#
# BP demo code
# 
# reference: 
#   https://www.2cto.com/kf/201612/543750.html
#

import numpy as np
from sklearn import datasets, linear_model
import matplotlib.pyplot as plt

 
class Config:
    nn_input_dim = 2
    nn_output_dim = 2 
    epsilon = 0.01 
    reg_lambda = 0.01 
 
 
def generate_data():
    np.random.seed(0)
    X, y = datasets.make_moons(200, noise=0.20)
    return X, y
 
 
def visualize(X, y, model):
    plot_decision_boundary(lambda x:predict(model,x), X, y)
    plt.title("Logistic Regression")
 
 
def plot_decision_boundary(pred_func, X, y):
    x_min, x_max = X[:, 0].min() - .5, X[:, 0].max() + .5
    y_min, y_max = X[:, 1].min() - .5, X[:, 1].max() + .5
    h = 0.01
    xx, yy = np.meshgrid(np.arange(x_min, x_max, h), np.arange(y_min, y_max, h))
    Z = pred_func(np.c_[xx.ravel(), yy.ravel()])
    Z = Z.reshape(xx.shape)
    plt.contourf(xx, yy, Z, cmap=plt.cm.Spectral)
    plt.scatter(X[:, 0], X[:, 1], c=y, cmap=plt.cm.Spectral)
    plt.show()
 
def predict(model, x):
    W1, b1, W2, b2 = model['W1'], model['b1'], model['W2'], model['b2']
    z1 = x.dot(W1) + b1
    a1 = np.tanh(z1)
    z2 = a1.dot(W2) + b2
    exp_scores = np.exp(z2)
    probs = exp_scores / np.sum(exp_scores, axis=1, keepdims=True)
    return np.argmax(probs, axis=1)
 
 
def build_model(X, y, nn_hdim, num_passes=20000, print_loss=False):
    num_examples = len(X)
    np.random.seed(0)
    W1 = np.random.randn(Config.nn_input_dim, nn_hdim) / np.sqrt(Config.nn_input_dim)
    b1 = np.zeros((1, nn_hdim))
    W2 = np.random.randn(nn_hdim, Config.nn_output_dim) / np.sqrt(nn_hdim)
    b2 = np.zeros((1, Config.nn_output_dim))
 
    model = {}
 
    for i in range(0, num_passes):
 
        z1 = X.dot(W1) + b1
        a1 = np.tanh(z1)
        z2 = a1.dot(W2) + b2
        exp_scores = np.exp(z2)
        probs = exp_scores / np.sum(exp_scores, axis=1, keepdims=True)
 
        delta3 = probs
        delta3[range(num_examples), y] -= 1
        dW2 = (a1.T).dot(delta3)
        db2 = np.sum(delta3, axis=0, keepdims=True)
        delta2 = delta3.dot(W2.T) * (1 - np.power(a1, 2))
        dW1 = np.dot(X.T, delta2)
        db1 = np.sum(delta2, axis=0)
 
        dW2 += Config.reg_lambda * W2
        dW1 += Config.reg_lambda * W1
 
        W1 += -Config.epsilon * dW1
        b1 += -Config.epsilon * db1
        W2 += -Config.epsilon * dW2
        b2 += -Config.epsilon * db2
 
        model = {'W1': W1, 'b1': b1, 'W2': W2, 'b2': b2}
 
    return model
 
 
def main():
    X, y = generate_data()
    model = build_model(X, y, 3)
    visualize(X, y, model)
 
 
if __name__ == "__main__":
    main()