[1] = src[1];
            buf[2] = src[2];
            buf[3] = src[3];
            buf[4] = src[4];
            buf[5] = src[5];
        }
        7 => {
            buf[0] = src[0];
            buf[1] = src[1];
            buf[2] = src[2];
            buf[3] = src[3];
            buf[4] = src[4];
            buf[5] = src[5];
            buf[6] = src[6];
        }
        _ => {
            buf[0] = src[0];
            buf[1] = src[1];
            buf[2] = src[2];
            buf[3] = src[3];
            buf[4] = src[4];
            buf[5] = src[5];
            buf[6] = src[6];
            buf[7] = src[7];
        }
    }
    ArrayVec::from_array_len(buf, n)
}

/// Heap-owned metadata used only above [`INLINE_RANK`].
#[derive(Clone, Debug)]
struct SpilledMeta {
    dims: Box<[usize]>,
    strides: Box<[isize]>,
}

/// Shape and strides of a view or array.
///
/// Rank `<= INLINE_RANK` is stored inline in two `ArrayVec`s and the
/// `spilled` slot is `None`, so cloning the whole metadata is a fixed-size
/// bitwise copy with one predictable `None` check. A higher rank keeps the
/// inline buffers empty and boxes both vectors.
#[derive(Debug)]
struct ViewMeta {
    inline_dims: ArrayVec<[usize; INLINE_RANK]>,
    inline_strides: ArrayVec<[isize; INLINE_RANK]>,
    spilled: Option<Box<SpilledMeta>>,
}

impl Clone for ViewMeta {
    #[inline]
    fn clone(&self) -> Self {
        Self {
            inline_dims: self.inline_dims,
            inline_strides: self.inline_strides,
            spilled: self.spilled.clone(),
        }
    }
}

impl ViewMeta {
    #[inline(always)]
    fn from_slices(dims: &[usize], strides: &[isize]) -> Self {
        debug_assert_eq!(dims.len(), strides.len());
        if dims.len() <= INLINE_RANK {
            Self {
                inline_dims: arrayvec_from_slice(dims),
                inline_strides: arrayvec_from_slice(strides),
                spilled: None,
            }
        } else {
            Self {
                inline_dims: ArrayVec::new(),
                inline_strides: ArrayVec::new(),
                spilled: Some(Box::new(SpilledMeta {
                    dims: dims.into(),
                    strides: strides.into(),
                })),
            }
        }
    }

    #[inline]
    fn dims(&self) -> &[usize] {
        match &self.spilled {
            Some(spilled) => &spilled.dims,
            None => self.inline_dims.as_slice(),
        }
    }

    #[inline]
    fn strides(&self) -> &[isize] {
        match &self.spilled {
            Some(spilled) => &spilled.strides,
            None => self.inline_strides.as_slice(),
        }
    }
}

#[inline]
fn empty_layout(dims: &[usize]) -> bool {
    dims.iter().any(|&dim| dim == 0)
}

#[inline]
fn empty_const_ptr<T>() -> *const T {
    core::ptr::NonNull::<T>::dangling().as_ptr()
}

#[inline]
fn empty_mut_ptr<T>() -> *mut T {
    core::ptr::NonNull::<T>::dangling().as_ptr()
}

// ============================================================================
// Validation helpers
// ============================================================================

/// Validate that all accessed offsets stay within `[0, len)`.
#[inline]
pub(crate) fn validate_bounds(
    len: usize,
    dims: &[usize],
    strides: &[isize],
    offset: isize,
) -> Result<()> {
    if dims.len() != strides.len() {
        return Err(StridedError::StrideLengthMismatch);
    }
    // Empty array - no access needed
    if dims.iter().any(|&d| d == 0) {
        return Ok(());
    }
    // Compute min and max offsets
    let mut min_offset = offset;
    let mut max_offset = offset;
    for (&dim, &stride) in dims.iter().zip(strides.iter()) {
        if dim > 1 {
            let last = dim
                .checked_sub(1)
                .and_then(|value| isize::try_from(value).ok())
                .ok_or(StridedError::OffsetOverflow)?;
            let end = stride
                .checked_mul(last)
                .ok_or(StridedError::OffsetOverflow)?;
            if end >= 0 {
                max_offset = max_offset
                    .checked_add(end)
                    .ok_or(StridedError::OffsetOverflow)?;
            } else {
                min_offset = min_offset
                    .checked_add(end)
                    .ok_or(StridedError::OffsetOverflow)?;
            }
        }
    }
    if min_offset < 0 || max_offset < 0 {
        return Err(StridedError::OffsetOverflow);
    }
    if max_offset as usize >= len {
        return Err(StridedError::OffsetOverflow);
    }
    Ok(())
}

/// Offset of `indices` inside `dims`/`strides`, relative to `base`.
///
/// # Panics
///
/// Panics when `indices` is not the full index for `dims`, an index is out of
/// bounds, or an indexed offset does not fit `isize`. Every panicking element
/// accessor below reports those conditions through this helper.
#[inline]
fn indexed_offset(dims: &[usize], strides: &[isize], indices: &[usize], base: isize) -> isize {
    assert_eq!(indices.len(), dims.len(), "wrong number of indices");
    let mut offset = base;
    for (i, &index) in indices.iter().enumerate() {
        assert!(
            index < dims[i],
            "index {} out of bounds for dim {}",
            index,
            dims[i]
        );
        offset = isize::try_from(index)
            .ok()
            .and_then(|index| index.checked_mul(strides[i]))
            .and_then(|step| offset.checked_add(step))
            .expect("index offset overflow");
    }
    offset
}

/// Compute column-major strides (Julia default: first index varies fastest).
///
/// Returns an inline [`StrideVec`]: the result does not allocate at rank
/// `<= INLINE_RANK` and spills above it.
///
/// # Examples
///
/// ```
/// use strided_view::col_major_strides;
///
/// // Julia default: first index varies fastest.
/// assert_eq!(col_major_strides(&[2, 3]).as_slice(), &[1, 2]);
/// ```
#[inline]
pub fn col_major_strides(dims: &[usize]) -> StrideVec {
    let rank = dims.len();
    if rank == 0 {
        return StrideVec::new();
    }
    let mut strides = StrideVec::new();
    strides.resize(rank, 1isize);
    for i in 1..rank {
        strides[i] = strides[i - 1] * dims[i - 1] as isize;
    }
    strides
}

/// Compute row-major strides (C default: last index varies fastest).
///
/// Returns an inline [`StrideVec`]: the result does not allocate at rank
/// `<= INLINE_RANK` and spills above it.
///
/// # Examples
///
/// ```
/// use strided_view::row_major_strides;
///
/// // C default: last index varies fastest.
/// assert_eq!(row_major_strides(&[2, 3]).as_slice(), &[3, 1]);
/// ```
#[inline]
pub fn row_major_strides(dims: &[usize]) -> StrideVec {
    let rank = dims.len();
    if rank == 0 {
        return StrideVec::new();
    }
    let mut strides = StrideVec::new();
    strides.resize(rank, 1isize);
    for i in (0..rank - 1).rev() {
        strides[i] = strides[i + 1] * dims[i + 1] as isize;
    }
    strides
}

// ============================================================================
// StridedView
// ============================================================================

/// Dynamic-rank immutable strided view with lazy element operations.
///
/// This is the Julia-equivalent `StridedView` type with:
/// - Dynamic rank, with metadata stored inline for up to [`INLINE_RANK`]
///   dimensions ([`StridedView::new`] copies the caller's layout, so the view
///   borrows only the element data)
/// - Lazy element operations via the `Op` type parameter
/// - Zero-copy transformations (permute, transpose, adjoint, conj)
///
/// # Type Parameters
/// - `'a`: Lifetime of the underlying data
/// - `T`: Element type
/// - `Op`: Element operation applied lazily on access (default: `Identity`)
///
/// # Size
///
/// The inline metadata makes the view 184 bytes on 64-bit targets for
/// `usize`/`isize` strides.
///
/// # Auto traits
///
/// The view borrows its elements, so it is `Send` only when `T: Sync` (a
/// shared slice is). A view of non-`Sync` data is not `Send`:
///
/// ```compile_fail
/// use std::cell::Cell;
/// use strided_view::StridedView;
///
/// fn assert_send<T: Send>() {}
/// assert_send::<StridedView<'static, Cell<i32>>>();
/// ```
pub struct StridedView<'a, T, Op = Identity> {
    ptr: *const T,
    data: &'a [T],
    meta: ViewMeta,
    offset: isize,
    _op: PhantomData<Op>,
}

unsafe impl<T: Sync, Op: Send> Send for StridedView<'_, T, Op> {}
unsafe impl<T: Sync, Op: Sync> Sync for StridedView<'_, T, Op> {}

impl<T, Op> Clone for StridedView<'_, T, Op> {
    #[inline]
    fn clone(&self) -> Self {
        Self {
            ptr: self.ptr,
            data: self.data,
            meta: self.meta.clone(),
            offset: self.offset,
            _op: PhantomData,
        }
    }
}

impl<T: std::fmt::Debug, Op> std::fmt::Debug for StridedView<'_, T, Op> {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        f.debug_struct("StridedView")
            .field("dims", &self.meta.dims())
            .field("strides", &self.meta.strides())
            .field("offset", &self.offset)
            .finish()
    }
}

/// The cached base pointer of a view over `data` at `offset`.
///
/// An empty layout gets the dangling pointer rather than `data + offset`: a
/// zero-sized extent may sit at an offset no element of `data` reaches.
#[inline]
fn base_ptr<T>(data: &[T], dims: &[usize], offset: isize) -> *const T {
    if empty_layout(dims) {
        empty_const_ptr()
    } else {
        unsafe { data.as_ptr().offset(offset) }
    }
}

/// A view from already-validated metadata, with the element op chosen here.
#[inline]
fn from_meta<'a, T, Op>(
    data: &'a [T],
    dims: &[usize],
    strides: &[isize],
    offset: isize,
) -> StridedView<'a, T, Op> {
    let meta = ViewMeta::from_slices(dims, strides);
    let ptr = base_ptr(data, meta.dims(), offset);
    StridedView {
        ptr,
        data,
        meta,
        offset,
        _op: PhantomData,
    }
}

impl<'a, T, Op> StridedView<'a, T, Op> {
    /// Create an immutable strided view, copying the caller's metadata inline.
    ///
    /// The layout is copied into the view's inline storage, so it does not
    /// need to outlive the view and no heap allocation happens at rank
    /// `<= INLINE_RANK`. The view's lifetime is tied to `data` only.
    ///
    /// # Examples
    ///
    /// ```
    /// use strided_view::StridedView;
    ///
    /// let data = [1.0f64, 2.0, 3.0, 4.0, 5.0, 6.0];
    /// let view = StridedView::<f64>::new(&data, &[2, 3], &[1, 2], 0).unwrap();
    /// assert_eq!(view.dims(), &[2, 3]);
    /// assert_eq!(view.get(&[1, 2]), 6.0);
    /// ```
    #[inline]
    pub fn new(data: &'a [T], dims: &[usize], strides: &[isize], offset: isize) -> Result<Self> {
        validate_bounds(data.len(), dims, strides, offset)?;
        Ok(from_meta(data, dims, strides, offset))
    }

    /// Create a view without bounds checking, copying the caller's metadata.
    ///
    /// # Safety
    /// The caller must ensure all index combinations stay within bounds.
    ///
    /// # Examples
    ///
    /// ```
    /// use strided_view::StridedView;
    ///
    /// let data = [1.0f64, 2.0, 3.0, 4.0];
    /// let view = unsafe { StridedView::<f64>::new_unchecked(&data, &[2, 2], &[1, 2], 0) };
    /// assert_eq!(view.ndim(), 2);
    /// ```
    #[inline]
    pub unsafe fn new_unchecked(
        data: &'a [T],
        dims: &[usize],
        strides: &[isize],
        offset: isize,
    ) -> Self {
        from_meta(data, dims, strides, offset)
    }

    /// Returns the shape (dimension sizes) of this view.
    #[inline]
    pub fn dims(&self) -> &[usize] {
        self.meta.dims()
    }

    /// Returns the strides (in units of `T`) for each dimension.
    #[inline]
    pub fn strides(&self) -> &[isize] {
        self.meta.strides()
    }

    /// Returns the byte offset into the backing data.
    #[inline]
    pub fn offset(&self) -> isize {
        self.offset
    }

    /// Returns the number of dimensions (rank).
    #[inline]
    pub fn ndim(&self) -> usize {
        self.dims().len()
    }

    /// Returns the total number of elements.
    #[inline]
    pub fn len(&self) -> usize {
        self.dims().iter().product()
    }

    /// Returns `true` if any dimension is zero.
    #[inline]
    pub fn is_empty(&self) -> bool {
        self.dims().iter().any(|&d| d == 0)
    }

    /// Returns a reference to the backing data slice.
    #[inline]
    pub fn data(&self) -> &'a [T] {
        self.data
    }

    /// Raw const pointer to element at the view's base offset.
    #[inline]
    pub fn ptr(&self) -> *const T {
        self.ptr
    }

    /// Permute dimensions.
    pub fn permute(&self, perm: &[usize]) -> Result<StridedView<'a, T, Op>> {
        let rank = self.dims().len();
        if perm.len() != rank {
            return Err(StridedError::RankMismatch(perm.len(), rank));
        }
        let mut seen: tinyvec::TinyVec<[bool; INLINE_RANK]> = tinyvec::TinyVec::new();
        seen.resize(rank, false);
        for &p in perm {
            if p >= rank {
                return Err(StridedError::InvalidAxis { axis: p, rank });
            }
            if seen[p] {
                return Err(StridedError::InvalidAxis { axis: p, rank });
            }
            seen[p] = true;
        }
        let new_dims: AxisVec = perm.iter().map(|&p| self.dims()[p]).collect();
        let new_strides: StrideVec = perm.iter().map(|&p| self.strides()[p]).collect();
        Ok(from_meta(self.data, &new_dims, &new_strides, self.offset))
    }

    /// Create a diagonal view by fusing repeated axis pairs via stride trick (zero-copy).
    ///
    /// For each pair `(a, b)`:
    /// - New stride = `strides[a] + strides[b]`
    /// - New dim = `min(dims[a], dims[b])`
    /// - The higher-numbered axis is removed
    /// - Pairs use **original** axis numbering
    ///
    /// # Errors
    ///
    /// Returns [`StridedError::InvalidAxis`] when a pair names an axis outside
    /// the rank or names the same axis twice, and
    /// [`StridedError::OffsetOverflow`] when a fused stride does not fit
    /// `isize` or the fused layout reaches past the backing allocation. Two
    /// pairs may share an axis (`A[i,i,i]` is `[(0, 1), (0, 2)]`), but a pair
    /// pattern that folds one axis into itself or into both ends of another
    /// pair can address elements outside the backing data and is rejected.
    ///
    /// # Example
    /// `A[i,i,j]` shape=`[n,n,m]` strides=`[s0,s1,s2]` -> shape=`[n,m]` strides=`[s0+s1, s2]`
    pub fn diagonal_view(&self, axis_pairs: &[(usize, usize)]) -> Result<StridedView<'a, T, Op>> {
        let ndim = self.ndim();
        let mut dims: AxisVec = AxisVec::new();
        dims.extend_from_slice(self.dims());
        let mut strides: StrideVec = StrideVec::new();
        strides.extend_from_slice(self.strides());

        let mut axes_to_remove: tinyvec::TinyVec<[usize; INLINE_RANK]> = tinyvec::TinyVec::new();
        for &(a, b) in axis_pairs {
            let (lo, hi) = if a < b { (a, b) } else { (b, a) };
            if hi >= ndim {
                return Err(StridedError::InvalidAxis {
                    axis: hi,
                    rank: ndim,
                });
            }
            if lo == hi {
                return Err(StridedError::InvalidAxis {
                    axis: lo,
                    rank: ndim,
                });
            }
            strides[lo] = strides[lo]
                .checked_add(strides[hi])
                .ok_or(StridedError::OffsetOverflow)?;
            dims[lo] = dims[lo].min(dims[hi]);
            // Insert each removal axis once. Pairs may share an axis, so a
            // duplicate would otherwise grow the list past the rank bound and
            // spill the inline vector for an otherwise in-bounds view.
            if !axes_to_remove.contains(&hi) {
                axes_to_remove.push(hi);
            }
        }

        // Remove in descending order so the lower indices stay valid. The list
        // is already unique, so no deduplication pass is needed.
        axes_to_remove.sort_unstable();
        for &ax in axes_to_remove.iter().rev() {
            dims.remove(ax);
            strides.remove(ax);
        }

        // The fusion re-uses an already updated stride when a pair shares an
        // axis with an earlier pair, so the result carries no bound proof of
        // its own and is validated here instead (tensor4all/strided-rs#204).
        validate_bounds(self.data().len(), &dims, &strides, self.offset())?;

        Ok(from_meta(self.data(), &dims, &strides, self.offset()))
    }

    /// Broadcast this view to a target shape.
    ///
    /// Size-1 dimensions are expanded (stride set to 0) to match target.
    pub fn broadcast(&self, target_dims: &[usize]) -> Result<StridedView<'a, T, Op>> {
        if self.dims().len() != target_dims.len() {
            return Err(StridedError::RankMismatch(
                self.dims().len(),
                target_dims.len(),
            ));
        }
        let mut new_strides = StrideVec::with_capacity(self.dims().len());
        for i in 0..self.dims().len() {
            if self.dims()[i] == target_dims[i] {
                new_strides.push(self.strides()[i]);
            } else if self.dims()[i] == 1 {
                new_strides.push(0);
            } else {
                return Err(StridedError::ShapeMismatch(
                    self.dims().to_vec(),
                    target_dims.to_vec(),
                ));
            }
        }
        Ok(from_meta(self.data, target_dims, &new_strides, self.offset))
    }
}

/// Composition methods: require `T: ElementOpApply` and `Op: ComposableElementOp<T>`.
impl<'a, T: Copy + ElementOpApply, Op: ComposableElementOp<T>> StridedView<'a, T, Op> {
    /// Transpose a 2D view: reverses dimensions and composes the Transpose element op.
    ///
    /// Julia equivalent: `Base.transpose(a::AbstractStridedView{<:Any, 2})`
    pub fn transpose_2d(&self) -> Result<StridedView<'a, T, Op::ComposeTranspose>> {
        if self.dims().len() != 2 {
            return Err(StridedError::RankMismatch(self.dims().len(), 2));
        }
        Ok(from_meta(
            self.data,
            &[self.dims()[1], self.dims()[0]],
            &[self.strides()[1], self.strides()[0]],
            self.offset,
        ))
    }

    /// Adjoint (conjugate transpose) of a 2D view.
    ///
    /// Julia equivalent: `Base.adjoint(a::AbstractStridedView{<:Any, 2})`
    pub fn adjoint_2d(&self) -> Result<StridedView<'a, T, Op::ComposeAdjoint>> {
        if self.dims().len() != 2 {
            return Err(StridedError::RankMismatch(self.dims().len(), 2));
        }
        Ok(from_meta(
            self.data,
            &[self.dims()[1], self.dims()[0]],
            &[self.strides()[1], self.strides()[0]],
            self.offset,
        ))
    }

    /// Complex conjugate (compose Conj without changing dims/strides).
    ///
    /// Julia equivalent: `Base.conj(a::AbstractStridedView)`
    #[inline]
    pub fn conj(&self) -> StridedView<'a, T, Op::ComposeConj> {
        StridedView {
            ptr: self.ptr,
            data: self.data,
            meta: self.meta.clone(),
            offset: self.offset,
            _op: PhantomData,
        }
    }
}

/// Element access: requires `Op: ElementOp<T>`.
impl<'a, T: Copy, Op: ElementOp<T>> StridedView<'a, T, Op> {
    /// Get an element with the element operation applied.
    ///
    /// # Panics
    ///
    /// Panics if `indices.len()` is not the view's rank, an index is out of
    /// bounds, or the indexed offset does not fit `isize`.
    pub fn get(&self, indices: &[usize]) -> T {
        let offset = indexed_offset(self.dims(), self.strides(), indices, 0);
        Op::apply(unsafe { *self.ptr.offset(offset) })
    }

    /// Get an element without bounds checking.
    ///
    /// # Safety
    /// Caller must ensure indices are within bounds.
    #[inline]
    pub unsafe fn get_unchecked(&self, indices: &[usize]) -> T {
        let mut idx = 0isize;
        for (i, &index) in indices.iter().enumerate() {
            idx += index as isize * self.strides()[i];
        }
        Op::apply(*self.ptr.offset(idx))
    }
}

// ============================================================================
// StridedViewMut
// ============================================================================

/// Dynamic-rank mutable strided view.
///
/// Always uses `Identity` element operation for write simplicity.
/// Julia typically applies ops on the read side.
pub struct StridedViewMut<'a, T> {
    ptr: *mut T,
    data: &'a mut [T],
    meta: ViewMeta,
    offset: isize,
}

unsafe impl<T: Send> Send for StridedViewMut<'_, T> {}

impl<T: std::fmt::Debug> std::fmt::Debug for StridedViewMut<'_, T> {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        f.debug_struct("StridedViewMut")
            .field("dims", &self.meta.dims())
            .field("strides", &self.meta.strides())
            .field("offset", &self.offset)
            .finish()
    }
}

impl<'a, T> StridedViewMut<'a, T> {
    /// A view from already-validated metadata.
    #[inline]
    fn from_meta(data: &'a mut [T], dims: &[usize], strides: &[isize], offset: isize) -> Self {
        let meta = ViewMeta::from_slices(dims, strides);
        let ptr = if empty_layout(meta.dims()) {
            empty_mut_ptr()
        } else {
            unsafe { data.as_mut_ptr().offset(offset) }
        };
        Self {
            ptr,
            data,
            meta,
            offset,
        }
    }

    /// Create a mutable view, copying the caller's metadata inline.
    ///
    /// The layout is copied, so it does not need to outlive the view and no
    /// heap allocation happens at rank `<= INLINE_RANK`.
    ///
    /// # Examples
    ///
    /// ```
    /// use strided_view::StridedViewMut;
    ///
    /// let mut data = [0.0f64; 4];
    /// let mut view = StridedViewMut::<f64>::new(&mut data, &[2, 2], &[1, 2], 0).unwrap();
    /// view.set(&[1, 1], 5.0);
    /// assert_eq!(data[3], 5.0);
    /// ```
    #[inline]
    pub fn new(
        data: &'a mut [T],
        dims: &[usize],
        strides: &[isize],
        offset: isize,
    ) -> Result<Self> {
        validate_bounds(data.len(), dims, strides, offset)?;
        Ok(Self::from_meta(data, dims, strides, offset))
    }

    /// Create without bounds checking, copying the caller's metadata.
    ///
    /// # Safety
    /// Caller must ensure all index combinations stay within bounds.
    ///
    /// # Examples
    ///
    /// ```
    /// use strided_view::StridedViewMut;
    ///
    /// let mut data = [0.0f64; 4];
    /// let view = unsafe { StridedViewMut::<f64>::new_unchecked(&mut data, &[2, 2], &[1, 2], 0) };
    /// assert_eq!(view.ndim(), 2);
    /// ```
    #[inline]
    pub unsafe fn new_unchecked(
        data: &'a mut [T],
        dims: &[usize],
        strides: &[isize],
        offset: isize,
    ) -> Self {
        Self::from_meta(data, dims, strides, offset)
    }

    /// Returns the shape (dimension sizes) of this view.
    #[inline]
    pub fn dims(&self) -> &[usize] {
        self.meta.dims()
    }

    /// Returns the strides (in units of `T`) for each dimension.
    #[inline]
    pub fn strides(&self) -> &[isize] {
        self.meta.strides()
    }

    /// Returns the byte offset into the backing data.
    #[inline]
    pub fn offset(&self) -> isize {
        self.offset
    }

    /// Returns the number of dimensions (rank).
    #[inline]
    pub fn ndim(&self) -> usize {
        self.dims().len()
    }

    /// Returns the total number of elements.
    #[inline]
    pub fn len(&self) -> usize {
        self.dims().iter().product()
    }

    /// Returns `true` if any dimension is zero.
    #[inline]
    pub fn is_empty(&self) -> bool {
        self.dims().iter().any(|&d| d == 0)
    }

    /// Raw const pointer to element at the view's base offset.
    #[inline]
    pub fn ptr(&self) -> *const T {
        self.ptr as *const T
    }

    /// Raw mutable pointer to element at the view's base offset.
    #[inline]
    pub fn as_mut_ptr(&self) -> *mut T {
        self.ptr
    }

    /// Returns a reference to the backing data slice.
    #[inline]
    pub fn data(&self) -> &[T] {
        &self.data
    }

    /// Returns a mutable reference to the backing data slice.
    #[inline]
    pub fn data_mut(&mut self) -> &mut [T] {
        &mut self.data
    }

    /// Permute dimensions, consuming the mutable view.
    ///
    /// Returns a new mutable view with reordered dimensions and strides.
    /// Takes `self` by value to prevent aliasing of mutable views.
    pub fn permute(self, perm: &[usize]) -> Result<StridedViewMut<'a, T>> {
        let rank = self.dims().len();
        if perm.len() != rank {
            return Err(StridedError::RankMismatch(perm.len(), rank));
        }
        let mut seen: tinyvec::TinyVec<[bool; INLINE_RANK]> = tinyvec::TinyVec::new();
        seen.resize(rank, false);
        for &p in perm {
            if p >= rank {
                return Err(StridedError::InvalidAxis { axis: p, rank });
            }
            if seen[p] {
                return Err(StridedError::InvalidAxis { axis: p, rank });
            }
            seen[p] = true;
        }
        let new_dims: AxisVec = perm.iter().map(|&p| self.dims()[p]).collect();
        let new_strides: StrideVec = perm.iter().map(|&p| self.strides()[p]).collect();
        Ok(Self::from_meta(
            self.data,
            &new_dims,
            &new_strides,
            self.offset,
        ))
    }

    /// Reborrow as an immutable view.
    ///
    /// The reborrow copies this view's metadata into the immutable view's
    /// inline storage, which allocates nothing up to [`INLINE_RANK`] and
    /// spills above it (the mutable owner always keeps its own copy).
    ///
    /// # Examples
    ///
    /// ```
    /// use strided_view::StridedViewMut;
    ///
    /// let mut data = [1.0f64, 2.0, 3.0];
    /// let view = StridedViewMut::<f64>::new(&mut data, &[3], &[1], 0).unwrap();
    /// let read = view.as_view();
    /// assert_eq!(read.get(&[2]), 3.0);
    /// ```
    #[inline]
    pub fn as_view(&self) -> StridedView<'_, T, Identity> {
        StridedView {
            ptr: self.ptr as *const T,
            data: unsafe { std::slice::from_raw_parts(self.data.as_ptr(), self.data.len()) },
            meta: self.meta.clone(),
            offset: self.offset,
            _op: PhantomData,
        }
    }
}

impl<'a, T: Copy> StridedViewMut<'a, T> {
    /// Get an element.
    ///
    /// # Panics
    ///
    /// Panics if `indices.len()` is not the view's rank, an index is out of
    /// bounds, or the indexed offset does not fit `isize`.
    pub fn get(&self, indices: &[usize]) -> T {
        let offset = indexed_offset(self.dims(), self.strides(), indices, 0);
        unsafe { *self.ptr.offset(offset) }
    }

    /// Set an element.
    ///
    /// # Panics
    ///
    /// Panics if `indices.len()` is not the view's rank, an index is out of
    /// bounds, or the indexed offset does not fit `isize`.
    pub fn set(&mut self, indices: &[usize], value: T) {
        let offset = indexed_offset(self.dims(), self.strides(), indices, 0);
        unsafe {
            *self.ptr.offset(offset) = value;
        }
    }
}

// ============================================================================
// StridedArray
// ============================================================================

/// Owned strided multidimensional array.
///
/// Supports both column-major (Julia default) and row-major (C default) layouts.
pub struct StridedArray<T> {
    data: Vec<T>,
    meta: ViewMeta,
    offset: isize,
}

impl<T: std::fmt::Debug> std::fmt::Debug for StridedArray<T> {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        f.debug_struct("StridedArray")
            .field("dims", &self.meta.dims())
            .field("strides", &self.meta.strides())
            .field("offset", &self.offset)
            .finish()
    }
}

impl<T: Clone> Clone for StridedArray<T> {
    fn clone(&self) -> Self {
        Self {
            data: self.data.clone(),
            meta: self.meta.clone(),
            offset: self.offset,
        }
    }
}

impl<T: Clone + Default> StridedArray<T> {
    /// Create a column-major (Julia default) tensor filled with Default values.
    ///
    /// The shape and strides are stored inline, so a rank `<= INLINE_RANK`
    /// tensor allocates only its element buffer.
    ///
    /// # Examples
    ///
    /// ```
    /// use strided_view::StridedArray;
    ///
    /// let a = StridedArray::<f64>::col_major(&[2, 3]);
    /// assert_eq!(a.dims(), &[2, 3]);
    /// assert_eq!(a.strides(), &[1, 2]);
    /// ```
    pub fn col_major(dims: &[usize]) -> Self {
        let total: usize = dims.iter().product();
        let data = vec![T::default(); total];
        let strides = col_major_strides(dims);
        Self {
            data,
            meta: ViewMeta::from_slices(dims, &strides),
            offset: 0,
        }
    }

    /// Create a row-major (C default) tensor filled with Default values.
    ///
    /// The shape and strides are stored inline, so a rank `<= INLINE_RANK`
    /// tensor allocates only its element buffer.
    ///
    /// # Examples
    ///
    /// ```
    /// use strided_view::StridedArray;
    ///
    /// let a = StridedArray::<f64>::row_major(&[2, 3]);
    /// assert_eq!(a.dims(), &[2, 3]);
    /// assert_eq!(a.strides(), &[3, 1]);
    /// ```
    pub fn row_major(dims: &[usize]) -> Self {
        let total: usize = dims.iter().product();
        let data = vec![T::default(); total];
        let strides = row_major_strides(dims);
        Self {
            data,
            meta: ViewMeta::from_slices(dims, &strides),
            offset: 0,
        }
    }

    /// Create a column-major tensor with values produced by a function.
    ///
    /// The function is called with indices in column-major iteration order.
    pub fn from_fn_col_major(dims: &[usize], mut f: impl FnMut(&[usize]) -> T) -> Self {
        let total: usize = dims.iter().product();
        let strides = col_major_strides(dims);
        let rank = dims.len();
        let mut data = Vec::with_capacity(total);
        let mut idx: AxisVec = AxisVec::new();
        idx.resize(rank, 0usize);
        // Borrow the cursor once; indexing the `TinyVec` per access would pay
        // its inline/heap match in the element-count loop.
        let idx: &mut [usize] = &mut idx[..];
        for _ in 0..total {
            data.push(f(idx));
            for d in 0..rank {
                idx[d] += 1;
                if idx[d] < dims[d] {
                    break;
                }
                idx[d] = 0;
            }
        }
        Self {
            data,
            meta: ViewMeta::from_slices(dims, &strides),
            offset: 0,
        }
    }

    /// Create a row-major tensor with values produced by a function.
    ///
    /// The function is called with indices in row-major iteration order.
    pub fn from_fn_row_major(dims: &[usize], mut f: impl FnMut(&[usize]) -> T) -> Self {
        let total: usize = dims.iter().product();
        let strides = row_major_strides(dims);
        let rank = dims.len();
        let mut data = Vec::with_capacity(total);
        let mut idx: AxisVec = AxisVec::new();
        idx.resize(rank, 0usize);
        // Borrow the cursor once; see `from_fn_col_major`.
        let idx: &mut [usize] = &mut idx[..];
        for _ in 0..total {
            data.push(f(idx));
            for d in (0..rank).rev() {
                idx[d] += 1;
                if idx[d] < dims[d] {
                    break;
                }
                idx[d] = 0;
            }
        }
        Self {
            data,
            meta: ViewMeta::from_slices(dims, &strides),
            offset: 0,
        }
    }
}

impl<T> StridedArray<T> {
    /// Create from raw parts.
    pub fn from_parts(
        data: Vec<T>,
        dims: &[usize],
        strides: &[isize],
        offset: isize,
    ) -> Result<Self> {
        validate_bounds(data.len(), dims, strides, offset)?;
        Ok(Self {
            data,
            meta: ViewMeta::from_slices(dims, strides),
            offset,
        })
    }

    /// Returns the shape (dimension sizes) of this array.
    #[inline]
    pub fn dims(&self) -> &[usize] {
        self.meta.dims()
    }

    /// Returns the strides (in units of `T`) for each dimension.
    #[inline]
    pub fn strides(&self) -> &[isize] {
        self.meta.strides()
    }

    /// Returns the number of dimensions (rank).
    #[inline]
    pub fn ndim(&self) -> usize {
        self.dims().len()
    }

    /// Returns the total number of elements.
    #[inline]
    pub fn len(&self) -> usize {
        self.dims().iter().product()
    }

    /// Returns `true` if any dimension is zero.
    #[inline]
    pub fn is_empty(&self) -> bool {
        self.dims().iter().any(|&d| d == 0)
    }

    /// Returns a reference to the backing data slice.
    #[inline]
    pub fn data(&self) -> &[T] {
        &self.data
    }

    /// Returns a mutable reference to the backing data slice.
    #[inline]
    pub fn data_mut(&mut self) -> &mut [T] {
        &mut self.data
    }

    /// Create an immutable view over this tensor.
    #[inline]
    pub fn view(&self) -> StridedView<'_, T> {
        let ptr = if self.is_empty() {
            empty_const_ptr()
        } else {
            unsafe { self.data.as_ptr().offset(self.offset) }
        };
        StridedView {
            ptr,
            data: &self.data,
            meta: self.meta.clone(),
            offset: self.offset,
            _op: PhantomData,
        }
    }

    /// Create a mutable view over this tensor.
    #[inline]
    pub fn view_mut(&mut self) -> StridedViewMut<'_, T> {
        let ptr = if self.is_empty() {
            empty_mut_ptr()
        } else {
            unsafe { self.data.as_mut_ptr().offset(self.offset) }
        };
        StridedViewMut {
            ptr,
            data: &mut self.data,
            meta: self.meta.clone(),
            offset: self.offset,
        }
    }

    /// Permute dimensions (metadata-only reorder, no data copy).
    ///
    /// Returns a new array with reordered dims and strides.
    /// The underlying data buffer is not touched.
    pub fn permuted(self, perm: &[usize]) -> Result<Self> {
        let rank = self.dims().len();
        if perm.len() != rank {
            return Err(StridedError::RankMismatch(perm.len(), rank));
        }
        let mut seen: tinyvec::TinyVec<[bool; INLINE_RANK]> = tinyvec::TinyVec::new();
        seen.resize(rank, false);
        for &p in perm {
            if p >= rank {
                return Err(StridedError::InvalidAxis { axis: p, rank });
            }
            if seen[p] {
                return Err(StridedError::InvalidAxis { axis: p, rank });
            }
            seen[p] = true;
        }
        let new_dims: AxisVec = perm.iter().map(|&p| self.dims()[p]).collect();
        let new_strides: StrideVec = perm.iter().map(|&p| self.strides()[p]).collect();
        Ok(Self {
            data: self.data,
            meta: ViewMeta::from_slices(&new_dims, &new_strides),
            offset: self.offset,
        })
    }

    /// Consume the array and return the backing `Vec<T>`.
    pub fn into_data(self) -> Vec<T> {
        self.data
    }

    /// Iterate over all elements in memory order.
    pub fn iter(&self) -> std::slice::Iter<'_, T> {
        self.data.iter()
    }

    /// Mutable iteration over all elements in memory order.
    pub fn iter_mut(&mut self) -> std::slice::IterMut<'_, T> {
        self.data.iter_mut()
    }
}

impl<T: Default> StridedArray<T> {
    /// Create a column-major tensor reusing an existing buffer.
    ///
    /// If `buf` has at least `product(dims)` elements, it is truncated and
    /// zeroed.  Otherwise a fresh buffer is allocated.
    pub fn col_major_from_buffer(mut buf: Vec<T>, dims: &[usize]) -> Self {
        let total: usize = dims.iter().product();
        if buf.len() >= total {
            buf.truncate(total);
        } else {
            buf.resize_with(total, T::default);
        }
        // Zero the reused region
        for v in buf.iter_mut() {
            *v = T::default();
        }
        let strides = col_major_strides(dims);
        Self {
            data: buf,
            meta: ViewMeta::from_slices(dims, &strides),
            offset: 0,
        }
    }
}

impl<T: Copy> StridedArray<T> {
    /// Create a column-major tensor with **uninitialized** data.
    ///
    /// # Safety
    /// Caller must write every element before reading.
    pub unsafe fn col_major_uninit(dims: &[usize]) -> Self {
        let total: usize = dims.iter().product();
        let mut data = Vec::with_capacity(total);
        data.set_len(total);
        let strides = col_major_strides(dims);
        Self {
            data,
            meta: ViewMeta::from_slices(dims, &strides),
            offset: 0,
        }
    }

    /// Reuse an existing buffer as a column-major tensor **without zeroing**.
    ///
    /// # Safety
    /// Caller must write every element before reading.
    pub unsafe fn col_major_from_buffer_uninit(mut buf: Vec<T>, dims: &[usize]) -> Self {
        let total: usize = dims.iter().product();
        if buf.capacity() < total {
            buf.reserve(total - buf.len());
        }
        buf.set_len(total);
        let strides = col_major_strides(dims);
        Self {
            data: buf,
            meta: ViewMeta::from_slices(dims, &strides),
            offset: 0,
        }
    }
}

impl<T: Copy> StridedArray<T> {
    /// Get an element by multi-dimensional index.
    ///
    /// # Panics
    ///
    /// Panics if `indices.len()` is not the array's rank, an index is out of
    /// bounds, or the indexed offset does not fit `isize`.
    pub fn get(&self, indices: &[usize]) -> T {
        self.view().get(indices)
    }

    /// Set an element by multi-dimensional index.
    ///
    /// # Panics
    ///
    /// Panics if `indices.len()` is not the array's rank, an index is out of
    /// bounds, or the indexed offset does not fit `isize`.
    pub fn set(&mut self, indices: &[usize], value: T) {
        let offset = indexed_offset(self.dims(), self.strides(), indices, self.offset);
        self.data[offset as usize] = value;
    }
}

impl<T: Copy> Index<&[usize]> for StridedArray<T> {
    type Output = T;

    fn index(&self, indices: &[usize]) -> &T {
        let offset = indexed_offset(self.dims(), self.strides(), indices, self.offset);
        &self.data[offset as usize]
    }
}

impl<T: Copy> IndexMut<&[usize]> for StridedArray<T> {
    fn index_mut(&mut self, indices: &[usize]) -> &mut T {
        let offset = indexed_offset(self.dims(), self.strides(), indices, self.offset);
        &mut self.data[offset as usize]
    }
}

// ============================================================================
// Tests
// ============================================================================

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn empty_views_use_dangling_base_pointers_for_extreme_offsets() {
        let dims = [0usize];
        let strides = [1isize];
        let data: [f64; 0] = [];
        let view = StridedView::<f64>::new(&data, &dims, &strides, isize::MAX).unwrap();
        assert_eq!(view.ptr(), empty_const_ptr());

        let mut data: [f64; 0] = [];
        let view = StridedViewMut::new(&mut data, &dims, &strides, isize::MAX).unwrap();
        assert_eq!(view.ptr(), empty_const_ptr());
        assert_eq!(view.as_mut_ptr(), empty_mut_ptr());

        let array =
            StridedArray::<f64>::from_parts(Vec::new(), &dims, &strides, isize::MAX).unwrap();
        assert_eq!(array.view().ptr(), empty_const_ptr());

        let mut array =
            StridedArray::<f64>::from_parts(Vec::new(), &dims, &strides, isize::MAX).unwrap();
        let view = array.view_mut();
        assert_eq!(view.ptr(), empty_const_ptr());
        assert_eq!(view.as_mut_ptr(), empty_mut_ptr());
    }
    use num_complex::Complex64;

    #[test]
    fn test_col_major_strides() {
        assert_eq!(col_major_strides(&[3, 4]).as_slice(), &[1, 3]);
        assert_eq!(col_major_strides(&[2, 3, 4]).as_slice(), &[1, 2, 6]);
    }

    #[test]
    fn test_row_major_strides() {
        assert_eq!(row_major_strides(&[3, 4]).as_slice(), &[4, 1]);
        assert_eq!(row_major_strides(&[2, 3, 4]).as_slice(), &[12, 4, 1]);
    }

    #[test]
    fn test_strided_view_new() {
        let data = vec![1.0, 2.0, 3.0, 4.0, 5.0, 6.0];
        let view = StridedView::<f64>::new(&data, &[2, 3], &[3, 1], 0).unwrap();
        assert_eq!(view.ndim(), 2);
        assert_eq!(view.dims(), &[2, 3]);
        assert_eq!(view.strides(), &[3, 1]);
        assert_eq!(view.len(), 6);
    }

    #[test]
    fn test_strided_view_get() {
        let data = vec![1.0, 2.0, 3.0, 4.0, 5.0, 6.0];
        let view = StridedView::<f64>::new(&data, &[2, 3], &[3, 1], 0).unwrap();
        assert_eq!(view.get(&[0, 0]), 1.0);
        assert_eq!(view.get(&[0, 1]), 2.0);
        assert_eq!(view.get(&[0, 2]), 3.0);
        assert_eq!(view.get(&[1, 0]), 4.0);
        assert_eq!(view.get(&[1, 2]), 6.0);
    }

    #[test]
    fn test_strided_view_col_major() {
        // Column-major: strides [1, 2] for 2x3
        let data = vec![1.0, 2.0, 3.0, 4.0, 5.0, 6.0];
        let view = StridedView::<f64>::new(&data, &[2, 3], &[1, 2], 0).unwrap();
        assert_eq!(view.get(&[0, 0]), 1.0); // data[0]
        assert_eq!(view.get(&[1, 0]), 2.0); // data[1]
        assert_eq!(view.get(&[0, 1]), 3.0); // data[2]
        assert_eq!(view.get(&[1, 1]), 4.0); // data[3]
        assert_eq!(view.get(&[0, 2]), 5.0); // data[4]
        assert_eq!(view.get(&[1, 2]), 6.0); // data[5]
    }

    #[test]
    fn test_strided_view_permute() {
        let data = vec![1.0, 2.0, 3.0, 4.0, 5.0, 6.0];
        let view = StridedView::<f64>::new(&data, &[2, 3], &[3, 1], 0).unwrap();
        let perm = view.permute(&[1, 0]).unwrap();
        assert_eq!(perm.dims(), &[3, 2]);
        assert_eq!(perm.strides(), &[1, 3]);
        assert_eq!(perm.get(&[0, 0]), 1.0);
        assert_eq!(perm.get(&[1, 0]), 2.0);
        assert_eq!(perm.get(&[0, 1]), 4.0);
    }

    #[test]
    fn test_strided_view_transpose_2d() {
        let data = vec![1.0, 2.0, 3.0, 4.0, 5.0, 6.0];
        let view = StridedView::<f64>::new(&data, &[2, 3], &[3, 1], 0).unwrap();
        let t = view.transpose_2d().unwrap();
        assert_eq!(t.dims(), &[3, 2]);
        assert_eq!(t.get(&[0, 0]), 1.0);
        assert_eq!(t.get(&[1, 0]), 2.0);
        assert_eq!(t.get(&[0, 1]), 4.0);
    }

    #[test]
    fn test_strided_view_conj() {
        let data = vec![Complex64::new(1.0, 2.0), Complex64::new(3.0, 4.0)];
        let view = StridedView::<Complex64>::new(&data, &[2], &[1], 0).unwrap();
        let c = view.conj();
        assert_eq!(c.get(&[0]), Complex64::new(1.0, -2.0));
        assert_eq!(c.get(&[1]), Complex64::new(3.0, -4.0));
    }

    #[test]
    fn test_strided_view_adjoint_2d() {
        let data = vec![
            Complex64::new(1.0, 2.0),
            Complex64::new(3.0, 4.0),
            Complex64::new(5.0, 6.0),
            Complex64::new(7.0, 8.0),
        ];
        // 2x2 row-major
        let view = StridedView::<Complex64>::new(&data, &[2, 2], &[2, 1], 0).unwrap();
        let adj = view.adjoint_2d().unwrap();
        assert_eq!(adj.dims(), &[2, 2]);
        // Adjoint: conj + transpose
        assert_eq!(adj.get(&[0, 0]), Complex64::new(1.0, -2.0));
        assert_eq!(adj.get(&[1, 0]), Complex64::new(3.0, -4.0));
        assert_eq!(adj.get(&[0, 1]), Complex64::new(5.0, -6.0));
    }

    #[test]
    fn test_strided_view_broadcast() {
        let data = vec![1.0, 2.0, 3.0];
        let view = StridedView::<f64>::new(&data, &[1, 3], &[3, 1], 0).unwrap();
        let broad = view.broadcast(&[4, 3]).unwrap();
        assert_eq!(broad.dims(), &[4, 3]);
        for i in 0..4 {
            assert_eq!(broad.get(&[i, 0]), 1.0);
            assert_eq!(broad.get(&[i, 1]), 2.0);
            assert_eq!(broad.get(&[i, 2]), 3.0);
        }
    }

    #[test]
    fn test_strided_view_mut() {
        let mut data = vec![0.0; 6];
        {
            let mut view = StridedViewMut::<f64>::new(&mut data, &[2, 3], &[3, 1], 0).unwrap();
            view.set(&[0, 0], 1.0);
            view.set(&[1, 2], 6.0);
        }
        assert_eq!(data[0], 1.0);
        assert_eq!(data[5], 6.0);
    }

    #[test]
    fn test_strided_view_mut_as_view() {
        let mut data = vec![1.0, 2.0, 3.0];
        let vm = StridedViewMut::<f64>::new(&mut data, &[3], &[1], 0).unwrap();
        let v = vm.as_view();
        assert_eq!(v.get(&[0]), 1.0);
        assert_eq!(v.get(&[2]), 3.0);
    }

    #[test]
    fn test_strided_tensor_col_major() {
        let t = StridedArray::<f64>::from_fn_col_major(&[2, 3], |idx| (idx[0] * 3 + idx[1]) as f64);
        assert_eq!(t.dims(), &[2, 3]);
        assert_eq!(t.strides(), &[1, 2]); // column-major
        assert_eq!(t.get(&[0, 0]), 0.0);
        assert_eq!(t.get(&[1, 0]), 3.0);
        assert_eq!(t.get(&[0, 1]), 1.0);
        assert_eq!(t.get(&[1, 2]), 5.0);
    }

    #[test]
    fn test_strided_tensor_row_major() {
        let t = StridedArray::<f64>::from_fn_row_major(&[2, 3], |idx| (idx[0] * 3 + idx[1]) as f64);
        assert_eq!(t.dims(), &[2, 3]);
        assert_eq!(t.strides(), &[3, 1]); // row-major
        assert_eq!(t.get(&[0, 0]), 0.0);
        assert_eq!(t.get(&[0, 1]), 1.0);
        assert_eq!(t.get(&[1, 0]), 3.0);
        assert_eq!(t.get(&[1, 2]), 5.0);
    }

    #[test]
    fn test_strided_tensor_view() {
        let t =
            StridedArray::<f64>::from_fn_col_major(&[2, 3], |idx| (idx[0] * 10 + idx[1]) as f64);
        let v = t.view();
        assert_eq!(v.get(&[0, 0]), 0.0);
        assert_eq!(v.get(&[1, 0]), 10.0);
        assert_eq!(v.get(&[0, 2]), 2.0);
    }

    #[test]
    fn test_strided_tensor_view_mut() {
        let mut t = StridedArray::<f64>::col_major(&[2, 3]);
        {
            let mut vm = t.view_mut();
            vm.set(&[1, 2], 42.0);
        }
        assert_eq!(t.get(&[1, 2]), 42.0);
    }

    #[test]
    fn test_strided_tensor_index() {
        let t =
            StridedArray::<f64>::from_fn_row_major(&[3, 4], |idx| (idx[0] * 10 + idx[1]) as f64);
        assert_eq!(t[&[0usize, 0] as &[usize]], 0.0);
        assert_eq!(t[&[2usize, 3] as &[usize]], 23.0);
    }

    #[test]
    fn test_strided_tensor_index_mut() {
        let mut t = StridedArray::<f64>::row_major(&[2, 3]);
        t[&[1usize, 2] as &[usize]] = 99.0;
        assert_eq!(t.get(&[1, 2]), 99.0);
    }

    #[test]
    fn test_validate_bounds_ok() {
        assert!(validate_bounds(6, &[2, 3], &[3, 1], 0).is_ok());
        assert!(validate_bounds(6, &[2, 3], &[1, 2], 0).is_ok());
    }

    #[test]
    fn test_validate_bounds_out_of_range() {
        assert!(validate_bounds(5, &[2, 3], &[3, 1], 0).is_err());
    }

    #[test]
    fn test_validate_bounds_empty() {
        assert!(validate_bounds(0, &[0, 3], &[3, 1], 0).is_ok());
    }

    #[test]
    fn test_validate_bounds_with_offset() {
        assert!(validate_bounds(7, &[2, 3], &[3, 1], 1).is_ok());
        assert!(validate_bounds(6, &[2, 3], &[3, 1], 1).is_err());
    }

    #[test]
    fn test_strided_tensor_3d() {
        let t = StridedArray::<f64>::from_fn_col_major(&[2, 3, 4], |idx| {
            (idx[0] * 100 + idx[1] * 10 + idx[2]) as f64
        });
        assert_eq!(t.ndim(), 3);
        assert_eq!(t.strides(), &[1, 2, 6]); // column-major
        assert_eq!(t.get(&[0, 0, 0]), 0.0);
        assert_eq!(t.get(&[1, 0, 0]), 100.0);
        assert_eq!(t.get(&[0, 1, 0]), 10.0);
        assert_eq!(t.get(&[0, 0, 1]), 1.0);
        assert_eq!(t.get(&[1, 2, 3]), 123.0);
    }

    #[test]
    fn test_diagonal_view_2d() {
        // A[i,i] shape=[3,3] row-major strides=[3,1]
        // diagonal: shape=[3] strides=[4] (3+1)
        let data: Vec<f64> = (0..9).map(|x| x as f64).collect();
        let view = StridedView::<f64>::new(&data, &[3, 3], &[3, 1], 0).unwrap();
        let diag = view.diagonal_view(&[(0, 1)]).unwrap();
        assert_eq!(diag.dims(), &[3]);
        assert_eq!(diag.strides(), &[4]);
        assert_eq!(diag.get(&[0]), 0.0); // A[0,0]
        assert_eq!(diag.get(&[1]), 4.0); // A[1,1]
        assert_eq!(diag.get(&[2]), 8.0); // A[2,2]
    }

    #[test]
    fn test_diagonal_view_3d_adjacent() {
        // A[i,i,j] shape=[2,2,3] row-major strides=[6,3,1]
        // diagonal over (0,1): shape=[2,3] strides=[9,1] (6+3)
        let data: Vec<f64> = (0..12).map(|x| x as f64).collect();
        let view = StridedView::<f64>::new(&data, &[2, 2, 3], &[6, 3, 1], 0).unwrap();
        let diag = view.diagonal_view(&[(0, 1)]).unwrap();
        assert_eq!(diag.dims(), &[2, 3]);
        assert_eq!(diag.strides(), &[9, 1]);
        assert_eq!(diag.get(&[0, 0]), 0.0);
        assert_eq!(diag.get(&[0, 2]), 2.0);
        assert_eq!(diag.get(&[1, 0]), 9.0);
    }

    #[test]
    fn test_diagonal_view_3d_non_adjacent() {
        // A[i,j,i] shape=[2,3,2] row-major strides=[6,2,1]
        // diagonal over (0,2): shape=[2,3] strides=[7,2] (6+1)
        let data: Vec<f64> = (0..12).map(|x| x as f64).collect();
        let view = StridedView::<f64>::new(&data, &[2, 3, 2], &[6, 2, 1], 0).unwrap();
        let diag = view.diagonal_view(&[(0, 2)]).unwrap();
        assert_eq!(diag.dims(), &[2, 3]);
        assert_eq!(diag.strides(), &[7, 2]);
        assert_eq!(diag.get(&[0, 0]), 0.0);
        assert_eq!(diag.get(&[0, 1]), 2.0);
        assert_eq!(diag.get(&[1, 0]), 7.0);
        assert_eq!(diag.get(&[1, 2]), 11.0);
    }

    #[test]
    fn test_diagonal_view_two_pairs() {
        // A[i,j,i,j] shape=[2,3,2,3] -> A_diag[i,j] shape=[2,3]
        let data: Vec<f64> = (0..36).map(|x| x as f64).collect();
        let view = StridedView::<f64>::new(&data, &[2, 3, 2, 3], &[18, 6, 3, 1], 0).unwrap();
        let diag = view.diagonal_view(&[(0, 2), (1, 3)]).unwrap();
        assert_eq!(diag.dims(), &[2, 3]);
        assert_eq!(diag.strides(), &[21, 7]);
        assert_eq!(diag.get(&[0, 0]), 0.0);
        assert_eq!(diag.get(&[1, 1]), 28.0);
    }

    #[test]
    fn test_diagonal_view_rejects_pairs_that_leave_the_backing() {
        let data: Vec<f64> = (0..8).map(|x| x as f64).collect();
        let view = StridedView::<f64>::new(&data, &[2, 2, 2], &[4, 2, 1], 0).unwrap();
        // Axis 2 is folded into both axis 0 and axis 1, so the result reaches
        // offset 8 of a length-8 backing.
        assert!(view.diagonal_view(&[(0, 2), (1, 2)]).is_err());
        // The same pair twice doubles one axis's stride contribution.
        assert!(view.diagonal_view(&[(0, 1), (0, 1)]).is_err());
    }

    #[test]
    fn test_diagonal_view_fuses_a_shared_low_axis_in_bounds() {
        // A[i,i,i] is [(0, 1), (0, 2)]: axis 0 carries both fused strides and
        // the result stays inside the backing.
        let data: Vec<f64> = (0..8).map(|x| x as f64).collect();
        let view = StridedView::<f64>::new(&data, &[2, 2, 2], &[4, 2, 1], 0).unwrap();
        let diag = view.diagonal_view(&[(0, 1), (0, 2)]).unwrap();
        assert_eq!(diag.dims(), &[2]);
        assert_eq!(diag.strides(), &[7]);
        assert_eq!(diag.get(&[0]), 0.0);
        assert_eq!(diag.get(&[1]), 7.0);
    }

    #[test]
    fn test_custom_type_without_element_op_apply() {
        // Custom Copy type that does NOT implement ElementOpApply.
        // Should work with Identity views (StridedView<T> default).
        #[derive(Debug, Clone, Copy, PartialEq)]
        struct MyScalar(f64);

        impl Default for MyScalar {
            fn default() -> Self {
                MyScalar(0.0)
            }
        }

        // Create array with custom type
        let data = vec![MyScalar(1.0), MyScalar(2.0), MyScalar(3.0), MyScalar(4.0)];
        let arr = StridedArray::from_parts(data, &[2, 2], &[2, 1], 0).unwrap();

        // Access via Identity view (default)
        assert_eq!(arr.get(&[0, 0]), MyScalar(1.0));
        assert_eq!(arr.get(&[1, 0]), MyScalar(3.0));

        // Create view and access elements
        let view: StridedView<MyScalar> = arr.view();
        assert_eq!(view.get(&[0, 1]), MyScalar(2.0));

        // Permute view (metadata-only, no ElementOpApply needed)
        let perm = view.permute(&[1, 0]).unwrap();
        assert_eq!(perm.get(&[0, 0]), MyScalar(1.0));
        assert_eq!(perm.get(&[0, 1]), MyScalar(3.0)); // transposed
    }

    #[test]
    #[should_panic(expected = "wrong number of indices")]
    fn test_index_rejects_a_short_index() {
        let arr = StridedArray::from_parts(vec![0.0f64; 4], &[2, 2], &[2, 1], 0).unwrap();
        // Without the rank check this reads an element the index does not name.
        let _ = arr[&[1][..]];
    }

    #[test]
    #[should_panic(expected = "wrong number of indices")]
    fn test_view_get_rejects_a_short_index() {
        let data = vec![0.0f64; 4];
        let view = StridedView::<f64>::new(&data, &[2, 2], &[2, 1], 0).unwrap();
        let _ = view.get(&[1]);
    }

    #[test]
    #[should_panic(expected = "out of bounds")]
    fn test_view_set_rejects_an_out_of_bounds_index() {
        let mut data = vec![0.0f64; 4];
        let mut view = StridedViewMut::<f64>::new(&mut data, &[2, 2], &[2, 1], 0).unwrap();
        view.set(&[2, 0], 1.0);
    }
}