import numpy as np

from .autodiffutils import AutoDiffXd


def derivative(function, x):
    """Compute the derivative of the function evaluated at the scalar input x
    using Eigen's automatic differentiation.

    The function should be scalar-input and scalar-output.
    """
    x_ad = AutoDiffXd(value=x, size=1, offset=0)
    y_ad = function(x_ad)
    return y_ad.derivatives()[0]


def gradient(function, x):
    """Compute the gradient of the function evaluated at the vector input x
    using Eigen's automatic differentiation.

    ``function`` should be vector-input and be either a scalar output or a
    vector of size 1, where the element must be of type AutoDiffXd.
    """
    x = np.asarray(x)
    assert x.ndim == 1, "x must be a vector"
    x_ad = np.empty(x.shape, dtype=AutoDiffXd)
    for i in range(x.size):
        x_ad.flat[i] = AutoDiffXd(value=x.flat[i], size=x.size, offset=i)
    y_ad = np.asarray(function(x_ad))
    # TODO(eric.cousineau): Consider restricting this in the future to only be
    # a scalar.
    assert y_ad.size == 1 and y_ad.ndim <= 1, (
        "The output of `function` must be of a scalar or a vector of size 1")
    y_ad = y_ad.reshape(())  # To scalar.
    return y_ad.item().derivatives()


def jacobian(function, x):
    """Compute the jacobian of the function evaluated at the vector input x
    using Eigen's automatic differentiation. The dimension of the jacobian will
    be one more than the output of ``function``.

    ``function`` should be vector-input, and can be any dimension output, and
    must return an array with AutoDiffXd elements.
    """
    x = np.asarray(x)
    assert x.ndim == 1, "x must be a vector"
    x_ad = np.empty(x.shape, dtype=object)
    for i in range(x.size):
        x_ad.flat[i] = AutoDiffXd(value=x.flat[i], size=x.size, offset=i)
    y_ad = np.asarray(function(x_ad))
    # An AutoDiffXd variable with empty derivatives should be treated as
    # having all zero derivatives. Checking the length of the derivatives
    # vector, and replacing it with all zeros ensures np.vstack doesn't
    # throw an error when the shapes don't line up.
    return np.vstack(
        [y.derivatives() if len(y.derivatives()) > 0 else np.zeros(x.size)
            for y in y_ad.flat]).reshape(y_ad.shape + (-1,))


# Method overloads:
# These are obviously not a complete set of mathematical functions for
# autodiff numbers. Rather, they exist just as a demonstration of *how*
# to overload individual mathematical functions to work with both
# AutoDiff and numeric inputs.

def sin(x):
    if isinstance(x, AutoDiffXd):
        return x.sin()
    else:
        return np.sin(x)


def cos(x):
    if isinstance(x, AutoDiffXd):
        return x.cos()
    else:
        return np.cos(x)
