_funcs.py

from __future__ import annotations

import typing
import warnings

if typing.TYPE_CHECKING:
    from ._typing import Array, ModuleType

__all__ = ["atleast_nd", "cov", "create_diagonal", "expand_dims", "kron", "sinc"]


def atleast_nd(x: Array, /, *, ndim: int, xp: ModuleType) -> Array:
    """
    Recursively expand the dimension of an array to at least `ndim`.

    Parameters
    ----------
    x : array
    ndim : int
        The minimum number of dimensions for the result.
    xp : array_namespace
        The standard-compatible namespace for `x`.

    Returns
    -------
    res : array
        An array with ``res.ndim`` >= `ndim`.
        If ``x.ndim`` >= `ndim`, `x` is returned.
        If ``x.ndim`` < `ndim`, `x` is expanded by prepending new axes
        until ``res.ndim`` equals `ndim`.

    Examples
    --------
    >>> import array_api_strict as xp
    >>> import array_api_extra as xpx
    >>> x = xp.asarray([1])
    >>> xpx.atleast_nd(x, ndim=3, xp=xp)
    Array([[[1]]], dtype=array_api_strict.int64)

    >>> x = xp.asarray([[[1, 2],
    ...                  [3, 4]]])
    >>> xpx.atleast_nd(x, ndim=1, xp=xp) is x
    True

    """
    if x.ndim < ndim:
        x = xp.expand_dims(x, axis=0)
        x = atleast_nd(x, ndim=ndim, xp=xp)
    return x


def cov(m: Array, /, *, xp: ModuleType) -> Array:
    """
    Estimate a covariance matrix.

    Covariance indicates the level to which two variables vary together.
    If we examine N-dimensional samples, :math:`X = [x_1, x_2, ... x_N]^T`,
    then the covariance matrix element :math:`C_{ij}` is the covariance of
    :math:`x_i` and :math:`x_j`. The element :math:`C_{ii}` is the variance
    of :math:`x_i`.

    This provides a subset of the functionality of ``numpy.cov``.

    Parameters
    ----------
    m : array
        A 1-D or 2-D array containing multiple variables and observations.
        Each row of `m` represents a variable, and each column a single
        observation of all those variables.
    xp : array_namespace
        The standard-compatible namespace for `m`.

    Returns
    -------
    res : array
        The covariance matrix of the variables.

    Examples
    --------
    >>> import array_api_strict as xp
    >>> import array_api_extra as xpx

    Consider two variables, :math:`x_0` and :math:`x_1`, which
    correlate perfectly, but in opposite directions:

    >>> x = xp.asarray([[0, 2], [1, 1], [2, 0]]).T
    >>> x
    Array([[0, 1, 2],
           [2, 1, 0]], dtype=array_api_strict.int64)

    Note how :math:`x_0` increases while :math:`x_1` decreases. The covariance
    matrix shows this clearly:

    >>> xpx.cov(x, xp=xp)
    Array([[ 1., -1.],
           [-1.,  1.]], dtype=array_api_strict.float64)


    Note that element :math:`C_{0,1}`, which shows the correlation between
    :math:`x_0` and :math:`x_1`, is negative.

    Further, note how `x` and `y` are combined:

    >>> x = xp.asarray([-2.1, -1,  4.3])
    >>> y = xp.asarray([3,  1.1,  0.12])
    >>> X = xp.stack((x, y), axis=0)
    >>> xpx.cov(X, xp=xp)
    Array([[11.71      , -4.286     ],
           [-4.286     ,  2.14413333]], dtype=array_api_strict.float64)

    >>> xpx.cov(x, xp=xp)
    Array(11.71, dtype=array_api_strict.float64)

    >>> xpx.cov(y, xp=xp)
    Array(2.14413333, dtype=array_api_strict.float64)

    """
    m = xp.asarray(m, copy=True)
    dtype = (
        xp.float64 if xp.isdtype(m.dtype, "integral") else xp.result_type(m, xp.float64)
    )

    m = atleast_nd(m, ndim=2, xp=xp)
    m = xp.astype(m, dtype)

    avg = _mean(m, axis=1, xp=xp)
    fact = m.shape[1] - 1

    if fact <= 0:
        warnings.warn("Degrees of freedom <= 0 for slice", RuntimeWarning, stacklevel=2)
        fact = 0.0

    m -= avg[:, None]
    m_transpose = m.T
    if xp.isdtype(m_transpose.dtype, "complex floating"):
        m_transpose = xp.conj(m_transpose)
    c = m @ m_transpose
    c /= fact
    axes = tuple(axis for axis, length in enumerate(c.shape) if length == 1)
    return xp.squeeze(c, axis=axes)


def create_diagonal(x: Array, /, *, offset: int = 0, xp: ModuleType) -> Array:
    """
    Construct a diagonal array.

    Parameters
    ----------
    x : array
        A 1-D array
    offset : int, optional
        Offset from the leading diagonal (default is ``0``).
        Use positive ints for diagonals above the leading diagonal,
        and negative ints for diagonals below the leading diagonal.
    xp : array_namespace
        The standard-compatible namespace for `x`.

    Returns
    -------
    res : array
        A 2-D array with `x` on the diagonal (offset by `offset`).

    Examples
    --------
    >>> import array_api_strict as xp
    >>> import array_api_extra as xpx
    >>> x = xp.asarray([2, 4, 8])

    >>> xpx.create_diagonal(x, xp=xp)
    Array([[2, 0, 0],
           [0, 4, 0],
           [0, 0, 8]], dtype=array_api_strict.int64)

    >>> xpx.create_diagonal(x, offset=-2, xp=xp)
    Array([[0, 0, 0, 0, 0],
           [0, 0, 0, 0, 0],
           [2, 0, 0, 0, 0],
           [0, 4, 0, 0, 0],
           [0, 0, 8, 0, 0]], dtype=array_api_strict.int64)

    """
    if x.ndim != 1:
        err_msg = "`x` must be 1-dimensional."
        raise ValueError(err_msg)
    n = x.shape[0] + abs(offset)
    diag = xp.zeros(n**2, dtype=x.dtype)
    i = offset if offset >= 0 else abs(offset) * n
    diag[i : min(n * (n - offset), diag.shape[0]) : n + 1] = x
    return xp.reshape(diag, (n, n))


def _mean(
    x: Array,
    /,
    *,
    axis: int | tuple[int, ...] | None = None,
    keepdims: bool = False,
    xp: ModuleType,
) -> Array:
    """
    Complex mean, https://github.com/data-apis/array-api/issues/846.
    """
    if xp.isdtype(x.dtype, "complex floating"):
        x_real = xp.real(x)
        x_imag = xp.imag(x)
        mean_real = xp.mean(x_real, axis=axis, keepdims=keepdims)
        mean_imag = xp.mean(x_imag, axis=axis, keepdims=keepdims)
        return mean_real + (mean_imag * xp.asarray(1j))
    return xp.mean(x, axis=axis, keepdims=keepdims)


def expand_dims(
    a: Array, /, *, axis: int | tuple[int, ...] = (0,), xp: ModuleType
) -> Array:
    """
    Expand the shape of an array.

    Insert (a) new axis/axes that will appear at the position(s) specified by
    `axis` in the expanded array shape.

    This is ``xp.expand_dims`` for `axis` an int *or a tuple of ints*.
    Roughly equivalent to ``numpy.expand_dims`` for NumPy arrays.

    Parameters
    ----------
    a : array
    axis : int or tuple of ints, optional
        Position(s) in the expanded axes where the new axis (or axes) is/are placed.
        If multiple positions are provided, they should be unique (note that a position
        given by a positive index could also be referred to by a negative index -
        that will also result in an error).
        Default: ``(0,)``.
    xp : array_namespace
        The standard-compatible namespace for `a`.

    Returns
    -------
    res : array
        `a` with an expanded shape.

    Examples
    --------
    >>> import array_api_strict as xp
    >>> import array_api_extra as xpx
    >>> x = xp.asarray([1, 2])
    >>> x.shape
    (2,)

    The following is equivalent to ``x[xp.newaxis, :]`` or ``x[xp.newaxis]``:

    >>> y = xpx.expand_dims(x, axis=0, xp=xp)
    >>> y
    Array([[1, 2]], dtype=array_api_strict.int64)
    >>> y.shape
    (1, 2)

    The following is equivalent to ``x[:, xp.newaxis]``:

    >>> y = xpx.expand_dims(x, axis=1, xp=xp)
    >>> y
    Array([[1],
           [2]], dtype=array_api_strict.int64)
    >>> y.shape
    (2, 1)

    ``axis`` may also be a tuple:

    >>> y = xpx.expand_dims(x, axis=(0, 1), xp=xp)
    >>> y
    Array([[[1, 2]]], dtype=array_api_strict.int64)

    >>> y = xpx.expand_dims(x, axis=(2, 0), xp=xp)
    >>> y
    Array([[[1],
            [2]]], dtype=array_api_strict.int64)

    """
    if not isinstance(axis, tuple):
        axis = (axis,)
    ndim = a.ndim + len(axis)
    if axis != () and (min(axis) < -ndim or max(axis) >= ndim):
        err_msg = (
            f"a provided axis position is out of bounds for array of dimension {a.ndim}"
        )
        raise IndexError(err_msg)
    axis = tuple(dim % ndim for dim in axis)
    if len(set(axis)) != len(axis):
        err_msg = "Duplicate dimensions specified in `axis`."
        raise ValueError(err_msg)
    for i in sorted(axis):
        a = xp.expand_dims(a, axis=i)
    return a


def kron(a: Array, b: Array, /, *, xp: ModuleType) -> Array:
    """
    Kronecker product of two arrays.

    Computes the Kronecker product, a composite array made of blocks of the
    second array scaled by the first.

    Equivalent to ``numpy.kron`` for NumPy arrays.

    Parameters
    ----------
    a, b : array
    xp : array_namespace
        The standard-compatible namespace for `a` and `b`.

    Returns
    -------
    res : array
        The Kronecker product of `a` and `b`.

    Notes
    -----
    The function assumes that the number of dimensions of `a` and `b`
    are the same, if necessary prepending the smallest with ones.
    If ``a.shape = (r0,r1,..,rN)`` and ``b.shape = (s0,s1,...,sN)``,
    the Kronecker product has shape ``(r0*s0, r1*s1, ..., rN*SN)``.
    The elements are products of elements from `a` and `b`, organized
    explicitly by::

        kron(a,b)[k0,k1,...,kN] = a[i0,i1,...,iN] * b[j0,j1,...,jN]

    where::

        kt = it * st + jt,  t = 0,...,N

    In the common 2-D case (N=1), the block structure can be visualized::

        [[ a[0,0]*b,   a[0,1]*b,  ... , a[0,-1]*b  ],
         [  ...                              ...   ],
         [ a[-1,0]*b,  a[-1,1]*b, ... , a[-1,-1]*b ]]


    Examples
    --------
    >>> import array_api_strict as xp
    >>> import array_api_extra as xpx
    >>> xpx.kron(xp.asarray([1, 10, 100]), xp.asarray([5, 6, 7]), xp=xp)
    Array([  5,   6,   7,  50,  60,  70, 500,
           600, 700], dtype=array_api_strict.int64)

    >>> xpx.kron(xp.asarray([5, 6, 7]), xp.asarray([1, 10, 100]), xp=xp)
    Array([  5,  50, 500,   6,  60, 600,   7,
            70, 700], dtype=array_api_strict.int64)

    >>> xpx.kron(xp.eye(2), xp.ones((2, 2)), xp=xp)
    Array([[1., 1., 0., 0.],
           [1., 1., 0., 0.],
           [0., 0., 1., 1.],
           [0., 0., 1., 1.]], dtype=array_api_strict.float64)


    >>> a = xp.reshape(xp.arange(100), (2, 5, 2, 5))
    >>> b = xp.reshape(xp.arange(24), (2, 3, 4))
    >>> c = xpx.kron(a, b, xp=xp)
    >>> c.shape
    (2, 10, 6, 20)
    >>> I = (1, 3, 0, 2)
    >>> J = (0, 2, 1)
    >>> J1 = (0,) + J             # extend to ndim=4
    >>> S1 = (1,) + b.shape
    >>> K = tuple(xp.asarray(I) * xp.asarray(S1) + xp.asarray(J1))
    >>> c[K] == a[I]*b[J]
    Array(True, dtype=array_api_strict.bool)

    """

    b = xp.asarray(b)
    singletons = (1,) * (b.ndim - a.ndim)
    a = xp.broadcast_to(xp.asarray(a), singletons + a.shape)

    nd_b, nd_a = b.ndim, a.ndim
    nd_max = max(nd_b, nd_a)
    if nd_a == 0 or nd_b == 0:
        return xp.multiply(a, b)

    a_shape = a.shape
    b_shape = b.shape

    # Equalise the shapes by prepending smaller one with 1s
    a_shape = (1,) * max(0, nd_b - nd_a) + a_shape
    b_shape = (1,) * max(0, nd_a - nd_b) + b_shape

    # Insert empty dimensions
    a_arr = expand_dims(a, axis=tuple(range(nd_b - nd_a)), xp=xp)
    b_arr = expand_dims(b, axis=tuple(range(nd_a - nd_b)), xp=xp)

    # Compute the product
    a_arr = expand_dims(a_arr, axis=tuple(range(1, nd_max * 2, 2)), xp=xp)
    b_arr = expand_dims(b_arr, axis=tuple(range(0, nd_max * 2, 2)), xp=xp)
    result = xp.multiply(a_arr, b_arr)

    # Reshape back and return
    a_shape = xp.asarray(a_shape)
    b_shape = xp.asarray(b_shape)
    return xp.reshape(result, tuple(xp.multiply(a_shape, b_shape)))


def sinc(x: Array, /, *, xp: ModuleType) -> Array:
    r"""
    Return the normalized sinc function.

    The sinc function is equal to :math:`\sin(\pi x)/(\pi x)` for any argument
    :math:`x\ne 0`. ``sinc(0)`` takes the limit value 1, making ``sinc`` not
    only everywhere continuous but also infinitely differentiable.

    .. note::

        Note the normalization factor of ``pi`` used in the definition.
        This is the most commonly used definition in signal processing.
        Use ``sinc(x / xp.pi)`` to obtain the unnormalized sinc function
        :math:`\sin(x)/x` that is more common in mathematics.

    Parameters
    ----------
    x : array
        Array (possibly multi-dimensional) of values for which to calculate
        ``sinc(x)``. Must have a real floating point dtype.
    xp : array_namespace
        The standard-compatible namespace for `x`.

    Returns
    -------
    res : array
        ``sinc(x)`` calculated elementwise, which has the same shape as the input.

    Notes
    -----
    The name sinc is short for "sine cardinal" or "sinus cardinalis".

    The sinc function is used in various signal processing applications,
    including in anti-aliasing, in the construction of a Lanczos resampling
    filter, and in interpolation.

    For bandlimited interpolation of discrete-time signals, the ideal
    interpolation kernel is proportional to the sinc function.

    References
    ----------
    .. [1] Weisstein, Eric W. "Sinc Function." From MathWorld--A Wolfram Web
           Resource. https://mathworld.wolfram.com/SincFunction.html
    .. [2] Wikipedia, "Sinc function",
           https://en.wikipedia.org/wiki/Sinc_function

    Examples
    --------
    >>> import array_api_strict as xp
    >>> import array_api_extra as xpx
    >>> x = xp.linspace(-4, 4, 41)
    >>> xpx.sinc(x, xp=xp)
    Array([-3.89817183e-17, -4.92362781e-02,
           -8.40918587e-02, -8.90384387e-02,
           -5.84680802e-02,  3.89817183e-17,
            6.68206631e-02,  1.16434881e-01,
            1.26137788e-01,  8.50444803e-02,
           -3.89817183e-17, -1.03943254e-01,
           -1.89206682e-01, -2.16236208e-01,
           -1.55914881e-01,  3.89817183e-17,
            2.33872321e-01,  5.04551152e-01,
            7.56826729e-01,  9.35489284e-01,
            1.00000000e+00,  9.35489284e-01,
            7.56826729e-01,  5.04551152e-01,
            2.33872321e-01,  3.89817183e-17,
           -1.55914881e-01, -2.16236208e-01,
           -1.89206682e-01, -1.03943254e-01,
           -3.89817183e-17,  8.50444803e-02,
            1.26137788e-01,  1.16434881e-01,
            6.68206631e-02,  3.89817183e-17,
           -5.84680802e-02, -8.90384387e-02,
           -8.40918587e-02, -4.92362781e-02,
           -3.89817183e-17], dtype=array_api_strict.float64)

    """
    if not xp.isdtype(x.dtype, "real floating"):
        err_msg = "`x` must have a real floating data type."
        raise ValueError(err_msg)
    # no scalars in `where` - array-api#807
    y = xp.pi * xp.where(
        x, x, xp.asarray(xp.finfo(x.dtype).smallest_normal, dtype=x.dtype)
    )
    return xp.sin(y) / y