#define _USE_MATH_DEFINES

#include <cmath>

#include <matplotlibcpp.h>

#include <chrono>

#include <xtensor/xbuilder.hpp>

#include <xtensor/xtensor.hpp>

namespace plt = matplotlibcpp;

template<class E1, class E2>

void update_window(const double x, const double y, const double t,

E1& xt, E2& yt)

{

const double target_length = 300;

const double half_win = (target_length/(2.*sqrt(1.+t*t)));

xt[0] = x - half_win;

xt[1] = x + half_win;

yt[0] = y - half_win*t;

yt[1] = y + half_win*t;

}

int main()

{

size_t n = 1000;

const double w = 0.05;

const double a = n/2;

xt::xtensor<double, 1> x = xt::linspace<double>(0, n-1, n);

xt::xtensor<double, 1> y = a * xt::sin(w * x);

xt::xtensor<double, 1> xt{0,0};

xt::xtensor<double, 1> yt{0,0};

plt::title("Tangent of a sine curve");

// Plot sin once and for all.

plt::named_plot("sin", x, y);

plt::xlim(x[0], x[n-1]);

plt::ylim(-a, a);

plt::axis("equal");

// Prepare plotting the tangent.

plt::Plot plot("tangent");

plt::legend();

for (size_t i=0; i<n; i++) {

if (i % 10 == 0) {

update_window(x[i], y[i], a*w*cos(w*x[i]), xt, yt);

// Just update data for this plot.

plot.update(xt, yt);

// Small pause so the viewer has a chance to enjoy the animation.

plt::pause(0.1);

}

}

}