ferray_core/manipulation/

mod.rs

// ferray-core: Shape manipulation functions (REQ-20, REQ-21, REQ-22)
//
// Mirrors numpy's shape manipulation routines: reshape, ravel, flatten,
// concatenate, stack, transpose, flip, etc.
//
// ## REQ status (shape manipulation, NumPy parity)
//  - REQ-20 (shape methods) — SHIPPED: `reshape`/`ravel`/`flatten`/`squeeze`/
//    `expand_dims`/`broadcast_to` (this file; `broadcast_to` re-exports the
//    `dimension::broadcast` primitive). Non-test consumer: `array/arc.rs`
//    materializes via `manipulation::broadcast_to`.
//  - REQ-21 (join/split) — SHIPPED: `concatenate`/`stack`/`vstack`/`hstack`/
//    `dstack`/`column_stack`/`row_stack`/`block`/`split`/`array_split`/
//    `array_split_n`/`vsplit`/`hsplit`/`dsplit` (this file).
//  - REQ-22 (transpose/reorder) — SHIPPED: `transpose`/`swapaxes`/`moveaxis`/
//    `rollaxis`/`flip`/`fliplr`/`flipud`/`rot90`/`roll` (this file). Non-test
//    consumer: `array/arc.rs` calls `manipulation::transpose`.
//  - `r_`/`c_` (NumPy index-expression concatenation builders) — SHIPPED:
//    `r_`/`c_` (this file).

pub mod extended;

use crate::array::owned::Array;
use crate::dimension::{Dimension, Ix1, IxDyn};
use crate::dtype::Element;
use crate::error::{FerrayError, FerrayResult};

// ============================================================================
// REQ-20: Shape methods
// ============================================================================

/// Reshape an array to a new shape (returns a new owned array).
///
/// The total number of elements must remain the same.
///
/// Analogous to `numpy.reshape()`. Delegates to ndarray's `to_shape`,
/// which re-uses the existing buffer (zero copy on the data side) for
/// contiguous inputs and only does a bulk memcpy via `to_owned` — the
/// old path ran an element-by-element `iter().cloned().collect()`
/// instead (issue #82).
///
/// # Errors
/// Returns `FerrayError::ShapeMismatch` if the new shape has a different
/// total number of elements.
pub fn reshape<T: Element, D: Dimension>(
    a: &Array<T, D>,
    new_shape: &[usize],
) -> FerrayResult<Array<T, IxDyn>> {
    let old_size = a.size();
    let new_size: usize = new_shape.iter().product();
    if old_size != new_size {
        return Err(FerrayError::shape_mismatch(format!(
            "cannot reshape array of size {old_size} into shape {new_shape:?} (size {new_size})",
        )));
    }
    let view = a.inner.view().into_dyn();
    let reshaped = view
        .to_shape(ndarray::IxDyn(new_shape))
        .map_err(|e| FerrayError::shape_mismatch(e.to_string()))?;
    // `as_standard_layout` ensures the final buffer is C-contiguous so
    // `as_slice()` works for callers, even when the source view was
    // F-contiguous / strided.
    Ok(Array::from_ndarray(
        reshaped.as_standard_layout().into_owned(),
    ))
}

/// Return a flattened (1-D) copy of the array.
///
/// Analogous to `numpy.ravel()`. Uses ndarray's `to_shape` rather than
/// the element-wise collect path (issue #82).
pub fn ravel<T: Element, D: Dimension>(a: &Array<T, D>) -> FerrayResult<Array<T, Ix1>> {
    let n = a.size();
    let view = a.inner.view().into_dyn();
    let reshaped = view
        .to_shape(ndarray::IxDyn(&[n]))
        .expect("1-D reshape always succeeds for a size-preserving target");
    let standard = reshaped.as_standard_layout().into_owned();
    let one_d = standard
        .into_dimensionality::<ndarray::Ix1>()
        .expect("reshape result has ndim == 1 by construction");
    Ok(Array::from_ndarray(one_d))
}

/// Return a flattened (1-D) copy of the array.
///
/// Identical to `ravel()` — analogous to `ndarray.flatten()`.
pub fn flatten<T: Element, D: Dimension>(a: &Array<T, D>) -> FerrayResult<Array<T, Ix1>> {
    ravel(a)
}

/// Remove axes of length 1 from the shape.
///
/// If `axis` is `None`, all length-1 axes are removed.
/// If `axis` is `Some(ax)`, only that axis is removed (errors if it is not length 1).
///
/// Analogous to `numpy.squeeze()`.
///
/// # Errors
/// Returns `FerrayError::AxisOutOfBounds` if the axis is invalid, or
/// `FerrayError::InvalidValue` if the specified axis has size != 1.
pub fn squeeze<T: Element, D: Dimension>(
    a: &Array<T, D>,
    axis: Option<usize>,
) -> FerrayResult<Array<T, IxDyn>> {
    let shape = a.shape();
    if let Some(ax) = axis {
        if ax >= shape.len() {
            return Err(FerrayError::axis_out_of_bounds(ax, shape.len()));
        }
        if shape[ax] != 1 {
            return Err(FerrayError::invalid_value(format!(
                "cannot select axis {} with size {} for squeeze (must be 1)",
                ax, shape[ax],
            )));
        }
        let new_shape: Vec<usize> = shape
            .iter()
            .enumerate()
            .filter(|&(i, _)| i != ax)
            .map(|(_, &s)| s)
            .collect();
        let data: Vec<T> = a.iter().cloned().collect();
        Array::from_vec(IxDyn::new(&new_shape), data)
    } else {
        // Remove every length-1 axis. When *all* axes are length 1 the
        // result collapses to a 0-D array (shape `[]`), matching numpy:
        // `np.squeeze(np.ones((1,1,1))).shape == ()` — numpy
        // _core/fromnumeric.py:1615 ("if all axes are squeezed, the result
        // is a 0d array and not a scalar"). An empty `new_shape` already
        // encodes the 0-D case, so no special-casing is needed.
        let new_shape: Vec<usize> = shape.iter().copied().filter(|&s| s != 1).collect();
        let data: Vec<T> = a.iter().cloned().collect();
        Array::from_vec(IxDyn::new(&new_shape), data)
    }
}

/// Insert a new axis of length 1 at the given position.
///
/// Analogous to `numpy.expand_dims()`.
///
/// # Errors
/// Returns `FerrayError::AxisOutOfBounds` if `axis > ndim`.
pub fn expand_dims<T: Element, D: Dimension>(
    a: &Array<T, D>,
    axis: usize,
) -> FerrayResult<Array<T, IxDyn>> {
    let ndim = a.ndim();
    if axis > ndim {
        return Err(FerrayError::axis_out_of_bounds(axis, ndim + 1));
    }
    let mut new_shape: Vec<usize> = a.shape().to_vec();
    new_shape.insert(axis, 1);
    let data: Vec<T> = a.iter().cloned().collect();
    Array::from_vec(IxDyn::new(&new_shape), data)
}

/// Broadcast an array to a new shape (returns a new owned array).
///
/// The array is replicated along size-1 dimensions to match the target shape.
///
/// Analogous to `numpy.broadcast_to()`. Uses ndarray's `broadcast` to
/// produce a zero-copy stride-0 view and then materializes it with a
/// single `to_owned` — the prior implementation allocated a source
/// buffer via `iter().cloned().collect()` and walked every output
/// index in a hand-rolled nested loop (issue #81).
///
/// # Errors
/// Returns `FerrayError::BroadcastFailure` if the shapes are incompatible.
pub fn broadcast_to<T: Element, D: Dimension>(
    a: &Array<T, D>,
    new_shape: &[usize],
) -> FerrayResult<Array<T, IxDyn>> {
    let src_shape = a.shape();
    let dyn_view = a.inner.view().into_dyn();
    let broadcast_view = dyn_view
        .broadcast(ndarray::IxDyn(new_shape))
        .ok_or_else(|| FerrayError::BroadcastFailure {
            shape_a: src_shape.to_vec(),
            shape_b: new_shape.to_vec(),
        })?;
    // `as_standard_layout` turns the stride-0 broadcast view into a
    // proper C-contiguous owned buffer in one pass; the previous
    // hand-rolled loop walked every destination index separately.
    Ok(Array::from_ndarray(
        broadcast_view.as_standard_layout().into_owned(),
    ))
}

// ============================================================================
// REQ-21: Join/split
// ============================================================================

/// Join a sequence of arrays along an existing axis.
///
/// Analogous to `numpy.concatenate()`.
///
/// # Errors
/// Returns `FerrayError::InvalidValue` if the array list is empty.
/// Returns `FerrayError::ShapeMismatch` if shapes differ on non-concatenation axes.
/// Returns `FerrayError::AxisOutOfBounds` if axis is out of bounds.
pub fn concatenate<T: Element>(
    arrays: &[Array<T, IxDyn>],
    axis: usize,
) -> FerrayResult<Array<T, IxDyn>> {
    if arrays.is_empty() {
        return Err(FerrayError::invalid_value(
            "concatenate: need at least one array",
        ));
    }
    let ndim = arrays[0].ndim();
    if axis >= ndim {
        return Err(FerrayError::axis_out_of_bounds(axis, ndim));
    }
    let base_shape = arrays[0].shape();

    // Validate all arrays have same ndim and matching shapes on non-concat axes
    let mut total_along_axis = 0usize;
    for arr in arrays {
        if arr.ndim() != ndim {
            return Err(FerrayError::shape_mismatch(format!(
                "all arrays must have same ndim; got {} and {}",
                ndim,
                arr.ndim(),
            )));
        }
        for (i, (&s, &base)) in arr.shape().iter().zip(base_shape.iter()).enumerate() {
            if i != axis && s != base {
                return Err(FerrayError::shape_mismatch(format!(
                    "shape mismatch on axis {i}: {s} vs {base}",
                )));
            }
        }
        total_along_axis += arr.shape()[axis];
    }

    // Build new shape
    let mut new_shape = base_shape.to_vec();
    new_shape[axis] = total_along_axis;
    let total: usize = new_shape.iter().product();
    let mut data = Vec::with_capacity(total);

    // Pre-collect each source array into a contiguous Vec to avoid O(n) iter().nth() per element
    let src_vecs: Vec<Vec<T>> = arrays.iter().map(|a| a.iter().cloned().collect()).collect();

    // Compute strides for the output array (C-order)
    let mut out_strides = vec![1usize; ndim];
    for i in (0..ndim - 1).rev() {
        out_strides[i] = out_strides[i + 1] * new_shape[i + 1];
    }

    // For each position in the output, figure out which source array and offset
    for flat_idx in 0..total {
        // Convert flat index to nd-index
        let mut rem = flat_idx;
        let mut nd_idx = vec![0usize; ndim];
        for i in 0..ndim {
            nd_idx[i] = rem / out_strides[i];
            rem %= out_strides[i];
        }

        // Find which source array this position belongs to
        let concat_idx = nd_idx[axis];
        let mut offset = 0;
        let mut src_arr_idx = 0;
        for (k, arr) in arrays.iter().enumerate() {
            let len_along = arr.shape()[axis];
            if concat_idx < offset + len_along {
                src_arr_idx = k;
                break;
            }
            offset += len_along;
        }
        let local_concat_idx = concat_idx - offset;

        // Build source flat index (C-order)
        let src_shape = arrays[src_arr_idx].shape();
        let mut src_flat = 0usize;
        let mut src_mul = 1usize;
        for i in (0..ndim).rev() {
            let idx = if i == axis {
                local_concat_idx
            } else {
                nd_idx[i]
            };
            src_flat += idx * src_mul;
            src_mul *= src_shape[i];
        }

        let elem = src_vecs[src_arr_idx].get(src_flat).ok_or_else(|| {
            FerrayError::invalid_value(format!(
                "concatenate: internal index {} out of range for source array of length {}",
                src_flat,
                src_vecs[src_arr_idx].len(),
            ))
        })?;
        data.push(elem.clone());
    }

    Array::from_vec(IxDyn::new(&new_shape), data)
}

/// Join a sequence of arrays along a **new** axis.
///
/// All arrays must have the same shape. The result has one more dimension
/// than the inputs.
///
/// Analogous to `numpy.stack()`.
///
/// # Errors
/// Returns `FerrayError::InvalidValue` if the array list is empty.
/// Returns `FerrayError::ShapeMismatch` if shapes differ.
/// Returns `FerrayError::AxisOutOfBounds` if axis > ndim.
pub fn stack<T: Element>(arrays: &[Array<T, IxDyn>], axis: usize) -> FerrayResult<Array<T, IxDyn>> {
    if arrays.is_empty() {
        return Err(FerrayError::invalid_value("stack: need at least one array"));
    }
    let base_shape = arrays[0].shape();
    let ndim = base_shape.len();

    if axis > ndim {
        return Err(FerrayError::axis_out_of_bounds(axis, ndim + 1));
    }

    for arr in &arrays[1..] {
        if arr.shape() != base_shape {
            return Err(FerrayError::shape_mismatch(format!(
                "all input arrays must have the same shape; got {:?} and {:?}",
                base_shape,
                arr.shape(),
            )));
        }
    }

    // Expand each array along the new axis, then concatenate
    let mut expanded = Vec::with_capacity(arrays.len());
    for arr in arrays {
        expanded.push(expand_dims(arr, axis)?);
    }
    concatenate(&expanded, axis)
}

/// Stack arrays vertically (row-wise). Equivalent to `concatenate` along axis 0
/// for 2-D+ arrays, or equivalent to stacking 1-D arrays as rows.
///
/// Analogous to `numpy.vstack()`.
pub fn vstack<T: Element>(arrays: &[Array<T, IxDyn>]) -> FerrayResult<Array<T, IxDyn>> {
    if arrays.is_empty() {
        return Err(FerrayError::invalid_value(
            "vstack: need at least one array",
        ));
    }
    // For 1-D arrays, reshape to (1, N) then concatenate along axis 0
    let ndim = arrays[0].ndim();
    if ndim == 1 {
        let mut reshaped = Vec::with_capacity(arrays.len());
        for arr in arrays {
            let n = arr.shape()[0];
            reshaped.push(reshape(arr, &[1, n])?);
        }
        concatenate(&reshaped, 0)
    } else {
        concatenate(arrays, 0)
    }
}

/// Stack arrays horizontally (column-wise). Equivalent to `concatenate` along
/// axis 1 for 2-D+ arrays, or along axis 0 for 1-D arrays.
///
/// Analogous to `numpy.hstack()`.
pub fn hstack<T: Element>(arrays: &[Array<T, IxDyn>]) -> FerrayResult<Array<T, IxDyn>> {
    if arrays.is_empty() {
        return Err(FerrayError::invalid_value(
            "hstack: need at least one array",
        ));
    }
    let ndim = arrays[0].ndim();
    if ndim == 1 {
        concatenate(arrays, 0)
    } else {
        concatenate(arrays, 1)
    }
}

/// Stack arrays along the third axis (depth-wise).
///
/// For 1-D arrays of shape `(N,)`, reshapes to `(1, N, 1)`.
/// For 2-D arrays of shape `(M, N)`, reshapes to `(M, N, 1)`.
/// Then concatenates along axis 2.
///
/// Analogous to `numpy.dstack()`.
pub fn dstack<T: Element>(arrays: &[Array<T, IxDyn>]) -> FerrayResult<Array<T, IxDyn>> {
    if arrays.is_empty() {
        return Err(FerrayError::invalid_value(
            "dstack: need at least one array",
        ));
    }
    let mut expanded = Vec::with_capacity(arrays.len());
    for arr in arrays {
        let shape = arr.shape();
        match shape.len() {
            1 => {
                let n = shape[0];
                expanded.push(reshape(arr, &[1, n, 1])?);
            }
            2 => {
                let (m, n) = (shape[0], shape[1]);
                expanded.push(reshape(arr, &[m, n, 1])?);
            }
            _ => {
                // Already 3-D+, just use as-is
                let data: Vec<T> = arr.iter().cloned().collect();
                expanded.push(Array::from_vec(IxDyn::new(shape), data)?);
            }
        }
    }
    concatenate(&expanded, 2)
}

/// Stack 1-D arrays as columns into a 2-D array.
///
/// Each input becomes one column of the output. For 2-D+ inputs, this is
/// equivalent to [`hstack`].
///
/// Analogous to `numpy.column_stack()`.
///
/// # Errors
/// Returns `FerrayError::InvalidValue` if the input is empty,
/// or `FerrayError::ShapeMismatch` if 1-D inputs have different lengths.
pub fn column_stack<T: Element>(arrays: &[Array<T, IxDyn>]) -> FerrayResult<Array<T, IxDyn>> {
    if arrays.is_empty() {
        return Err(FerrayError::invalid_value(
            "column_stack: need at least one array",
        ));
    }
    let first_ndim = arrays[0].ndim();
    if first_ndim == 1 {
        // Convert each 1-D array of length N into a (N, 1) column, then hstack.
        let n = arrays[0].shape()[0];
        let mut reshaped = Vec::with_capacity(arrays.len());
        for arr in arrays {
            if arr.ndim() != 1 {
                return Err(FerrayError::shape_mismatch(
                    "column_stack: all inputs must have the same ndim",
                ));
            }
            if arr.shape()[0] != n {
                return Err(FerrayError::shape_mismatch(format!(
                    "column_stack: 1-D inputs must have the same length; got {} and {}",
                    n,
                    arr.shape()[0],
                )));
            }
            reshaped.push(reshape(arr, &[n, 1])?);
        }
        concatenate(&reshaped, 1)
    } else {
        // 2-D+: same as hstack
        hstack(arrays)
    }
}

/// Stack arrays in sequence vertically (row-wise). Alias for [`vstack`].
///
/// Analogous to `numpy.row_stack()` (deprecated alias for `vstack` in `NumPy` 2.0
/// but still widely used).
pub fn row_stack<T: Element>(arrays: &[Array<T, IxDyn>]) -> FerrayResult<Array<T, IxDyn>> {
    vstack(arrays)
}

/// Assemble an array from nested blocks.
///
/// Simplified version: takes a 2-D grid of arrays (as Vec<Vec<...>>)
/// and assembles them by stacking rows horizontally, then all rows vertically.
///
/// Analogous to `numpy.block()`.
///
/// # Errors
/// Returns errors on shape mismatches.
pub fn block<T: Element>(blocks: &[Vec<Array<T, IxDyn>>]) -> FerrayResult<Array<T, IxDyn>> {
    if blocks.is_empty() {
        return Err(FerrayError::invalid_value("block: empty input"));
    }
    let mut rows = Vec::with_capacity(blocks.len());
    for row in blocks {
        if row.is_empty() {
            return Err(FerrayError::invalid_value("block: empty row"));
        }
        // Concatenate along axis 1 (columns within each row)
        let row_arr = if row.len() == 1 {
            let data: Vec<T> = row[0].iter().cloned().collect();
            Array::from_vec(IxDyn::new(row[0].shape()), data)?
        } else {
            hstack(row)?
        };
        rows.push(row_arr);
    }
    if rows.len() == 1 {
        // SAFETY: just checked len() == 1, so pop() always returns Some
        Ok(rows.pop().unwrap_or_else(|| unreachable!()))
    } else {
        vstack(&rows)
    }
}

/// Split an array into equal-sized sub-arrays.
///
/// `n_sections` must evenly divide the size along `axis`.
///
/// Analogous to `numpy.split()`.
///
/// # Errors
/// Returns `FerrayError::InvalidValue` if the axis cannot be evenly split.
pub fn split<T: Element>(
    a: &Array<T, IxDyn>,
    n_sections: usize,
    axis: usize,
) -> FerrayResult<Vec<Array<T, IxDyn>>> {
    let shape = a.shape();
    if axis >= shape.len() {
        return Err(FerrayError::axis_out_of_bounds(axis, shape.len()));
    }
    let axis_len = shape[axis];
    if n_sections == 0 {
        return Err(FerrayError::invalid_value("split: n_sections must be > 0"));
    }
    if !axis_len.is_multiple_of(n_sections) {
        return Err(FerrayError::invalid_value(format!(
            "array of size {axis_len} along axis {axis} cannot be evenly split into {n_sections} sections",
        )));
    }
    let chunk_size = axis_len / n_sections;
    let indices: Vec<usize> = (1..n_sections).map(|i| i * chunk_size).collect();
    array_split(a, &indices, axis)
}

/// Split an array into sub-arrays at the given indices along `axis`.
///
/// Unlike `split()`, this does not require even division.
///
/// Analogous to `numpy.array_split()` (with explicit split points).
///
/// # Errors
/// Returns `FerrayError::AxisOutOfBounds` if axis is invalid.
pub fn array_split<T: Element>(
    a: &Array<T, IxDyn>,
    indices: &[usize],
    axis: usize,
) -> FerrayResult<Vec<Array<T, IxDyn>>> {
    let shape = a.shape();
    let ndim = shape.len();
    if axis >= ndim {
        return Err(FerrayError::axis_out_of_bounds(axis, ndim));
    }
    let axis_len = shape[axis];
    let src_data: Vec<T> = a.iter().cloned().collect();

    // Build split points including 0 and axis_len
    let mut splits = Vec::with_capacity(indices.len() + 2);
    splits.push(0);
    for &idx in indices {
        splits.push(idx.min(axis_len));
    }
    splits.push(axis_len);

    // Compute source strides (C-order)
    let mut src_strides = vec![1usize; ndim];
    for i in (0..ndim - 1).rev() {
        src_strides[i] = src_strides[i + 1] * shape[i + 1];
    }

    let mut result = Vec::with_capacity(splits.len() - 1);
    for w in splits.windows(2) {
        let start = w[0];
        let end = w[1];
        let chunk_len = end - start;

        let mut sub_shape = shape.to_vec();
        sub_shape[axis] = chunk_len;
        let sub_total: usize = sub_shape.iter().product();

        // Compute sub strides
        let mut sub_strides = vec![1usize; ndim];
        for i in (0..ndim - 1).rev() {
            sub_strides[i] = sub_strides[i + 1] * sub_shape[i + 1];
        }

        let mut sub_data = Vec::with_capacity(sub_total);
        for flat in 0..sub_total {
            // Convert to nd-index in sub array
            let mut rem = flat;
            let mut src_flat = 0usize;
            for i in 0..ndim {
                let idx = rem / sub_strides[i];
                rem %= sub_strides[i];
                let src_idx = if i == axis { idx + start } else { idx };
                src_flat += src_idx * src_strides[i];
            }
            sub_data.push(src_data[src_flat].clone());
        }
        result.push(Array::from_vec(IxDyn::new(&sub_shape), sub_data)?);
    }

    Ok(result)
}

/// Split an array into `n` sub-arrays along `axis`, allowing uneven sections.
///
/// Unlike [`split`], this never errors on uneven division: the first
/// `axis_len % n` sections have one extra element. This matches `NumPy`'s
/// `numpy.array_split(ary, n, axis)` (integer-section variant).
///
/// # Errors
/// Returns `FerrayError::AxisOutOfBounds` if `axis >= ndim`.
/// Returns `FerrayError::InvalidValue` if `n == 0`.
pub fn array_split_n<T: Element>(
    a: &Array<T, IxDyn>,
    n: usize,
    axis: usize,
) -> FerrayResult<Vec<Array<T, IxDyn>>> {
    if n == 0 {
        return Err(FerrayError::invalid_value("array_split_n: n must be > 0"));
    }
    let shape = a.shape();
    if axis >= shape.len() {
        return Err(FerrayError::axis_out_of_bounds(axis, shape.len()));
    }
    let axis_len = shape[axis];

    // Build split indices following NumPy's array_split:
    // First (axis_len % n) sections get (axis_len / n + 1) elements,
    // remaining sections get (axis_len / n) elements.
    let base = axis_len / n;
    let extra = axis_len % n;
    let mut indices = Vec::with_capacity(n.saturating_sub(1));
    let mut cum = 0usize;
    for i in 0..n - 1 {
        cum += if i < extra { base + 1 } else { base };
        indices.push(cum);
    }
    array_split(a, &indices, axis)
}

/// Split array along axis 0 (vertical split). Equivalent to `split(a, n, 0)`.
///
/// Analogous to `numpy.vsplit()`.
pub fn vsplit<T: Element>(
    a: &Array<T, IxDyn>,
    n_sections: usize,
) -> FerrayResult<Vec<Array<T, IxDyn>>> {
    split(a, n_sections, 0)
}

/// Split array along axis 1 (horizontal split). Equivalent to `split(a, n, 1)`.
///
/// Analogous to `numpy.hsplit()`.
pub fn hsplit<T: Element>(
    a: &Array<T, IxDyn>,
    n_sections: usize,
) -> FerrayResult<Vec<Array<T, IxDyn>>> {
    split(a, n_sections, 1)
}

/// Split array along axis 2 (depth split). Equivalent to `split(a, n, 2)`.
///
/// Analogous to `numpy.dsplit()`.
pub fn dsplit<T: Element>(
    a: &Array<T, IxDyn>,
    n_sections: usize,
) -> FerrayResult<Vec<Array<T, IxDyn>>> {
    split(a, n_sections, 2)
}

// ============================================================================
// REQ-22: Transpose / reorder
// ============================================================================

/// Permute the axes of an array.
///
/// `axes` specifies the new ordering. For a 2-D array, `[1, 0]` transposes.
/// If `axes` is `None`, reverses the order of all axes.
///
/// Analogous to `numpy.transpose()`.
///
/// # Errors
/// Returns `FerrayError::InvalidValue` if `axes` is the wrong length or
/// contains invalid/duplicate axis indices.
pub fn transpose<T: Element, D: Dimension>(
    a: &Array<T, D>,
    axes: Option<&[usize]>,
) -> FerrayResult<Array<T, IxDyn>> {
    let ndim = a.ndim();
    let perm: Vec<usize> = match axes {
        Some(ax) => {
            if ax.len() != ndim {
                return Err(FerrayError::invalid_value(format!(
                    "axes must have length {} but got {}",
                    ndim,
                    ax.len(),
                )));
            }
            // Validate: each axis appears exactly once
            let mut seen = vec![false; ndim];
            for &a in ax {
                if a >= ndim {
                    return Err(FerrayError::axis_out_of_bounds(a, ndim));
                }
                if seen[a] {
                    return Err(FerrayError::invalid_value(format!(
                        "duplicate axis {a} in transpose",
                    )));
                }
                seen[a] = true;
            }
            ax.to_vec()
        }
        None => (0..ndim).rev().collect(),
    };

    // Permute axes is a zero-copy stride rearrangement in ndarray; we
    // then call `as_standard_layout` which returns a borrowed view when
    // already C-contiguous or materializes a single standard-layout
    // owned buffer otherwise. The old implementation walked every
    // output index in a hand-rolled scatter loop (issue #82). Plain
    // `to_owned()` would preserve the F-contiguous stride pattern from
    // the permutation, which downstream callers don't want because
    // `as_slice()` only succeeds on standard layouts.
    let permuted = a
        .inner
        .view()
        .into_dyn()
        .permuted_axes(ndarray::IxDyn(&perm));
    Ok(Array::from_ndarray(
        permuted.as_standard_layout().into_owned(),
    ))
}

/// Swap two axes of an array.
///
/// Analogous to `numpy.swapaxes()`.
///
/// # Errors
/// Returns `FerrayError::AxisOutOfBounds` if either axis is out of bounds.
pub fn swapaxes<T: Element, D: Dimension>(
    a: &Array<T, D>,
    axis1: usize,
    axis2: usize,
) -> FerrayResult<Array<T, IxDyn>> {
    let ndim = a.ndim();
    if axis1 >= ndim {
        return Err(FerrayError::axis_out_of_bounds(axis1, ndim));
    }
    if axis2 >= ndim {
        return Err(FerrayError::axis_out_of_bounds(axis2, ndim));
    }
    let mut perm: Vec<usize> = (0..ndim).collect();
    perm.swap(axis1, axis2);
    transpose(a, Some(&perm))
}

/// Move an axis to a new position.
///
/// Analogous to `numpy.moveaxis()`.
///
/// # Errors
/// Returns `FerrayError::AxisOutOfBounds` if either axis is out of bounds.
pub fn moveaxis<T: Element, D: Dimension>(
    a: &Array<T, D>,
    source: usize,
    destination: usize,
) -> FerrayResult<Array<T, IxDyn>> {
    let ndim = a.ndim();
    if source >= ndim {
        return Err(FerrayError::axis_out_of_bounds(source, ndim));
    }
    if destination >= ndim {
        return Err(FerrayError::axis_out_of_bounds(destination, ndim));
    }
    // Build permutation by removing source and inserting at destination
    let mut order: Vec<usize> = (0..ndim).filter(|&x| x != source).collect();
    order.insert(destination, source);
    transpose(a, Some(&order))
}

/// Roll an axis to a new position (similar to moveaxis).
///
/// Analogous to `numpy.rollaxis()`.
///
/// # Errors
/// Returns `FerrayError::AxisOutOfBounds` if `axis >= ndim` or `start > ndim`.
pub fn rollaxis<T: Element, D: Dimension>(
    a: &Array<T, D>,
    axis: usize,
    start: usize,
) -> FerrayResult<Array<T, IxDyn>> {
    let ndim = a.ndim();
    if axis >= ndim {
        return Err(FerrayError::axis_out_of_bounds(axis, ndim));
    }
    if start > ndim {
        return Err(FerrayError::axis_out_of_bounds(start, ndim + 1));
    }
    let dst = if start > axis { start - 1 } else { start };
    if axis == dst {
        // No-op: return a copy
        let data: Vec<T> = a.iter().cloned().collect();
        return Array::from_vec(IxDyn::new(a.shape()), data);
    }
    moveaxis(a, axis, dst)
}

/// Reverse the order of elements along the given axis.
///
/// Analogous to `numpy.flip()`.
///
/// # Errors
/// Returns `FerrayError::AxisOutOfBounds` if axis is out of bounds.
pub fn flip<T: Element, D: Dimension>(
    a: &Array<T, D>,
    axis: usize,
) -> FerrayResult<Array<T, IxDyn>> {
    let shape = a.shape();
    let ndim = shape.len();
    if axis >= ndim {
        return Err(FerrayError::axis_out_of_bounds(axis, ndim));
    }
    let src_data: Vec<T> = a.iter().cloned().collect();
    let total = src_data.len();

    // Compute strides (C-order)
    let mut strides = vec![1usize; ndim];
    for i in (0..ndim.saturating_sub(1)).rev() {
        strides[i] = strides[i + 1] * shape[i + 1];
    }

    let mut data = Vec::with_capacity(total);
    for flat in 0..total {
        let mut rem = flat;
        let mut src_flat = 0usize;
        for i in 0..ndim {
            let idx = rem / strides[i];
            rem %= strides[i];
            let src_idx = if i == axis { shape[i] - 1 - idx } else { idx };
            src_flat += src_idx * strides[i];
        }
        data.push(src_data[src_flat].clone());
    }
    Array::from_vec(IxDyn::new(shape), data)
}

/// Flip array left-right (reverse axis 1).
///
/// Analogous to `numpy.fliplr()`.
///
/// # Errors
/// Returns `FerrayError::InvalidValue` if the array has fewer than 2 dimensions.
pub fn fliplr<T: Element, D: Dimension>(a: &Array<T, D>) -> FerrayResult<Array<T, IxDyn>> {
    if a.ndim() < 2 {
        return Err(FerrayError::invalid_value(
            "fliplr: array must be at least 2-D",
        ));
    }
    flip(a, 1)
}

/// Flip array up-down (reverse axis 0).
///
/// Analogous to `numpy.flipud()`.
///
/// # Errors
/// Returns `FerrayError::InvalidValue` if the array has 0 dimensions.
pub fn flipud<T: Element, D: Dimension>(a: &Array<T, D>) -> FerrayResult<Array<T, IxDyn>> {
    if a.ndim() < 1 {
        return Err(FerrayError::invalid_value(
            "flipud: array must be at least 1-D",
        ));
    }
    flip(a, 0)
}

/// Rotate array 90 degrees counterclockwise in the plane defined by axes (0, 1).
///
/// `k` specifies the number of 90-degree rotations (can be negative).
///
/// Analogous to `numpy.rot90()`.
///
/// # Errors
/// Returns `FerrayError::InvalidValue` if the array has fewer than 2 dimensions.
pub fn rot90<T: Element, D: Dimension>(a: &Array<T, D>, k: i32) -> FerrayResult<Array<T, IxDyn>> {
    if a.ndim() < 2 {
        return Err(FerrayError::invalid_value(
            "rot90: array must be at least 2-D",
        ));
    }
    // Normalize k to [0, 4)
    let k = k.rem_euclid(4);
    let shape = a.shape();
    let data: Vec<T> = a.iter().cloned().collect();

    // We work with the IxDyn representation
    let as_dyn = Array::from_vec(IxDyn::new(shape), data)?;

    match k {
        0 => Ok(as_dyn),
        1 => {
            // rot90 once: flip axis 1, then transpose axes 0,1
            let flipped = flip(&as_dyn, 1)?;
            swapaxes(&flipped, 0, 1)
        }
        2 => {
            // rot180: flip both axes
            let f1 = flip(&as_dyn, 0)?;
            flip(&f1, 1)
        }
        3 => {
            // rot270: transpose, then flip axis 1
            let transposed = swapaxes(&as_dyn, 0, 1)?;
            flip(&transposed, 1)
        }
        _ => unreachable!(),
    }
}

/// Roll elements along an axis. Elements that roll past the end
/// are re-introduced at the beginning.
///
/// If `axis` is `None`, the array is flattened first, then rolled.
///
/// Analogous to `numpy.roll()`.
///
/// # Errors
/// Returns `FerrayError::AxisOutOfBounds` if axis is out of bounds.
pub fn roll<T: Element, D: Dimension>(
    a: &Array<T, D>,
    shift: isize,
    axis: Option<usize>,
) -> FerrayResult<Array<T, IxDyn>> {
    match axis {
        None => {
            // Flatten, roll, reshape back
            let data: Vec<T> = a.iter().cloned().collect();
            let n = data.len();
            if n == 0 {
                return Array::from_vec(IxDyn::new(a.shape()), data);
            }
            let shift = ((shift % n as isize) + n as isize) as usize % n;
            let mut rolled = Vec::with_capacity(n);
            for i in 0..n {
                rolled.push(data[(n + i - shift) % n].clone());
            }
            Array::from_vec(IxDyn::new(a.shape()), rolled)
        }
        Some(ax) => {
            let shape = a.shape();
            let ndim = shape.len();
            if ax >= ndim {
                return Err(FerrayError::axis_out_of_bounds(ax, ndim));
            }
            let axis_len = shape[ax];
            if axis_len == 0 {
                let data: Vec<T> = a.iter().cloned().collect();
                return Array::from_vec(IxDyn::new(shape), data);
            }
            let shift = ((shift % axis_len as isize) + axis_len as isize) as usize % axis_len;
            let src_data: Vec<T> = a.iter().cloned().collect();
            let total = src_data.len();

            // Compute strides (C-order)
            let mut strides = vec![1usize; ndim];
            for i in (0..ndim.saturating_sub(1)).rev() {
                strides[i] = strides[i + 1] * shape[i + 1];
            }

            let mut data = Vec::with_capacity(total);
            for flat in 0..total {
                let mut rem = flat;
                let mut src_flat = 0usize;
                #[allow(clippy::needless_range_loop)]
                for i in 0..ndim {
                    let idx = rem / strides[i];
                    rem %= strides[i];
                    let src_idx = if i == ax {
                        (axis_len + idx - shift) % axis_len
                    } else {
                        idx
                    };
                    src_flat += src_idx * strides[i];
                }
                data.push(src_data[src_flat].clone());
            }
            Array::from_vec(IxDyn::new(shape), data)
        }
    }
}

// ============================================================================
// REQ: r_ / c_ — quick row/column builders (functional analogues of NumPy's
//                np.r_ / np.c_ index_exp slicing-tuple machinery)
// ============================================================================

/// Concatenate a sequence of 1-D arrays into a single 1-D array.
///
/// This is the function-form of `numpy.r_[a, b, c, ...]` for the simple
/// case of building up a 1-D vector. NumPy's `r_` also accepts slice
/// objects (`r_[1:5, 8:12]`) which Rust syntax cannot replicate cleanly;
/// for that, build the components with [`arange`](crate::creation::arange) /
/// [`linspace`](crate::creation::linspace) first, then call `r_`.
///
/// # Errors
/// Returns `FerrayError::InvalidValue` if `arrays` is empty.
pub fn r_<T: Element>(arrays: &[Array<T, Ix1>]) -> FerrayResult<Array<T, Ix1>> {
    if arrays.is_empty() {
        return Err(FerrayError::invalid_value("r_: need at least one array"));
    }
    let total: usize = arrays.iter().map(|a| a.shape()[0]).sum();
    let mut data = Vec::with_capacity(total);
    for a in arrays {
        data.extend(a.iter().cloned());
    }
    Array::from_vec(Ix1::new([total]), data)
}

/// Stack a sequence of 1-D arrays as columns of a 2-D array.
///
/// All inputs must have the same length `n`. The result has shape
/// `(n, k)` where `k = arrays.len()`.
///
/// Function-form of `numpy.c_[a, b, c]` when each input is 1-D.
///
/// # Errors
/// Returns `FerrayError::InvalidValue` if `arrays` is empty.
/// Returns `FerrayError::ShapeMismatch` if the inputs differ in length.
pub fn c_<T: Element>(arrays: &[Array<T, Ix1>]) -> FerrayResult<Array<T, IxDyn>> {
    if arrays.is_empty() {
        return Err(FerrayError::invalid_value("c_: need at least one array"));
    }
    let n = arrays[0].shape()[0];
    for a in &arrays[1..] {
        if a.shape()[0] != n {
            return Err(FerrayError::shape_mismatch(format!(
                "c_: all 1-D inputs must have the same length; got {} and {}",
                n,
                a.shape()[0],
            )));
        }
    }
    let k = arrays.len();
    // Output is row-major (n, k); element (i, j) comes from arrays[j][i].
    let cols: Vec<Vec<T>> = arrays.iter().map(|a| a.iter().cloned().collect()).collect();
    let mut data = Vec::with_capacity(n * k);
    for i in 0..n {
        for col in &cols {
            data.push(col[i].clone());
        }
    }
    Array::from_vec(IxDyn::new(&[n, k]), data)
}

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

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

    fn dyn_arr(shape: &[usize], data: Vec<f64>) -> Array<f64, IxDyn> {
        Array::from_vec(IxDyn::new(shape), data).unwrap()
    }

    // -- REQ-20 --

    #[test]
    fn test_reshape() {
        let a = dyn_arr(&[2, 3], vec![1.0, 2.0, 3.0, 4.0, 5.0, 6.0]);
        let b = reshape(&a, &[3, 2]).unwrap();
        assert_eq!(b.shape(), &[3, 2]);
        let data: Vec<f64> = b.iter().copied().collect();
        assert_eq!(data, vec![1.0, 2.0, 3.0, 4.0, 5.0, 6.0]);
    }

    #[test]
    fn test_reshape_size_mismatch() {
        let a = dyn_arr(&[2, 3], vec![1.0; 6]);
        assert!(reshape(&a, &[2, 4]).is_err());
    }

    #[test]
    fn test_ravel() {
        let a = dyn_arr(&[2, 3], vec![1.0, 2.0, 3.0, 4.0, 5.0, 6.0]);
        let b = ravel(&a).unwrap();
        assert_eq!(b.shape(), &[6]);
        assert_eq!(b.as_slice().unwrap(), &[1.0, 2.0, 3.0, 4.0, 5.0, 6.0]);
    }

    #[test]
    fn test_flatten() {
        let a = dyn_arr(&[2, 3], vec![1.0, 2.0, 3.0, 4.0, 5.0, 6.0]);
        let b = flatten(&a).unwrap();
        assert_eq!(b.shape(), &[6]);
    }

    #[test]
    fn test_squeeze() {
        let a = dyn_arr(&[1, 3, 1], vec![1.0, 2.0, 3.0]);
        let b = squeeze(&a, None).unwrap();
        assert_eq!(b.shape(), &[3]);
    }

    #[test]
    fn test_squeeze_specific_axis() {
        let a = dyn_arr(&[1, 3, 1], vec![1.0, 2.0, 3.0]);
        let b = squeeze(&a, Some(0)).unwrap();
        assert_eq!(b.shape(), &[3, 1]);
    }

    #[test]
    fn test_squeeze_not_size_1() {
        let a = dyn_arr(&[2, 3], vec![1.0; 6]);
        assert!(squeeze(&a, Some(0)).is_err());
    }

    #[test]
    fn test_expand_dims() {
        let a = dyn_arr(&[3], vec![1.0, 2.0, 3.0]);
        let b = expand_dims(&a, 0).unwrap();
        assert_eq!(b.shape(), &[1, 3]);
        let c = expand_dims(&a, 1).unwrap();
        assert_eq!(c.shape(), &[3, 1]);
    }

    #[test]
    fn test_expand_dims_oob() {
        let a = dyn_arr(&[3], vec![1.0, 2.0, 3.0]);
        assert!(expand_dims(&a, 3).is_err());
    }

    #[test]
    fn test_broadcast_to() {
        let a = dyn_arr(&[1, 3], vec![1.0, 2.0, 3.0]);
        let b = broadcast_to(&a, &[3, 3]).unwrap();
        assert_eq!(b.shape(), &[3, 3]);
        let data: Vec<f64> = b.iter().copied().collect();
        assert_eq!(data, vec![1.0, 2.0, 3.0, 1.0, 2.0, 3.0, 1.0, 2.0, 3.0]);
    }

    #[test]
    fn test_broadcast_to_1d_to_2d() {
        let a = dyn_arr(&[3], vec![1.0, 2.0, 3.0]);
        let b = broadcast_to(&a, &[2, 3]).unwrap();
        assert_eq!(b.shape(), &[2, 3]);
    }

    #[test]
    fn test_broadcast_to_incompatible() {
        let a = dyn_arr(&[4], vec![1.0, 2.0, 3.0, 4.0]);
        assert!(broadcast_to(&a, &[3]).is_err());
    }

    #[test]
    fn test_broadcast_to_from_non_contiguous_source() {
        // Issue #133: broadcast_to's source may itself be non-contiguous
        // (e.g., transposed). Since broadcast_to now delegates to
        // ndarray's `broadcast` which handles any stride pattern, the
        // result should materialize correctly.
        let a = dyn_arr(&[2, 3], vec![1.0, 2.0, 3.0, 4.0, 5.0, 6.0]);
        // Transpose gives us a 3x2 logical view — then broadcast to (2, 3, 2).
        let t = transpose(&a, None).unwrap();
        let b = broadcast_to(&t, &[2, 3, 2]).unwrap();
        assert_eq!(b.shape(), &[2, 3, 2]);
        // Both outer slices should be identical.
        let data: Vec<f64> = b.iter().copied().collect();
        assert_eq!(&data[..6], &data[6..12]);
    }

    // -- REQ-21 --

    #[test]
    fn test_concatenate() {
        let a = dyn_arr(&[2, 3], vec![1.0, 2.0, 3.0, 4.0, 5.0, 6.0]);
        let b = dyn_arr(&[2, 3], vec![7.0, 8.0, 9.0, 10.0, 11.0, 12.0]);
        let c = concatenate(&[a, b], 0).unwrap();
        assert_eq!(c.shape(), &[4, 3]);
    }

    #[test]
    fn test_concatenate_axis1() {
        let a = dyn_arr(&[2, 2], vec![1.0, 2.0, 3.0, 4.0]);
        let b = dyn_arr(&[2, 3], vec![5.0, 6.0, 7.0, 8.0, 9.0, 10.0]);
        let c = concatenate(&[a, b], 1).unwrap();
        assert_eq!(c.shape(), &[2, 5]);
    }

    #[test]
    fn test_concatenate_shape_mismatch() {
        let a = dyn_arr(&[2, 3], vec![1.0; 6]);
        let b = dyn_arr(&[3, 3], vec![1.0; 9]);
        // Axis 0: different sizes on axis 1? No — axis 1 is same (3).
        // But axis 0 concat: shapes are [2,3] and [3,3], axis 0 can differ.
        // Non-concat axis (1) matches.
        let c = concatenate(&[a, b], 0).unwrap();
        assert_eq!(c.shape(), &[5, 3]);
    }

    #[test]
    fn test_concatenate_empty() {
        let v: Vec<Array<f64, IxDyn>> = vec![];
        assert!(concatenate(&v, 0).is_err());
    }

    #[test]
    fn test_stack() {
        let a = dyn_arr(&[3], vec![1.0, 2.0, 3.0]);
        let b = dyn_arr(&[3], vec![4.0, 5.0, 6.0]);
        let c = stack(&[a, b], 0).unwrap();
        assert_eq!(c.shape(), &[2, 3]);
        let data: Vec<f64> = c.iter().copied().collect();
        assert_eq!(data, vec![1.0, 2.0, 3.0, 4.0, 5.0, 6.0]);
    }

    #[test]
    fn test_stack_axis1() {
        let a = dyn_arr(&[3], vec![1.0, 2.0, 3.0]);
        let b = dyn_arr(&[3], vec![4.0, 5.0, 6.0]);
        let c = stack(&[a, b], 1).unwrap();
        assert_eq!(c.shape(), &[3, 2]);
        let data: Vec<f64> = c.iter().copied().collect();
        assert_eq!(data, vec![1.0, 4.0, 2.0, 5.0, 3.0, 6.0]);
    }

    #[test]
    fn test_vstack() {
        let a = dyn_arr(&[3], vec![1.0, 2.0, 3.0]);
        let b = dyn_arr(&[3], vec![4.0, 5.0, 6.0]);
        let c = vstack(&[a, b]).unwrap();
        assert_eq!(c.shape(), &[2, 3]);
    }

    #[test]
    fn test_hstack() {
        let a = dyn_arr(&[3], vec![1.0, 2.0, 3.0]);
        let b = dyn_arr(&[3], vec![4.0, 5.0, 6.0]);
        let c = hstack(&[a, b]).unwrap();
        assert_eq!(c.shape(), &[6]);
    }

    #[test]
    fn test_hstack_2d() {
        let a = dyn_arr(&[2, 2], vec![1.0, 2.0, 3.0, 4.0]);
        let b = dyn_arr(&[2, 3], vec![5.0, 6.0, 7.0, 8.0, 9.0, 10.0]);
        let c = hstack(&[a, b]).unwrap();
        assert_eq!(c.shape(), &[2, 5]);
    }

    #[test]
    fn test_dstack() {
        let a = dyn_arr(&[2, 2], vec![1.0, 2.0, 3.0, 4.0]);
        let b = dyn_arr(&[2, 2], vec![5.0, 6.0, 7.0, 8.0]);
        let c = dstack(&[a, b]).unwrap();
        assert_eq!(c.shape(), &[2, 2, 2]);
    }

    #[test]
    fn test_block() {
        let a = dyn_arr(&[2, 2], vec![1.0, 2.0, 3.0, 4.0]);
        let b = dyn_arr(&[2, 2], vec![5.0, 6.0, 7.0, 8.0]);
        let c = dyn_arr(&[2, 2], vec![9.0, 10.0, 11.0, 12.0]);
        let d = dyn_arr(&[2, 2], vec![13.0, 14.0, 15.0, 16.0]);
        let result = block(&[vec![a, b], vec![c, d]]).unwrap();
        assert_eq!(result.shape(), &[4, 4]);
    }

    #[test]
    fn test_split() {
        let a = dyn_arr(&[6], vec![1.0, 2.0, 3.0, 4.0, 5.0, 6.0]);
        let parts = split(&a, 3, 0).unwrap();
        assert_eq!(parts.len(), 3);
        assert_eq!(parts[0].shape(), &[2]);
        assert_eq!(parts[1].shape(), &[2]);
        assert_eq!(parts[2].shape(), &[2]);
    }

    #[test]
    fn test_split_uneven() {
        let a = dyn_arr(&[5], vec![1.0, 2.0, 3.0, 4.0, 5.0]);
        assert!(split(&a, 3, 0).is_err()); // 5 not divisible by 3
    }

    #[test]
    fn test_array_split() {
        let a = dyn_arr(&[5], vec![1.0, 2.0, 3.0, 4.0, 5.0]);
        let parts = array_split(&a, &[2, 4], 0).unwrap();
        assert_eq!(parts.len(), 3);
        assert_eq!(parts[0].shape(), &[2]); // [1,2]
        assert_eq!(parts[1].shape(), &[2]); // [3,4]
        assert_eq!(parts[2].shape(), &[1]); // [5]
    }

    #[test]
    fn test_vsplit() {
        let a = dyn_arr(&[4, 2], vec![1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0]);
        let parts = vsplit(&a, 2).unwrap();
        assert_eq!(parts.len(), 2);
        assert_eq!(parts[0].shape(), &[2, 2]);
    }

    #[test]
    fn test_hsplit() {
        let a = dyn_arr(&[2, 4], vec![1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0]);
        let parts = hsplit(&a, 2).unwrap();
        assert_eq!(parts.len(), 2);
        assert_eq!(parts[0].shape(), &[2, 2]);
    }

    // -- REQ-22 --

    #[test]
    fn test_transpose_2d() {
        let a = dyn_arr(&[2, 3], vec![1.0, 2.0, 3.0, 4.0, 5.0, 6.0]);
        let b = transpose(&a, None).unwrap();
        assert_eq!(b.shape(), &[3, 2]);
        let data: Vec<f64> = b.iter().copied().collect();
        assert_eq!(data, vec![1.0, 4.0, 2.0, 5.0, 3.0, 6.0]);
    }

    #[test]
    fn test_transpose_explicit() {
        let a = dyn_arr(&[2, 3], vec![1.0, 2.0, 3.0, 4.0, 5.0, 6.0]);
        let b = transpose(&a, Some(&[1, 0])).unwrap();
        assert_eq!(b.shape(), &[3, 2]);
    }

    #[test]
    fn test_transpose_bad_axes() {
        let a = dyn_arr(&[2, 3], vec![1.0; 6]);
        assert!(transpose(&a, Some(&[0])).is_err()); // wrong length
    }

    #[test]
    fn test_swapaxes() {
        let a = dyn_arr(&[2, 3, 4], vec![0.0; 24]);
        let b = swapaxes(&a, 0, 2).unwrap();
        assert_eq!(b.shape(), &[4, 3, 2]);
    }

    #[test]
    fn test_moveaxis() {
        let a = dyn_arr(&[2, 3, 4], vec![0.0; 24]);
        let b = moveaxis(&a, 0, 2).unwrap();
        assert_eq!(b.shape(), &[3, 4, 2]);
    }

    #[test]
    fn test_rollaxis() {
        let a = dyn_arr(&[2, 3, 4], vec![0.0; 24]);
        let b = rollaxis(&a, 2, 0).unwrap();
        assert_eq!(b.shape(), &[4, 2, 3]);
    }

    #[test]
    fn test_flip() {
        let a = dyn_arr(&[3], vec![1.0, 2.0, 3.0]);
        let b = flip(&a, 0).unwrap();
        let data: Vec<f64> = b.iter().copied().collect();
        assert_eq!(data, vec![3.0, 2.0, 1.0]);
    }

    #[test]
    fn test_flip_2d() {
        let a = dyn_arr(&[2, 3], vec![1.0, 2.0, 3.0, 4.0, 5.0, 6.0]);
        let b = flip(&a, 0).unwrap();
        let data: Vec<f64> = b.iter().copied().collect();
        assert_eq!(data, vec![4.0, 5.0, 6.0, 1.0, 2.0, 3.0]);

        let c = flip(&a, 1).unwrap();
        let data2: Vec<f64> = c.iter().copied().collect();
        assert_eq!(data2, vec![3.0, 2.0, 1.0, 6.0, 5.0, 4.0]);
    }

    #[test]
    fn test_fliplr() {
        let a = dyn_arr(&[2, 3], vec![1.0, 2.0, 3.0, 4.0, 5.0, 6.0]);
        let b = fliplr(&a).unwrap();
        let data: Vec<f64> = b.iter().copied().collect();
        assert_eq!(data, vec![3.0, 2.0, 1.0, 6.0, 5.0, 4.0]);
    }

    #[test]
    fn test_flipud() {
        let a = dyn_arr(&[2, 3], vec![1.0, 2.0, 3.0, 4.0, 5.0, 6.0]);
        let b = flipud(&a).unwrap();
        let data: Vec<f64> = b.iter().copied().collect();
        assert_eq!(data, vec![4.0, 5.0, 6.0, 1.0, 2.0, 3.0]);
    }

    #[test]
    fn test_fliplr_1d_err() {
        let a = dyn_arr(&[3], vec![1.0, 2.0, 3.0]);
        assert!(fliplr(&a).is_err());
    }

    #[test]
    fn test_rot90_once() {
        // [[1, 2], [3, 4]] -> [[2, 4], [1, 3]]
        let a = dyn_arr(&[2, 2], vec![1.0, 2.0, 3.0, 4.0]);
        let b = rot90(&a, 1).unwrap();
        assert_eq!(b.shape(), &[2, 2]);
        let data: Vec<f64> = b.iter().copied().collect();
        assert_eq!(data, vec![2.0, 4.0, 1.0, 3.0]);
    }

    #[test]
    fn test_rot90_twice() {
        let a = dyn_arr(&[2, 2], vec![1.0, 2.0, 3.0, 4.0]);
        let b = rot90(&a, 2).unwrap();
        let data: Vec<f64> = b.iter().copied().collect();
        assert_eq!(data, vec![4.0, 3.0, 2.0, 1.0]);
    }

    #[test]
    fn test_rot90_four_is_identity() {
        let a = dyn_arr(&[2, 3], vec![1.0, 2.0, 3.0, 4.0, 5.0, 6.0]);
        let b = rot90(&a, 4).unwrap();
        let data_a: Vec<f64> = a.iter().copied().collect();
        let data_b: Vec<f64> = b.iter().copied().collect();
        assert_eq!(data_a, data_b);
        assert_eq!(a.shape(), b.shape());
    }

    #[test]
    fn test_roll_flat() {
        let a = dyn_arr(&[5], vec![1.0, 2.0, 3.0, 4.0, 5.0]);
        let b = roll(&a, 2, None).unwrap();
        let data: Vec<f64> = b.iter().copied().collect();
        assert_eq!(data, vec![4.0, 5.0, 1.0, 2.0, 3.0]);
    }

    #[test]
    fn test_roll_negative() {
        let a = dyn_arr(&[5], vec![1.0, 2.0, 3.0, 4.0, 5.0]);
        let b = roll(&a, -2, None).unwrap();
        let data: Vec<f64> = b.iter().copied().collect();
        assert_eq!(data, vec![3.0, 4.0, 5.0, 1.0, 2.0]);
    }

    #[test]
    fn test_roll_axis() {
        let a = dyn_arr(&[2, 3], vec![1.0, 2.0, 3.0, 4.0, 5.0, 6.0]);
        let b = roll(&a, 1, Some(1)).unwrap();
        let data: Vec<f64> = b.iter().copied().collect();
        assert_eq!(data, vec![3.0, 1.0, 2.0, 6.0, 4.0, 5.0]);
    }

    // -----------------------------------------------------------------------
    // column_stack / row_stack / array_split_n / to_dyn (issue #362)
    // -----------------------------------------------------------------------

    #[test]
    fn test_column_stack_1d() {
        // 3 1-D arrays of length 4 -> (4, 3)
        let a = dyn_arr(&[4], vec![1.0, 2.0, 3.0, 4.0]);
        let b = dyn_arr(&[4], vec![10.0, 20.0, 30.0, 40.0]);
        let c = dyn_arr(&[4], vec![100.0, 200.0, 300.0, 400.0]);
        let result = column_stack(&[a, b, c]).unwrap();
        assert_eq!(result.shape(), &[4, 3]);
        assert_eq!(
            result.iter().copied().collect::<Vec<_>>(),
            vec![
                1.0, 10.0, 100.0, // row 0
                2.0, 20.0, 200.0, // row 1
                3.0, 30.0, 300.0, // row 2
                4.0, 40.0, 400.0, // row 3
            ]
        );
    }

    #[test]
    fn test_column_stack_2d_same_as_hstack() {
        let a = dyn_arr(&[2, 2], vec![1.0, 2.0, 3.0, 4.0]);
        let b = dyn_arr(&[2, 2], vec![5.0, 6.0, 7.0, 8.0]);
        let result = column_stack(&[a, b]).unwrap();
        assert_eq!(result.shape(), &[2, 4]);
        assert_eq!(
            result.iter().copied().collect::<Vec<_>>(),
            vec![1.0, 2.0, 5.0, 6.0, 3.0, 4.0, 7.0, 8.0]
        );
    }

    #[test]
    fn test_column_stack_length_mismatch() {
        let a = dyn_arr(&[3], vec![1.0, 2.0, 3.0]);
        let b = dyn_arr(&[4], vec![1.0, 2.0, 3.0, 4.0]);
        assert!(column_stack(&[a, b]).is_err());
    }

    #[test]
    fn test_row_stack_is_vstack() {
        let a = dyn_arr(&[3], vec![1.0, 2.0, 3.0]);
        let b = dyn_arr(&[3], vec![4.0, 5.0, 6.0]);
        let row = row_stack(&[a.clone(), b.clone()]).unwrap();
        let v = vstack(&[a, b]).unwrap();
        assert_eq!(row.shape(), v.shape());
        assert_eq!(
            row.iter().copied().collect::<Vec<_>>(),
            v.iter().copied().collect::<Vec<_>>()
        );
    }

    #[test]
    fn test_array_split_n_uneven() {
        // 7 elements split into 3 sections -> [3, 2, 2]
        let a = dyn_arr(&[7], vec![1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0]);
        let parts = array_split_n(&a, 3, 0).unwrap();
        assert_eq!(parts.len(), 3);
        assert_eq!(
            parts[0].iter().copied().collect::<Vec<_>>(),
            vec![1.0, 2.0, 3.0]
        );
        assert_eq!(parts[1].iter().copied().collect::<Vec<_>>(), vec![4.0, 5.0]);
        assert_eq!(parts[2].iter().copied().collect::<Vec<_>>(), vec![6.0, 7.0]);
    }

    #[test]
    fn test_array_split_n_even() {
        let a = dyn_arr(&[6], vec![1.0, 2.0, 3.0, 4.0, 5.0, 6.0]);
        let parts = array_split_n(&a, 3, 0).unwrap();
        assert_eq!(parts.len(), 3);
        for (i, expected) in [vec![1.0, 2.0], vec![3.0, 4.0], vec![5.0, 6.0]]
            .iter()
            .enumerate()
        {
            assert_eq!(&parts[i].iter().copied().collect::<Vec<_>>(), expected);
        }
    }

    #[test]
    fn test_array_split_n_more_sections_than_elements() {
        // NumPy's behavior: 3 elements split into 5 sections gives 5 parts,
        // first 3 have 1 element each, last 2 are empty.
        let a = dyn_arr(&[3], vec![1.0, 2.0, 3.0]);
        let parts = array_split_n(&a, 5, 0).unwrap();
        assert_eq!(parts.len(), 5);
        assert_eq!(parts[0].iter().copied().collect::<Vec<_>>(), vec![1.0]);
        assert_eq!(parts[1].iter().copied().collect::<Vec<_>>(), vec![2.0]);
        assert_eq!(parts[2].iter().copied().collect::<Vec<_>>(), vec![3.0]);
        assert_eq!(
            parts[3].iter().copied().collect::<Vec<_>>(),
            Vec::<f64>::new()
        );
        assert_eq!(
            parts[4].iter().copied().collect::<Vec<_>>(),
            Vec::<f64>::new()
        );
    }

    #[test]
    fn test_to_dyn_from_typed() {
        use crate::Array;
        use crate::dimension::Ix2;
        let typed =
            Array::<f64, Ix2>::from_vec(Ix2::new([2, 3]), vec![1.0, 2.0, 3.0, 4.0, 5.0, 6.0])
                .unwrap();
        let dy = typed.to_dyn();
        assert_eq!(dy.shape(), &[2, 3]);
        assert_eq!(
            dy.iter().copied().collect::<Vec<_>>(),
            vec![1.0, 2.0, 3.0, 4.0, 5.0, 6.0]
        );
    }

    #[test]
    fn test_concatenate_typed_via_to_dyn() {
        // Demonstrates the typical end-user flow: have Array<T, Ix2>, want to
        // concatenate, route through to_dyn().
        use crate::Array;
        use crate::dimension::Ix2;
        let a = Array::<f64, Ix2>::from_vec(Ix2::new([2, 2]), vec![1.0, 2.0, 3.0, 4.0]).unwrap();
        let b = Array::<f64, Ix2>::from_vec(Ix2::new([2, 2]), vec![5.0, 6.0, 7.0, 8.0]).unwrap();
        let result = concatenate(&[a.to_dyn(), b.to_dyn()], 0).unwrap();
        assert_eq!(result.shape(), &[4, 2]);
        assert_eq!(
            result.iter().copied().collect::<Vec<_>>(),
            vec![1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0]
        );
    }

    #[test]
    fn test_r_concatenates_1d() {
        use crate::Array;
        let a = Array::<i32, Ix1>::from_vec(Ix1::new([3]), vec![1, 2, 3]).unwrap();
        let b = Array::<i32, Ix1>::from_vec(Ix1::new([2]), vec![4, 5]).unwrap();
        let c = Array::<i32, Ix1>::from_vec(Ix1::new([1]), vec![6]).unwrap();
        let r = r_(&[a, b, c]).unwrap();
        assert_eq!(
            r.iter().copied().collect::<Vec<_>>(),
            vec![1, 2, 3, 4, 5, 6]
        );
    }

    #[test]
    fn test_r_empty_input_errs() {
        let r = r_::<f64>(&[]);
        assert!(r.is_err());
    }

    #[test]
    fn test_c_columns_to_2d() {
        use crate::Array;
        let a = Array::<i32, Ix1>::from_vec(Ix1::new([3]), vec![1, 2, 3]).unwrap();
        let b = Array::<i32, Ix1>::from_vec(Ix1::new([3]), vec![10, 20, 30]).unwrap();
        let r = c_(&[a, b]).unwrap();
        assert_eq!(r.shape(), &[3, 2]);
        // Row-major: (0,0)=1 (0,1)=10 (1,0)=2 (1,1)=20 (2,0)=3 (2,1)=30
        assert_eq!(
            r.iter().copied().collect::<Vec<_>>(),
            vec![1, 10, 2, 20, 3, 30],
        );
    }

    #[test]
    fn test_c_length_mismatch_errs() {
        use crate::Array;
        let a = Array::<i32, Ix1>::from_vec(Ix1::new([3]), vec![1, 2, 3]).unwrap();
        let b = Array::<i32, Ix1>::from_vec(Ix1::new([2]), vec![10, 20]).unwrap();
        assert!(c_(&[a, b]).is_err());
    }
}