from scipy.special import expit
import numpy as np

try:
    from torch import sigmoid, tanh, eye, diag, zeros, exp, max, matmul, identity, ones

    allow_torch = True
except ImportError:
    print(
        "torch installation could not be found, only numpy activation functions allowed"
    )
    allow_torch = False


def fill_diagonal_torch(arr, x):
    """Fill diagonal elements of torch Tensor

    Elements in the [i, i] position of a torch Tensor are replaced by
    the passed elements. 
    
    Args:
        arr (torch.Tensor): Tensor which diagonal elements must be replaced.
        x (torch.Tensor): 1D-Tensor to replace the diagonal elements of arr

    Returns:
        torch.Tensor: arr Tensor with diagonal elements replaced by x.
    
    Limitation:
        x.size(0) must be equal to arr.size(0). 
        
    Examples:
        >>> z = torch.zeros((3,3,3))
        >>> diagonal = torch.Tensor([1, 2, -1])
        >>> z = fill_diagonal_torch(z, diagonal)
        >>> z
        tensor([[[ 1.,  0.,  0.],
                [ 0.,  0.,  0.],
                [ 0.,  0.,  0.]],

                [[ 0.,  0.,  0.],
                [ 0.,  2.,  0.],
                [ 0.,  0.,  0.]],

                [[ 0.,  0.,  0.],
                [ 0.,  0.,  0.],
                [ 0.,  0., -1.]]])
    """
    k = range(arr.size(0))
    arr[[k] * len(arr.size())] = x
    return arr


def activation_function(func: str, use_torch: bool = False):
    """Obtain activation function for a given function name

    Obtain activation function for further use in calculating the output
    of a given neural network layer.
    
    Args:
        func (str): name of the activation function.
        use_torch (bool, optional): flag to make the activation function 
          return torch.Tensor instead of numpy.array. Defaults to False.

    Returns:
        function: activation function.
    
    Limitation:
        The only accepted activation functions name are: 
            * logistic
            * sigmoid
            * expit
            * identity
            * tanh
            * relu
            * softmax
        
    Examples:
        >>> import numpy as np
        >>> ac_fct = activation_function("sigmoid")
        >>> out = ac_fct(np.array([0.8, -0.1, 0.3]))
        >>> out
        array([0.68997448, 0.47502081, 0.57444252])
    """
    def identity(x):
        return x

    def relu(x):
        return x * (x > 0)

    if not use_torch or not allow_torch:

        def logistic(x):
            return expit(x)

        def stanh(x):
            return np.tanh(x)
        
        def softmax(x):
            return np.exp(x)/sum(np.exp(x))

    elif use_torch and allow_torch:

        def logistic(x):
            return sigmoid(x)

        def stanh(x):
            return tanh(x)

        def softmax(x):
            return exp(x)/sum(exp(x))
    actfunc = {
        "logistic": logistic,
        "sigmoid": logistic,
        "expit": logistic,
        "identity": identity,
        "tanh": stanh,
        "relu": relu,
        "softmax": softmax
    }
    return actfunc[func]


def der_activation_function(func: str, use_torch: bool = False):
    """Obtain derivative of activation function for a given function name

    Obtain derivative of activation function for further use in calculating 
    the derivative of a given neural network layer.
    
    Args:
        func (str): name of the activation function.
        use_torch (bool, optional): flag to make the activation function 
            return torch.Tensor instead of numpy.array. Defaults to False.

    Returns:
        function: derivative of activation function.
    
    Limitation:
        The only accepted activation functions name are: 
            * logistic
            * sigmoid
            * expit
            * identity
            * tanh
            * relu
            * softmax
        
    Examples:
        >>> import numpy as np
        >>> der_ac_fct = der_activation_function("sigmoid")
        >>> out = der_ac_fct(np.array([0.8, -0.1, 0.3]))
        >>> out
        array([[0.2139097 , 0.        , 0.        ],
            [0.        , 0.24937604, 0.        ],
            [0.        , 0.        , 0.24445831]])
    """
    if not use_torch or not allow_torch:

        def logistic(x):
            fx = expit(x)
            return np.diag(fx * (1 - fx))

        def identity(x):
            return np.identity(x.shape[0], dtype=int)

        def stanh(x):
            return np.diag(1 - np.tanh(x) ** 2)

        def relu(x):
            return np.diag(x > 0)
        
        def softmax(x):
            x = np.exp(x - np.max(x)) / np.sum(np.exp(x - np.max(x))) # numerical stability
            return np.matmul(x, np.ones((1, x.shape[0]))) * (np.identity(x.shape[0]) - np.matmul(np.ones((x.shape[0], 1)), x.T))

    elif use_torch and allow_torch:

        def logistic(x):
            fx = sigmoid(x)
            return diag(fx * (1 - fx)).float()

        def identity(x):
            return eye(x.size(0), dtype=int).float()

        def stanh(x):
            return diag(1 - np.tanh(x) ** 2).float()

        def relu(x):
            return diag(x > 0).float()
        
        def softmax(x):
            x = exp(x - max(x)) / sum(exp(x - max(x))) # numerical stability
            return matmul(x, ones((1, x.size(0)))) * (identity(x.size(0)) - matmul(ones((x.size(0), 1)), x.T))

    deractfunc = {
        "logistic": logistic,
        "sigmoid": logistic,
        "expit": logistic,
        "identity": identity,
        "tanh": stanh,
        "relu": relu,
        "softmax": softmax
    }
    return deractfunc[func]


def der_2_activation_function(func: str, use_torch: bool = False):
    """Obtain second derivative of activation function for a given function name

    Obtain second derivative of activation function for further use in calculating 
    the hessian of a given neural network layer.
    
    Args:
        func (str): name of the activation function.
        use_torch (bool, optional): flag to make the activation function 
            return torch.Tensor instead of numpy.array. Defaults to False.

    Returns:
        function: second derivative of activation function.
    
    Limitation:
        The only accepted activation functions name are: 
            * logistic
            * sigmoid
            * expit
            * identity
            * tanh
            * relu
        
    Examples:
        >>> import numpy as np
        >>> der_2_ac_fct = der_2_activation_function("sigmoid")
        >>> out = der_2_ac_fct(np.array([0.8, -0.1, 0.3]))
        >>> out
        array([[[-0.08127477,  0.        ,  0.        ],
                [ 0.        ,  0.        ,  0.        ],
                [ 0.        ,  0.        ,  0.        ]],

                [[ 0.        ,  0.        ,  0.        ],
                [ 0.        ,  0.01245842,  0.        ],
                [ 0.        ,  0.        ,  0.        ]],

                [[ 0.        ,  0.        ,  0.        ],
                [ 0.        ,  0.        ,  0.        ],
                [ 0.        ,  0.        , -0.03639618]]])
    """
    if not use_torch or not allow_torch:

        def logistic(x):
            zeros_3d = np.zeros((x.shape[0], x.shape[0], x.shape[0]), dtype=float)
            fx = expit(x)
            np.fill_diagonal(zeros_3d, fx * (1 - fx) * (1 - 2 * fx))
            return zeros_3d

        def identity(x):
            return np.zeros((x.shape[0], x.shape[0], x.shape[0]), dtype=int)

        def stanh(x):
            zeros_3d = np.zeros((x.shape[0], x.shape[0], x.shape[0]), dtype=float)
            fx = tanh(x)
            np.fill_diagonal(zeros_3d, -2 * fx * (1 - fx ^ 2))
            return zeros_3d

        def relu(x):
            return np.zeros((x.shape[0], x.shape[0], x.shape[0]), dtype=int)
        
        def softmax(x):
            x = np.exp(x - np.max(x)) / np.sum(np.exp(x - np.max(x))) # numerical stability
            # build 'delta' arrays
            d_i_m = np.broadcast_to(np.eye(x.shape[0]),(x.shape[0],)+np.eye(x.shape[0]).shape)
            d_m_p = d_i_m.transpose((1,2,0))
            d_i_p = d_i_m.transpose((1,0,2))
            # Build 'a' arrays
            am = np.broadcast_to(x @ d_i_m,(x.shape[0],)+np.eye(x.shape[0]).shape)
            ai = am.transpose((1,2,0))
            ap = am.transpose((2,1,0))
            # Create second derivative array
            return ai * ((d_i_p - ap) * (d_i_m - am) - am * (d_m_p * ap))
    elif use_torch and allow_torch:

        def logistic(x):
            zeros_3d = zeros((x.size(0), x.size(0), x.size(0))).float()
            fx = sigmoid(x)
            return fill_diagonal_torch(zeros_3d, fx * (1 - fx) * (1 - 2 * fx))

        def identity(x):
            return zeros((x.size(0), x.size(0), x.size(0)), dtype=int)

        def stanh(x):
            zeros_3d = zeros((x.size(0), x.size(0), x.size(0))).float()
            fx = tanh(x)
            return fill_diagonal_torch(zeros_3d, -2 * fx * (1 - fx ^ 2))

        def relu(x):
            return zeros((x.size(0), x.size(0), x.size(0)), dtype=int)
        
        def softmax(x):
            x = exp(x - max(x)) / sum(exp(x - max(x))) # numerical stability

    der2actfunc = {
        "logistic": logistic,
        "sigmoid": logistic,
        "expit": logistic,
        "identity": identity,
        "tanh": stanh,
        "relu": relu,
        "softmax": softmax
    }
    return der2actfunc[func]


def der_3_activation_function(func: str, use_torch: bool = False):
    """Obtain third derivative of activation function for a given function name

    Obtain third derivative of activation function for further use in calculating 
    the jerkian of a given neural network layer.
    
    Args:
        func (str): name of the activation function.
        use_torch (bool, optional): flag to make the activation function 
            return torch.Tensor instead of numpy.array. Defaults to False.

    Returns:
        function: third derivative of activation function.
    
    Limitation:
        The only accepted activation functions name are: 
            * logistic
            * sigmoid
            * expit
            * identity
            * tanh
            * relu
        
    Examples:
        >>> import numpy as np
        >>> der_3_ac_fct = der_3_activation_function("sigmoid")
        >>> out = der_3_ac_fct(np.array([0.8, -0.1, 0.3]))
        >>> out
        array([[[[ 0.08695778,  0.        ,  0.        ],
                [ 0.        ,  0.        ,  0.        ],
                [ 0.        ,  0.        ,  0.        ]],

                [[ 0.        ,  0.        ,  0.        ],
                [ 0.        ,  0.        ,  0.        ],
                [ 0.        ,  0.        ,  0.        ]],

                [[ 0.        ,  0.        ,  0.        ],
                [ 0.        ,  0.        ,  0.        ],
                [ 0.        ,  0.        ,  0.        ]]],


            [[[ 0.        ,  0.        ,  0.        ],
                [ 0.        ,  0.        ,  0.        ],
                [ 0.        ,  0.        ,  0.        ]],

                [[ 0.        ,  0.        ,  0.        ],
                [ 0.        , -0.00529561,  0.        ],
                [ 0.        ,  0.        ,  0.        ]],

                [[ 0.        ,  0.        ,  0.        ],
                [ 0.        ,  0.        ,  0.        ],
                [ 0.        ,  0.        ,  0.        ]]],


            [[[ 0.        ,  0.        ,  0.        ],
                [ 0.        ,  0.        ,  0.        ],
                [ 0.        ,  0.        ,  0.        ]],

                [[ 0.        ,  0.        ,  0.        ],
                [ 0.        ,  0.        ,  0.        ],
                [ 0.        ,  0.        ,  0.        ]],

                [[ 0.        ,  0.        ,  0.        ],
                [ 0.        ,  0.        ,  0.        ],
                [ 0.        ,  0.        ,  0.02632636]]]])
    """
    if not use_torch or not allow_torch:

        def logistic(x):
            zeros_4d = np.zeros(
                (x.shape[0], x.shape[0], x.shape[0], x.shape[0]), dtype=float
            )
            fx = np.exp(x)
            np.fill_diagonal(zeros_4d, fx / (fx + 1) ** 2 - 6 * fx ** 2 / (fx + 1) ** 3 + 6 * fx ** 3 / (fx + 1) ** 4)
            return zeros_4d

        def identity(x):
            return np.zeros((x.shape[0], x.shape[0], x.shape[0], x.shape[0]), dtype=int)

        def stanh(x):
            zeros_4d = np.zeros(
                (x.shape[0], x.shape[0], x.shape[0], x.shape[0]), dtype=float
            )
            fx = tanh(x)
            np.fill_diagonal(zeros_4d, -2 * (1 - 4 * fx ** 2 + fx ** 4))
            return zeros_4d

        def relu(x):
            return np.zeros((x.shape[0], x.shape[0], x.shape[0], x.shape[0]), dtype=int)

    elif use_torch and allow_torch:

        def logistic(x):
            zeros_4d = zeros((x.size(0), x.size(0), x.size(0), x.size(0))).float()
            fx = exp(-x)
            return fill_diagonal_torch(
                zeros_4d, fx / (fx + 1) ** 2 - 6 * fx ** 2 / (fx + 1) ** 3 + 6 * fx ** 3 / (fx + 1) ** 4
            )

        def identity(x):
            return zeros((x.size(0), x.size(0), x.size(0), x.size(0)), dtype=int)

        def stanh(x):
            zeros_4d = zeros((x.size(0), x.size(0), x.size(0), x.size(0))).float()
            fx = tanh(x)
            return fill_diagonal_torch(zeros_4d, -2 * (1 - 4 * fx ** 2 + fx ** 4))

        def relu(x):
            return zeros((x.size(0), x.size(0), x.size(0), x.size(0)), dtype=int)

    der3actfunc = {
        "logistic": logistic,
        "sigmoid": logistic,
        "expit": logistic,
        "identity": identity,
        "tanh": stanh,
        "relu": relu,
    }
    return der3actfunc[func]
