#include <iostream>
#include <cstdlib>
#include <cmath>

struct Vec2
{
  double x = 0.;
  double y = 0.;
};

  double cos_(double rw)
  {
    return(cos((rw*M_PI) / 180.));
  }

  double sin_(double rw)
  {
    return(sin((rw*M_PI) / 180.));
  }

double atan1_(double y, double x)
{
  double rw, eps = 0.0001;
  if ((x > -eps) && (x < eps)) {
    if ((y > -eps) && (y < eps)) rw = 0.;
    else if (y > 0.) rw = 90.;
    else if (y < 0.) rw = -90.;
  }
  else {
    rw = atan(y / x)*180. / M_PI;
    if ((y >= 0.) && (x < 0.)) { rw = rw + 180.; }
    else if ((y < 0.) && (x < 0.)) { rw = rw - 180.; }
  }
  return(rw);
}

double AngleX(const Vec2 &aVec) {
    double ret = atan1_(aVec.y, aVec.x);
    if (ret < 0.)
        ret += 360.;
    if (ret > 360.)
        ret -= 360.;
    return ret;
}

double AngleBetween(const Vec2 &aV1, const Vec2 &aV2) {
    double a1 = AngleX(aV1);
    double a2 = AngleX(aV2);
    if (a2 < a1) // ensure non-negative ccw angle
        a2 += 360.;
    double ret = a2 - a1;
    return ret;
}

Vec2 AngleToDir(const double aAngle)
{
  return {cos_(aAngle), sin_(aAngle)};
}

void doAndPrint(double x, double y) {
    std::cout << "AngleX(" << y << "," << x << "): " << AngleX({x,y}) << std::endl;
}

void doAngleBetween(const Vec2 &aV1, const Vec2 &aV2) {
    std::cout << "AngleBetween( (" << aV1.x << ", " << aV1.y << "), (" << aV2.x << ", " << aV2.y << ") = " << AngleBetween(aV1, aV2) << std::endl;
}


void doAngleToDir(const double aAngle) {
    Vec2 a = AngleToDir(aAngle);
    std::cout << "AngleToDir("<<aAngle<<") = ("<< a.x << ", "<<a.y<<")"<<std::endl;
}


int main()
{
    doAngleBetween({0.,-1.}, {-1., 1.});
    doAngleBetween({-1., 1.}, {0.,-1.});

    doAngleBetween({1., -1.}, {1.,1.});
    doAngleBetween({1.,1.}, {1., -1.});

    doAngleToDir(0.);
    doAngleToDir(45.);
    doAngleToDir(90.);
    doAngleToDir(135.);
    doAngleToDir(180.);
    doAngleToDir(225.);
    doAngleToDir(270.);
    doAngleToDir(315.);
    doAngleToDir(360.);
}