# Copyright 2015 The TensorFlow Authors. All Rights Reserved.
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#     http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
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# ==============================================================================


"""Constraints: functions that impose constraints on weight values."""

import tensorflow.compat.v2 as tf

from keras import backend
from keras.utils.generic_utils import deserialize_keras_object
from keras.utils.generic_utils import serialize_keras_object

# isort: off
from tensorflow.python.util.tf_export import keras_export
from tensorflow.tools.docs import doc_controls


@keras_export("keras.constraints.Constraint")
class Constraint:
    """Base class for weight constraints.

    A `Constraint` instance works like a stateless function.
    Users who subclass this
    class should override the `__call__` method, which takes a single
    weight parameter and return a projected version of that parameter
    (e.g. normalized or clipped). Constraints can be used with various Keras
    layers via the `kernel_constraint` or `bias_constraint` arguments.

    Here's a simple example of a non-negative weight constraint:

    >>> class NonNegative(tf.keras.constraints.Constraint):
    ...
    ...  def __call__(self, w):
    ...    return w * tf.cast(tf.math.greater_equal(w, 0.), w.dtype)

    >>> weight = tf.constant((-1.0, 1.0))
    >>> NonNegative()(weight)
    <tf.Tensor: shape=(2,), dtype=float32, numpy=array([0.,  1.],
    dtype=float32)>

    >>> tf.keras.layers.Dense(4, kernel_constraint=NonNegative())
    """

    def __call__(self, w):
        """Applies the constraint to the input weight variable.

        By default, the inputs weight variable is not modified.
        Users should override this method to implement their own projection
        function.

        Args:
          w: Input weight variable.

        Returns:
          Projected variable (by default, returns unmodified inputs).
        """
        return w

    def get_config(self):
        """Returns a Python dict of the object config.

        A constraint config is a Python dictionary (JSON-serializable) that can
        be used to reinstantiate the same object.

        Returns:
          Python dict containing the configuration of the constraint object.
        """
        return {}


@keras_export("keras.constraints.MaxNorm", "keras.constraints.max_norm")
class MaxNorm(Constraint):
    """MaxNorm weight constraint.

    Constrains the weights incident to each hidden unit
    to have a norm less than or equal to a desired value.

    Also available via the shortcut function `tf.keras.constraints.max_norm`.

    Args:
      max_value: the maximum norm value for the incoming weights.
      axis: integer, axis along which to calculate weight norms.
        For instance, in a `Dense` layer the weight matrix
        has shape `(input_dim, output_dim)`,
        set `axis` to `0` to constrain each weight vector
        of length `(input_dim,)`.
        In a `Conv2D` layer with `data_format="channels_last"`,
        the weight tensor has shape
        `(rows, cols, input_depth, output_depth)`,
        set `axis` to `[0, 1, 2]`
        to constrain the weights of each filter tensor of size
        `(rows, cols, input_depth)`.

    """

    def __init__(self, max_value=2, axis=0):
        self.max_value = max_value
        self.axis = axis

    @doc_controls.do_not_generate_docs
    def __call__(self, w):
        norms = backend.sqrt(
            tf.reduce_sum(tf.square(w), axis=self.axis, keepdims=True)
        )
        desired = backend.clip(norms, 0, self.max_value)
        return w * (desired / (backend.epsilon() + norms))

    @doc_controls.do_not_generate_docs
    def get_config(self):
        return {"max_value": self.max_value, "axis": self.axis}


@keras_export("keras.constraints.NonNeg", "keras.constraints.non_neg")
class NonNeg(Constraint):
    """Constrains the weights to be non-negative.

    Also available via the shortcut function `tf.keras.constraints.non_neg`.
    """

    def __call__(self, w):
        return w * tf.cast(tf.greater_equal(w, 0.0), backend.floatx())


@keras_export("keras.constraints.UnitNorm", "keras.constraints.unit_norm")
class UnitNorm(Constraint):
    """Constrains the weights incident to each hidden unit to have unit norm.

    Also available via the shortcut function `tf.keras.constraints.unit_norm`.

    Args:
      axis: integer, axis along which to calculate weight norms.
        For instance, in a `Dense` layer the weight matrix
        has shape `(input_dim, output_dim)`,
        set `axis` to `0` to constrain each weight vector
        of length `(input_dim,)`.
        In a `Conv2D` layer with `data_format="channels_last"`,
        the weight tensor has shape
        `(rows, cols, input_depth, output_depth)`,
        set `axis` to `[0, 1, 2]`
        to constrain the weights of each filter tensor of size
        `(rows, cols, input_depth)`.
    """

    def __init__(self, axis=0):
        self.axis = axis

    @doc_controls.do_not_generate_docs
    def __call__(self, w):
        return w / (
            backend.epsilon()
            + backend.sqrt(
                tf.reduce_sum(tf.square(w), axis=self.axis, keepdims=True)
            )
        )

    @doc_controls.do_not_generate_docs
    def get_config(self):
        return {"axis": self.axis}


@keras_export("keras.constraints.MinMaxNorm", "keras.constraints.min_max_norm")
class MinMaxNorm(Constraint):
    """MinMaxNorm weight constraint.

    Constrains the weights incident to each hidden unit
    to have the norm between a lower bound and an upper bound.

    Also available via the shortcut function
    `tf.keras.constraints.min_max_norm`.

    Args:
      min_value: the minimum norm for the incoming weights.
      max_value: the maximum norm for the incoming weights.
      rate: rate for enforcing the constraint: weights will be
        rescaled to yield
        `(1 - rate) * norm + rate * norm.clip(min_value, max_value)`.
        Effectively, this means that rate=1.0 stands for strict
        enforcement of the constraint, while rate<1.0 means that
        weights will be rescaled at each step to slowly move
        towards a value inside the desired interval.
      axis: integer, axis along which to calculate weight norms.
        For instance, in a `Dense` layer the weight matrix
        has shape `(input_dim, output_dim)`,
        set `axis` to `0` to constrain each weight vector
        of length `(input_dim,)`.
        In a `Conv2D` layer with `data_format="channels_last"`,
        the weight tensor has shape
        `(rows, cols, input_depth, output_depth)`,
        set `axis` to `[0, 1, 2]`
        to constrain the weights of each filter tensor of size
        `(rows, cols, input_depth)`.
    """

    def __init__(self, min_value=0.0, max_value=1.0, rate=1.0, axis=0):
        self.min_value = min_value
        self.max_value = max_value
        self.rate = rate
        self.axis = axis

    @doc_controls.do_not_generate_docs
    def __call__(self, w):
        norms = backend.sqrt(
            tf.reduce_sum(tf.square(w), axis=self.axis, keepdims=True)
        )
        desired = (
            self.rate * backend.clip(norms, self.min_value, self.max_value)
            + (1 - self.rate) * norms
        )
        return w * (desired / (backend.epsilon() + norms))

    @doc_controls.do_not_generate_docs
    def get_config(self):
        return {
            "min_value": self.min_value,
            "max_value": self.max_value,
            "rate": self.rate,
            "axis": self.axis,
        }


@keras_export(
    "keras.constraints.RadialConstraint", "keras.constraints.radial_constraint"
)
class RadialConstraint(Constraint):
    """Constrains `Conv2D` kernel weights to be the same for each radius.

    Also available via the shortcut function
    `tf.keras.constraints.radial_constraint`.

    For example, the desired output for the following 4-by-4 kernel:

    ```
        kernel = [[v_00, v_01, v_02, v_03],
                  [v_10, v_11, v_12, v_13],
                  [v_20, v_21, v_22, v_23],
                  [v_30, v_31, v_32, v_33]]
    ```

    is this::

    ```
        kernel = [[v_11, v_11, v_11, v_11],
                  [v_11, v_33, v_33, v_11],
                  [v_11, v_33, v_33, v_11],
                  [v_11, v_11, v_11, v_11]]
    ```

    This constraint can be applied to any `Conv2D` layer version, including
    `Conv2DTranspose` and `SeparableConv2D`, and with either `"channels_last"`
    or `"channels_first"` data format. The method assumes the weight tensor is
    of shape `(rows, cols, input_depth, output_depth)`.
    """

    @doc_controls.do_not_generate_docs
    def __call__(self, w):
        w_shape = w.shape
        if w_shape.rank is None or w_shape.rank != 4:
            raise ValueError(
                "The weight tensor must have rank 4. "
                f"Received weight tensor with shape: {w_shape}"
            )

        height, width, channels, kernels = w_shape
        w = backend.reshape(w, (height, width, channels * kernels))
        # TODO(cpeter): Switch map_fn for a faster tf.vectorized_map once
        # backend.switch is supported.
        w = backend.map_fn(
            self._kernel_constraint,
            backend.stack(tf.unstack(w, axis=-1), axis=0),
        )
        return backend.reshape(
            backend.stack(tf.unstack(w, axis=0), axis=-1),
            (height, width, channels, kernels),
        )

    def _kernel_constraint(self, kernel):
        """Radially constraints a kernel with shape (height, width,
        channels)."""
        padding = backend.constant([[1, 1], [1, 1]], dtype="int32")

        kernel_shape = backend.shape(kernel)[0]
        start = backend.cast(kernel_shape / 2, "int32")

        kernel_new = backend.switch(
            backend.cast(tf.math.floormod(kernel_shape, 2), "bool"),
            lambda: kernel[start - 1 : start, start - 1 : start],
            lambda: kernel[start - 1 : start, start - 1 : start]
            + backend.zeros((2, 2), dtype=kernel.dtype),
        )
        index = backend.switch(
            backend.cast(tf.math.floormod(kernel_shape, 2), "bool"),
            lambda: backend.constant(0, dtype="int32"),
            lambda: backend.constant(1, dtype="int32"),
        )
        while_condition = lambda index, *args: backend.less(index, start)

        def body_fn(i, array):
            return i + 1, tf.pad(
                array, padding, constant_values=kernel[start + i, start + i]
            )

        _, kernel_new = tf.compat.v1.while_loop(
            while_condition,
            body_fn,
            [index, kernel_new],
            shape_invariants=[index.get_shape(), tf.TensorShape([None, None])],
        )
        return kernel_new


# Aliases.

max_norm = MaxNorm
non_neg = NonNeg
unit_norm = UnitNorm
min_max_norm = MinMaxNorm
radial_constraint = RadialConstraint

# Legacy aliases.
maxnorm = max_norm
nonneg = non_neg
unitnorm = unit_norm


@keras_export("keras.constraints.serialize")
def serialize(constraint):
    return serialize_keras_object(constraint)


@keras_export("keras.constraints.deserialize")
def deserialize(config, custom_objects=None):
    return deserialize_keras_object(
        config,
        module_objects=globals(),
        custom_objects=custom_objects,
        printable_module_name="constraint",
    )


@keras_export("keras.constraints.get")
def get(identifier):
    """Retrieves a Keras constraint function."""
    if identifier is None:
        return None
    if isinstance(identifier, dict):
        return deserialize(identifier)
    elif isinstance(identifier, str):
        config = {"class_name": str(identifier), "config": {}}
        return deserialize(config)
    elif callable(identifier):
        return identifier
    else:
        raise ValueError(
            f"Could not interpret constraint function identifier: {identifier}"
        )