Area of Intersection of Circles; given by coordinates and diameters

IntersectionOfCircles.java

/**
 * calculates area of intersections of circles
 * 
 * @param x1,y1,r1 first circle
 * @param x2,y2,r2 second circle
 */
public double solution(int x1, int y1, int r1, int x2, int y2, int r2) {
    double sR1 = power(r1, 2);
    double sR2 = power(r2, 2);
    double d = distance(x1, y1, x2, y2);

    if (r1 == 0 || r2 == 0) {
        return 0.0D;
    } else if (d > r1 + r2) {
        return 0.0D;
    } else if (d == 0) {
        if (r1 >= r2) {
            return Math.PI * sR1;
        } else {
            return Math.PI * sR2;
        }
    } else if (d <= Math.abs(r1 - r2) && r1 >= r2) {
        return Math.PI * sR1;
    } else if (d <= Math.abs(r1 - r2) && r1 < r2) {
        return Math.PI * sR2;
    } else {
        double phi = (Math.acos((sR1 + (d * d) - sR2) / (2 * r1 * d))) * 2;
        double theta = (Math.acos((sR2 + (d * d) - sR1) / (2 * r2 * d))) * 2;
        double area1 = 0.5 * theta * sR2 - 0.5 * sR2 * Math.sin(theta);
        double area2 = 0.5 * phi * sR1 - 0.5 * sR1 * Math.sin(phi);

        return area1 + area2;
    }
}

private double power(int base, int power) {
    if (power <= 0) {
        return 1;
    } else if (power == 1) {
        return base;
    } else {
        return base * power(base, power - 1);
    }
}

private double distance(int x1, int y1, int x2, int y2) {
    long xD = x1 - x2;
    long yD = y1 - y2;
    return Math.sqrt(xD * xD + yD * yD);
}