The code below is my attempt at triangulation. It outputs the wrong angles (it read a square's angles as 90, 90. 90, 176) and draws the wrong shapes. What am I doing wrong?

//use earclipping to generate a list of triangles to draw

std::vector<vec> calcTriDraw(std::vector<vec> poly)
{
    std::vector<double> polyAngles;
    //get angles
    for(unsigned int i = 0;i < poly.size();i++)
    {

        int p1 = i - 1;
        int p2 = i;
        int p3 = i + 1;

        if(p3 > int(poly.size()))
            p3 -= poly.size();
        if(p1 < 0)
            p1 += poly.size();

        //get the angle from 3 points
        double dx, dy;
        dx = poly[p2].x - poly[p1].x;
        dy = poly[p2].y - poly[p1].y;
        double a = atan2(dy,dx);

        dx = poly[p3].x - poly[p2].x;
        dy = poly[p3].y - poly[p2].y;
        double b = atan2(dy,dx);

        polyAngles.push_back((a-b)*180/PI);
    }
    std::vector<vec> triList;
    for(unsigned int i = 0;i < poly.size() && poly.size() > 2;i++)
    {
        int p1 = i - 1;
        int p2 = i;
        int p3 = i + 1;

        if(p3 > int(poly.size()))
            p3 -= poly.size();
        if(p1 < 0)
            p1 += poly.size();

        if(polyAngles[p2] >= 180)
        {
            continue;
        }
        else
        {
            triList.push_back(poly[p1]);
            triList.push_back(poly[p2]);
            triList.push_back(poly[p3]);
            poly.erase(poly.begin()+p2);
            std::vector<vec> add = calcTriDraw(poly);
            triList.insert(triList.end(), add.begin(), add.end());
            break;
        }
    }

    return triList;
}