由平面圆弧上两点坐标及圆半径，计算圆心坐标

[Sogrey]

function Point(x, y) {
    this.x = x;
    this.y = y;
    if ((typeof (x) == "undefined") || (typeof (y) == "undefined")) {
        this.x = 0;
        this.y = 0;
    }

    this.distance = function (otherPoint) {
        var self = this;
        if (typeof (otherPoint) == "undefined") {
            return; // The return obejct is undefined.
        }
        if ((typeof (self.x) == "undefined") || (typeof (self.y) == "undefined")) {
            return;
        }
        if ((typeof (otherPoint.x) == "undefined") || (typeof (otherPoint.y) == "undefined")) {
            return;
        }
        var dx = parseFloat(self.x) - parseFloat(otherPoint.x);
        var dy = parseFloat(self.y) - parseFloat(otherPoint.y);
        return (Math.sqrt(parseFloat(dx) * parseFloat(dx) + parseFloat(dy) * parseFloat(dy)));
    }

    this.middle = function (otherPoint) {
        var self = this;
        if (typeof (otherPoint) == "undefined") {
            return; // The return obejct is undefined.
        }
        if ((typeof (self.x) == "undefined") || (typeof (self.y) == "undefined")) {
            return;
        }
        if ((typeof (otherPoint.x) == "undefined") || (typeof (otherPoint.y) == "undefined")) {
            return;
        }
        var thex = (parseFloat(self.x) + parseFloat(otherPoint.x)) / 2.0;
        var they = (parseFloat(self.y) + parseFloat(otherPoint.y)) / 2.0;
        return (new Point(thex, they));
    }

    this.vector = function (otherPoint) {
        var self = this;
        if (typeof (otherPoint) == "undefined") {
            return; // The return obejct is undefined.
        }
        if ((typeof (self.x) == "undefined") || (typeof (self.y) == "undefined")) {
            return;
        }
        if ((typeof (otherPoint.x) == "undefined") || (typeof (otherPoint.y) == "undefined")) {
            return;
        }
        var dx = parseFloat(otherPoint.x) - parseFloat(self.x);
        var dy = parseFloat(otherPoint.y) - parseFloat(self.y);
        return (new Point(dx, dy));
    }

    this.moveto = function (theVector) {
        var self = this;
        if (typeof (theVector) == "undefined") {
            return; // The return obejct is undefined.
        }
        if ((typeof (self.x) == "undefined") || (typeof (self.y) == "undefined")) {
            return;
        }
        if ((typeof (theVector.x) == "undefined") || (typeof (theVector.y) == "undefined")) {
            return;
        }
        var thex = parseFloat(theVector.x) + parseFloat(self.x);
        var they = parseFloat(theVector.y) + parseFloat(self.y);
        return (new Point(thex, they));
    }
}

function calCircleCenter(p1, p2, r) {
    // Checking parameters
    if (typeof (p1) == "undefined") {
        console.error("Parameter point 1 is undefined");
        return;
    }
    if (typeof (p2) == "undefined") {
        console.error("Parameter point 2 is undefined");
        return;
    }
    if (typeof (r) == "undefined") {
        console.error("Parameter radius is undefined");
        return;
    }

    if (parseFloat(r) < 0) {
        console.error("Parameter radius is less than zero");
        return;
    }

    var distance = p1.distance(p2);
    var half_dis = parseFloat(distance) / 2.0;
    if (parseFloat(r) < half_dis) {
        console.error("Parameter radius is too samll, please increass radius.");
        return;
    }
    if (parseFloat(half_dis) < 0.0001) {
        console.error("Distance between two point is too samll, please increase their distance.");
        return;
    }

    var v = p1.vector(p2);
    if (typeof (v) == "undefined") {
        console.error("Vector p1p2 is undefined");
        return;
    }

    var middle_point = p1.middle(p2);

    // Roating vector p1p2 90 degrees clockwise
    var new_v_x = v.y;
    var new_v_y = -v.x;

    // Unifing the new vector
    var origin = new Point(0.0, 0.0);
    var new_v = new Point(new_v_x, new_v_y);
    var l = origin.distance(new_v)
    var uni_v_x = parseFloat(new_v_x) / parseFloat(l);
    var uni_v_y = parseFloat(new_v_y) / parseFloat(l);

    var move_length = Math.sqrt(parseFloat(r) * parseFloat(r) - parseFloat(half_dis) * parseFloat(half_dis));
    var move_vector = new Point(parseFloat(uni_v_x) * parseFloat(move_length), parseFloat(uni_v_y) * parseFloat(move_length));

    var r_1 = middle_point.moveto(move_vector);

    // Roating vector p1p2 90 degrees anti-clockwise
    move_vector = new Point(-parseFloat(uni_v_x) * parseFloat(move_length), -parseFloat(uni_v_y) * parseFloat(move_length));

    var r_2 = middle_point.moveto(move_vector);

    return ([r_1, r_2]);
}

// Testing function
function test() {
    var p = new Point(0,-3);
    var p1 = new Point(0,3);
    var result = calCircleCenter(p, p1, 5)
    console.log('result = ', result);
}

test();