import numpy as np
import scipy.sparse as sps
from ._numdiff import approx_derivative, group_columns
from ._hessian_update_strategy import HessianUpdateStrategy
from scipy.sparse.linalg import LinearOperator
from scipy._lib._array_api import atleast_nd, array_namespace


FD_METHODS = ('2-point', '3-point', 'cs')


class ScalarFunction:
    """Scalar function and its derivatives.

    This class defines a scalar function F: R^n->R and methods for
    computing or approximating its first and second derivatives.

    Parameters
    ----------
    fun : callable
        evaluates the scalar function. Must be of the form ``fun(x, *args)``,
        where ``x`` is the argument in the form of a 1-D array and ``args`` is
        a tuple of any additional fixed parameters needed to completely specify
        the function. Should return a scalar.
    x0 : array-like
        Provides an initial set of variables for evaluating fun. Array of real
        elements of size (n,), where 'n' is the number of independent
        variables.
    args : tuple, optional
        Any additional fixed parameters needed to completely specify the scalar
        function.
    grad : {callable, '2-point', '3-point', 'cs'}
        Method for computing the gradient vector.
        If it is a callable, it should be a function that returns the gradient
        vector:

            ``grad(x, *args) -> array_like, shape (n,)``

        where ``x`` is an array with shape (n,) and ``args`` is a tuple with
        the fixed parameters.
        Alternatively, the keywords  {'2-point', '3-point', 'cs'} can be used
        to select a finite difference scheme for numerical estimation of the
        gradient with a relative step size. These finite difference schemes
        obey any specified `bounds`.
    hess : {callable, '2-point', '3-point', 'cs', HessianUpdateStrategy}
        Method for computing the Hessian matrix. If it is callable, it should
        return the  Hessian matrix:

            ``hess(x, *args) -> {LinearOperator, spmatrix, array}, (n, n)``

        where x is a (n,) ndarray and `args` is a tuple with the fixed
        parameters. Alternatively, the keywords {'2-point', '3-point', 'cs'}
        select a finite difference scheme for numerical estimation. Or, objects
        implementing `HessianUpdateStrategy` interface can be used to
        approximate the Hessian.
        Whenever the gradient is estimated via finite-differences, the Hessian
        cannot be estimated with options {'2-point', '3-point', 'cs'} and needs
        to be estimated using one of the quasi-Newton strategies.
    finite_diff_rel_step : None or array_like
        Relative step size to use. The absolute step size is computed as
        ``h = finite_diff_rel_step * sign(x0) * max(1, abs(x0))``, possibly
        adjusted to fit into the bounds. For ``method='3-point'`` the sign
        of `h` is ignored. If None then finite_diff_rel_step is selected
        automatically,
    finite_diff_bounds : tuple of array_like
        Lower and upper bounds on independent variables. Defaults to no bounds,
        (-np.inf, np.inf). Each bound must match the size of `x0` or be a
        scalar, in the latter case the bound will be the same for all
        variables. Use it to limit the range of function evaluation.
    epsilon : None or array_like, optional
        Absolute step size to use, possibly adjusted to fit into the bounds.
        For ``method='3-point'`` the sign of `epsilon` is ignored. By default
        relative steps are used, only if ``epsilon is not None`` are absolute
        steps used.

    Notes
    -----
    This class implements a memoization logic. There are methods `fun`,
    `grad`, hess` and corresponding attributes `f`, `g` and `H`. The following
    things should be considered:

        1. Use only public methods `fun`, `grad` and `hess`.
        2. After one of the methods is called, the corresponding attribute
           will be set. However, a subsequent call with a different argument
           of *any* of the methods may overwrite the attribute.
    """
    def __init__(self, fun, x0, args, grad, hess, finite_diff_rel_step,
                 finite_diff_bounds, epsilon=None):
        if not callable(grad) and grad not in FD_METHODS:
            raise ValueError(
                f"`grad` must be either callable or one of {FD_METHODS}."
            )

        if not (callable(hess) or hess in FD_METHODS
                or isinstance(hess, HessianUpdateStrategy)):
            raise ValueError(
                f"`hess` must be either callable, HessianUpdateStrategy"
                f" or one of {FD_METHODS}."
            )

        if grad in FD_METHODS and hess in FD_METHODS:
            raise ValueError("Whenever the gradient is estimated via "
                             "finite-differences, we require the Hessian "
                             "to be estimated using one of the "
                             "quasi-Newton strategies.")

        self.xp = xp = array_namespace(x0)
        _x = atleast_nd(x0, ndim=1, xp=xp)
        _dtype = xp.float64
        if xp.isdtype(_x.dtype, "real floating"):
            _dtype = _x.dtype

        # promotes to floating
        self.x = xp.astype(_x, _dtype)
        self.x_dtype = _dtype
        self.n = self.x.size
        self.nfev = 0
        self.ngev = 0
        self.nhev = 0
        self.f_updated = False
        self.g_updated = False
        self.H_updated = False

        self._lowest_x = None
        self._lowest_f = np.inf

        finite_diff_options = {}
        if grad in FD_METHODS:
            finite_diff_options["method"] = grad
            finite_diff_options["rel_step"] = finite_diff_rel_step
            finite_diff_options["abs_step"] = epsilon
            finite_diff_options["bounds"] = finite_diff_bounds
        if hess in FD_METHODS:
            finite_diff_options["method"] = hess
            finite_diff_options["rel_step"] = finite_diff_rel_step
            finite_diff_options["abs_step"] = epsilon
            finite_diff_options["as_linear_operator"] = True

        # Function evaluation
        def fun_wrapped(x):
            self.nfev += 1
            # Send a copy because the user may overwrite it.
            # Overwriting results in undefined behaviour because
            # fun(self.x) will change self.x, with the two no longer linked.
            fx = fun(np.copy(x), *args)
            # Make sure the function returns a true scalar
            if not np.isscalar(fx):
                try:
                    fx = np.asarray(fx).item()
                except (TypeError, ValueError) as e:
                    raise ValueError(
                        "The user-provided objective function "
                        "must return a scalar value."
                    ) from e

            if fx < self._lowest_f:
                self._lowest_x = x
                self._lowest_f = fx

            return fx

        def update_fun():
            self.f = fun_wrapped(self.x)

        self._update_fun_impl = update_fun
        self._update_fun()

        # Gradient evaluation
        if callable(grad):
            def grad_wrapped(x):
                self.ngev += 1
                return np.atleast_1d(grad(np.copy(x), *args))

            def update_grad():
                self.g = grad_wrapped(self.x)

        elif grad in FD_METHODS:
            def update_grad():
                self._update_fun()
                self.ngev += 1
                self.g = approx_derivative(fun_wrapped, self.x, f0=self.f,
                                           **finite_diff_options)

        self._update_grad_impl = update_grad
        self._update_grad()

        # Hessian Evaluation
        if callable(hess):
            self.H = hess(np.copy(x0), *args)
            self.H_updated = True
            self.nhev += 1

            if sps.issparse(self.H):
                def hess_wrapped(x):
                    self.nhev += 1
                    return sps.csr_matrix(hess(np.copy(x), *args))
                self.H = sps.csr_matrix(self.H)

            elif isinstance(self.H, LinearOperator):
                def hess_wrapped(x):
                    self.nhev += 1
                    return hess(np.copy(x), *args)

            else:
                def hess_wrapped(x):
                    self.nhev += 1
                    return np.atleast_2d(np.asarray(hess(np.copy(x), *args)))
                self.H = np.atleast_2d(np.asarray(self.H))

            def update_hess():
                self.H = hess_wrapped(self.x)

        elif hess in FD_METHODS:
            def update_hess():
                self._update_grad()
                self.H = approx_derivative(grad_wrapped, self.x, f0=self.g,
                                           **finite_diff_options)
                return self.H

            update_hess()
            self.H_updated = True
        elif isinstance(hess, HessianUpdateStrategy):
            self.H = hess
            self.H.initialize(self.n, 'hess')
            self.H_updated = True
            self.x_prev = None
            self.g_prev = None

            def update_hess():
                self._update_grad()
                self.H.update(self.x - self.x_prev, self.g - self.g_prev)

        self._update_hess_impl = update_hess

        if isinstance(hess, HessianUpdateStrategy):
            def update_x(x):
                self._update_grad()
                self.x_prev = self.x
                self.g_prev = self.g
                # ensure that self.x is a copy of x. Don't store a reference
                # otherwise the memoization doesn't work properly.

                _x = atleast_nd(x, ndim=1, xp=self.xp)
                self.x = self.xp.astype(_x, self.x_dtype)
                self.f_updated = False
                self.g_updated = False
                self.H_updated = False
                self._update_hess()
        else:
            def update_x(x):
                # ensure that self.x is a copy of x. Don't store a reference
                # otherwise the memoization doesn't work properly.
                _x = atleast_nd(x, ndim=1, xp=self.xp)
                self.x = self.xp.astype(_x, self.x_dtype)
                self.f_updated = False
                self.g_updated = False
                self.H_updated = False
        self._update_x_impl = update_x

    def _update_fun(self):
        if not self.f_updated:
            self._update_fun_impl()
            self.f_updated = True

    def _update_grad(self):
        if not self.g_updated:
            self._update_grad_impl()
            self.g_updated = True

    def _update_hess(self):
        if not self.H_updated:
            self._update_hess_impl()
            self.H_updated = True

    def fun(self, x):
        if not np.array_equal(x, self.x):
            self._update_x_impl(x)
        self._update_fun()
        return self.f

    def grad(self, x):
        if not np.array_equal(x, self.x):
            self._update_x_impl(x)
        self._update_grad()
        return self.g

    def hess(self, x):
        if not np.array_equal(x, self.x):
            self._update_x_impl(x)
        self._update_hess()
        return self.H

    def fun_and_grad(self, x):
        if not np.array_equal(x, self.x):
            self._update_x_impl(x)
        self._update_fun()
        self._update_grad()
        return self.f, self.g


class VectorFunction:
    """Vector function and its derivatives.

    This class defines a vector function F: R^n->R^m and methods for
    computing or approximating its first and second derivatives.

    Notes
    -----
    This class implements a memoization logic. There are methods `fun`,
    `jac`, hess` and corresponding attributes `f`, `J` and `H`. The following
    things should be considered:

        1. Use only public methods `fun`, `jac` and `hess`.
        2. After one of the methods is called, the corresponding attribute
           will be set. However, a subsequent call with a different argument
           of *any* of the methods may overwrite the attribute.
    """
    def __init__(self, fun, x0, jac, hess,
                 finite_diff_rel_step, finite_diff_jac_sparsity,
                 finite_diff_bounds, sparse_jacobian):
        if not callable(jac) and jac not in FD_METHODS:
            raise ValueError(f"`jac` must be either callable or one of {FD_METHODS}.")

        if not (callable(hess) or hess in FD_METHODS
                or isinstance(hess, HessianUpdateStrategy)):
            raise ValueError("`hess` must be either callable,"
                             f"HessianUpdateStrategy or one of {FD_METHODS}.")

        if jac in FD_METHODS and hess in FD_METHODS:
            raise ValueError("Whenever the Jacobian is estimated via "
                             "finite-differences, we require the Hessian to "
                             "be estimated using one of the quasi-Newton "
                             "strategies.")

        self.xp = xp = array_namespace(x0)
        _x = atleast_nd(x0, ndim=1, xp=xp)
        _dtype = xp.float64
        if xp.isdtype(_x.dtype, "real floating"):
            _dtype = _x.dtype

        # promotes to floating
        self.x = xp.astype(_x, _dtype)
        self.x_dtype = _dtype

        self.n = self.x.size
        self.nfev = 0
        self.njev = 0
        self.nhev = 0
        self.f_updated = False
        self.J_updated = False
        self.H_updated = False

        finite_diff_options = {}
        if jac in FD_METHODS:
            finite_diff_options["method"] = jac
            finite_diff_options["rel_step"] = finite_diff_rel_step
            if finite_diff_jac_sparsity is not None:
                sparsity_groups = group_columns(finite_diff_jac_sparsity)
                finite_diff_options["sparsity"] = (finite_diff_jac_sparsity,
                                                   sparsity_groups)
            finite_diff_options["bounds"] = finite_diff_bounds
            self.x_diff = np.copy(self.x)
        if hess in FD_METHODS:
            finite_diff_options["method"] = hess
            finite_diff_options["rel_step"] = finite_diff_rel_step
            finite_diff_options["as_linear_operator"] = True
            self.x_diff = np.copy(self.x)
        if jac in FD_METHODS and hess in FD_METHODS:
            raise ValueError("Whenever the Jacobian is estimated via "
                             "finite-differences, we require the Hessian to "
                             "be estimated using one of the quasi-Newton "
                             "strategies.")

        # Function evaluation
        def fun_wrapped(x):
            self.nfev += 1
            return np.atleast_1d(fun(x))

        def update_fun():
            self.f = fun_wrapped(self.x)

        self._update_fun_impl = update_fun
        update_fun()

        self.v = np.zeros_like(self.f)
        self.m = self.v.size

        # Jacobian Evaluation
        if callable(jac):
            self.J = jac(self.x)
            self.J_updated = True
            self.njev += 1

            if (sparse_jacobian or
                    sparse_jacobian is None and sps.issparse(self.J)):
                def jac_wrapped(x):
                    self.njev += 1
                    return sps.csr_matrix(jac(x))
                self.J = sps.csr_matrix(self.J)
                self.sparse_jacobian = True

            elif sps.issparse(self.J):
                def jac_wrapped(x):
                    self.njev += 1
                    return jac(x).toarray()
                self.J = self.J.toarray()
                self.sparse_jacobian = False

            else:
                def jac_wrapped(x):
                    self.njev += 1
                    return np.atleast_2d(jac(x))
                self.J = np.atleast_2d(self.J)
                self.sparse_jacobian = False

            def update_jac():
                self.J = jac_wrapped(self.x)

        elif jac in FD_METHODS:
            self.J = approx_derivative(fun_wrapped, self.x, f0=self.f,
                                       **finite_diff_options)
            self.J_updated = True

            if (sparse_jacobian or
                    sparse_jacobian is None and sps.issparse(self.J)):
                def update_jac():
                    self._update_fun()
                    self.J = sps.csr_matrix(
                        approx_derivative(fun_wrapped, self.x, f0=self.f,
                                          **finite_diff_options))
                self.J = sps.csr_matrix(self.J)
                self.sparse_jacobian = True

            elif sps.issparse(self.J):
                def update_jac():
                    self._update_fun()
                    self.J = approx_derivative(fun_wrapped, self.x, f0=self.f,
                                               **finite_diff_options).toarray()
                self.J = self.J.toarray()
                self.sparse_jacobian = False

            else:
                def update_jac():
                    self._update_fun()
                    self.J = np.atleast_2d(
                        approx_derivative(fun_wrapped, self.x, f0=self.f,
                                          **finite_diff_options))
                self.J = np.atleast_2d(self.J)
                self.sparse_jacobian = False

        self._update_jac_impl = update_jac

        # Define Hessian
        if callable(hess):
            self.H = hess(self.x, self.v)
            self.H_updated = True
            self.nhev += 1

            if sps.issparse(self.H):
                def hess_wrapped(x, v):
                    self.nhev += 1
                    return sps.csr_matrix(hess(x, v))
                self.H = sps.csr_matrix(self.H)

            elif isinstance(self.H, LinearOperator):
                def hess_wrapped(x, v):
                    self.nhev += 1
                    return hess(x, v)

            else:
                def hess_wrapped(x, v):
                    self.nhev += 1
                    return np.atleast_2d(np.asarray(hess(x, v)))
                self.H = np.atleast_2d(np.asarray(self.H))

            def update_hess():
                self.H = hess_wrapped(self.x, self.v)
        elif hess in FD_METHODS:
            def jac_dot_v(x, v):
                return jac_wrapped(x).T.dot(v)

            def update_hess():
                self._update_jac()
                self.H = approx_derivative(jac_dot_v, self.x,
                                           f0=self.J.T.dot(self.v),
                                           args=(self.v,),
                                           **finite_diff_options)
            update_hess()
            self.H_updated = True
        elif isinstance(hess, HessianUpdateStrategy):
            self.H = hess
            self.H.initialize(self.n, 'hess')
            self.H_updated = True
            self.x_prev = None
            self.J_prev = None

            def update_hess():
                self._update_jac()
                # When v is updated before x was updated, then x_prev and
                # J_prev are None and we need this check.
                if self.x_prev is not None and self.J_prev is not None:
                    delta_x = self.x - self.x_prev
                    delta_g = self.J.T.dot(self.v) - self.J_prev.T.dot(self.v)
                    self.H.update(delta_x, delta_g)

        self._update_hess_impl = update_hess

        if isinstance(hess, HessianUpdateStrategy):
            def update_x(x):
                self._update_jac()
                self.x_prev = self.x
                self.J_prev = self.J
                _x = atleast_nd(x, ndim=1, xp=self.xp)
                self.x = self.xp.astype(_x, self.x_dtype)
                self.f_updated = False
                self.J_updated = False
                self.H_updated = False
                self._update_hess()
        else:
            def update_x(x):
                _x = atleast_nd(x, ndim=1, xp=self.xp)
                self.x = self.xp.astype(_x, self.x_dtype)
                self.f_updated = False
                self.J_updated = False
                self.H_updated = False

        self._update_x_impl = update_x

    def _update_v(self, v):
        if not np.array_equal(v, self.v):
            self.v = v
            self.H_updated = False

    def _update_x(self, x):
        if not np.array_equal(x, self.x):
            self._update_x_impl(x)

    def _update_fun(self):
        if not self.f_updated:
            self._update_fun_impl()
            self.f_updated = True

    def _update_jac(self):
        if not self.J_updated:
            self._update_jac_impl()
            self.J_updated = True

    def _update_hess(self):
        if not self.H_updated:
            self._update_hess_impl()
            self.H_updated = True

    def fun(self, x):
        self._update_x(x)
        self._update_fun()
        return self.f

    def jac(self, x):
        self._update_x(x)
        self._update_jac()
        return self.J

    def hess(self, x, v):
        # v should be updated before x.
        self._update_v(v)
        self._update_x(x)
        self._update_hess()
        return self.H


class LinearVectorFunction:
    """Linear vector function and its derivatives.

    Defines a linear function F = A x, where x is N-D vector and
    A is m-by-n matrix. The Jacobian is constant and equals to A. The Hessian
    is identically zero and it is returned as a csr matrix.
    """
    def __init__(self, A, x0, sparse_jacobian):
        if sparse_jacobian or sparse_jacobian is None and sps.issparse(A):
            self.J = sps.csr_matrix(A)
            self.sparse_jacobian = True
        elif sps.issparse(A):
            self.J = A.toarray()
            self.sparse_jacobian = False
        else:
            # np.asarray makes sure A is ndarray and not matrix
            self.J = np.atleast_2d(np.asarray(A))
            self.sparse_jacobian = False

        self.m, self.n = self.J.shape

        self.xp = xp = array_namespace(x0)
        _x = atleast_nd(x0, ndim=1, xp=xp)
        _dtype = xp.float64
        if xp.isdtype(_x.dtype, "real floating"):
            _dtype = _x.dtype

        # promotes to floating
        self.x = xp.astype(_x, _dtype)
        self.x_dtype = _dtype

        self.f = self.J.dot(self.x)
        self.f_updated = True

        self.v = np.zeros(self.m, dtype=float)
        self.H = sps.csr_matrix((self.n, self.n))

    def _update_x(self, x):
        if not np.array_equal(x, self.x):
            _x = atleast_nd(x, ndim=1, xp=self.xp)
            self.x = self.xp.astype(_x, self.x_dtype)
            self.f_updated = False

    def fun(self, x):
        self._update_x(x)
        if not self.f_updated:
            self.f = self.J.dot(x)
            self.f_updated = True
        return self.f

    def jac(self, x):
        self._update_x(x)
        return self.J

    def hess(self, x, v):
        self._update_x(x)
        self.v = v
        return self.H


class IdentityVectorFunction(LinearVectorFunction):
    """Identity vector function and its derivatives.

    The Jacobian is the identity matrix, returned as a dense array when
    `sparse_jacobian=False` and as a csr matrix otherwise. The Hessian is
    identically zero and it is returned as a csr matrix.
    """
    def __init__(self, x0, sparse_jacobian):
        n = len(x0)
        if sparse_jacobian or sparse_jacobian is None:
            A = sps.eye(n, format='csr')
            sparse_jacobian = True
        else:
            A = np.eye(n)
            sparse_jacobian = False
        super().__init__(A, x0, sparse_jacobian)
