# Sebastian Raschka 2014-2020
# mlxtend Machine Learning Library Extensions
#
# Implementation of the mulitnomial logistic regression algorithm for
# classification.

# Author: Sebastian Raschka <sebastianraschka.com>
#
# License: BSD 3 clause

import numpy as np
from time import time
from .._base import _BaseModel
from .._base import _IterativeModel
from .._base import _MultiClass
from .._base import _Classifier


class SoftmaxRegression(_BaseModel, _IterativeModel,
                        _Classifier,  _MultiClass):

    """Softmax regression classifier.

    Parameters
    ------------
    eta : float (default: 0.01)
        Learning rate (between 0.0 and 1.0)
    epochs : int (default: 50)
        Passes over the training dataset.
        Prior to each epoch, the dataset is shuffled
        if `minibatches > 1` to prevent cycles in stochastic gradient descent.
    l2 : float
        Regularization parameter for L2 regularization.
        No regularization if l2=0.0.
    minibatches : int (default: 1)
        The number of minibatches for gradient-based optimization.
        If 1: Gradient Descent learning
        If len(y): Stochastic Gradient Descent (SGD) online learning
        If 1 < minibatches < len(y): SGD Minibatch learning
    n_classes : int (default: None)
        A positive integer to declare the number of class labels
        if not all class labels are present in a partial training set.
        Gets the number of class labels automatically if None.
    random_seed : int (default: None)
        Set random state for shuffling and initializing the weights.
    print_progress : int (default: 0)
        Prints progress in fitting to stderr.
        0: No output
        1: Epochs elapsed and cost
        2: 1 plus time elapsed
        3: 2 plus estimated time until completion

    Attributes
    -----------
    w_ : 2d-array, shape={n_features, 1}
      Model weights after fitting.
    b_ : 1d-array, shape={1,}
      Bias unit after fitting.
    cost_ : list
        List of floats, the average cross_entropy for each epoch.

    Examples
    -----------
    For usage examples, please see
    http://rasbt.github.io/mlxtend/user_guide/classifier/SoftmaxRegression/

    """
    def __init__(self, eta=0.01, epochs=50,
                 l2=0.0,
                 minibatches=1,
                 n_classes=None,
                 random_seed=None,
                 print_progress=0):

        _BaseModel.__init__(self)
        _IterativeModel.__init__(self)
        _Classifier.__init__(self)
        _MultiClass.__init__(self)

        self.eta = eta
        self.epochs = epochs
        self.l2 = l2
        self.minibatches = minibatches
        self.n_classes = n_classes
        self.random_seed = random_seed
        self.print_progress = print_progress
        self._is_fitted = False

    def _net_input(self, X, W, b):
        return (X.dot(W) + b)

    def _softmax(self, z):
        e_x = np.exp(z - z.max(axis=1, keepdims=True))
        out = e_x / e_x.sum(axis=1, keepdims=True)
        return out
        # return (np.exp(z.T) / np.sum(np.exp(z), axis=1)).T

    def _cross_entropy(self, output, y_target):
        return - np.sum(np.log(output) * (y_target), axis=1)

    def _cost(self, cross_entropy):
        L2_term = self.l2 * np.sum(self.w_ ** 2)
        cross_entropy = cross_entropy + L2_term
        return 0.5 * np.mean(cross_entropy)

    def _to_classlabels(self, z):
        return z.argmax(axis=1)

    def _fit(self, X, y, init_params=True):
        self._check_target_array(y)
        if init_params:
            if self.n_classes is None:
                self.n_classes = np.max(y) + 1
            self._n_features = X.shape[1]

            self.b_, self.w_ = self._init_params(
                weights_shape=(self._n_features, self.n_classes),
                bias_shape=(self.n_classes,),
                random_seed=self.random_seed)
            self.cost_ = []

        y_enc = self._one_hot(y=y, n_labels=self.n_classes, dtype=np.float)

        self.init_time_ = time()
        rgen = np.random.RandomState(self.random_seed)
        for i in range(self.epochs):
            for idx in self._yield_minibatches_idx(
                    rgen=rgen,
                    n_batches=self.minibatches,
                    data_ary=y,
                    shuffle=True):

                # givens:
                # w_ -> n_feat x n_classes
                # b_  -> n_classes

                # net_input, softmax and diff -> n_samples x n_classes:
                net = self._net_input(X[idx], self.w_, self.b_)
                softm = self._softmax(net)
                diff = softm - y_enc[idx]

                # gradient -> n_features x n_classes
                grad = np.dot(X[idx].T, diff)

                # update in opp. direction of the cost gradient
                self.w_ -= (self.eta * grad +
                            self.eta * self.l2 * self.w_)
                self.b_ -= (self.eta * np.sum(diff, axis=0))

            # compute cost of the whole epoch
            net = self._net_input(X, self.w_, self.b_)
            softm = self._softmax(net)
            cross_ent = self._cross_entropy(output=softm, y_target=y_enc)
            cost = self._cost(cross_ent)
            self.cost_.append(cost)

            if self.print_progress:
                self._print_progress(iteration=i + 1,
                                     n_iter=self.epochs,
                                     cost=cost)

        return self

    def predict_proba(self, X):
        """Predict class probabilities of X from the net input.

        Parameters
        ----------
        X : {array-like, sparse matrix}, shape = [n_samples, n_features]
            Training vectors, where n_samples is the number of samples and
            n_features is the number of features.

        Returns
        ----------
        Class probabilties : array-like, shape= [n_samples, n_classes]

        """
        net = self._net_input(X, self.w_, self.b_)
        softm = self._softmax(net)
        return softm

    def _predict(self, X):
        probas = self.predict_proba(X)
        return self._to_classlabels(probas)