C++3点座標による角度計算

計算角度ACBは、Cが中心点であり、A B Cの3点の座標がA(x1,y1)，B(x2,y2)，C(x3,y3)である.

#include 
#include 
using namespace std;

/*
get angle ACB, point C is the center point
A(x1,y1)
B(x2,y2)
C(x3,y3)
*/
double get_angle(double x1, double y1, double x2, double y2, double x3, double y3)
{
    double theta = atan2(x1 - x3, y1 - y3) - atan2(x2 - x3, y2 - y3);
	if (theta > M_PI)
		theta -= 2 * M_PI;
	if (theta < -M_PI)
		theta += 2 * M_PI;
 
	theta = abs(theta * 180.0 / M_PI);
	return theta;
}

int main()
{
    double x1 = 0;
	double y1 = 1;
	double x2 = 1;
	double y2 = 0;
	double x3 = -1;
	double y3 = -2;
	double angle1 = get_angle(x3, y3, x1, y1, x2, y2);
	double angle2 = get_angle(x1, y1, x2, y2, x3, y3);
	double angle3 = get_angle(x2, y2, x3, y3, x1, y1);
	cout << angle2 << endl;
	cout << angle3 << endl;
	cout << angle1 << endl;
	
    return 0;
}

参照:stackoverflow 3点間の挟み角を計算する