(function() {

if (typeof geo == 'undefined') {
    geo = {};
}

var tolerance = 1e-10;

function eq(a, b) {
    return (Math.abs(a - b) < tolerance);
}

function gt(a, b) {
    return (a - b > -tolerance);
}

function lt(a, b) {
    return gt(b, a);
}

geo.eq = eq;
geo.gt = gt;
geo.lt = lt;

geo.LineSegment = function(x1, y1, x2, y2) {
    this.x1 = x1;
    this.y1 = y1;
    this.x2 = x2;
    this.y2 = y2;

    // Ax + By = C
    this.a = y2 - y1;
    this.b = x1 - x2;
    this.c = x1 * this.a + y1 * this.b;

    if (eq(this.a, 0) && eq(this.b, 0)) {
        throw new Error(
            'Cannot construct a LineSegment with two equal endpoints.');
    }
};

geo.LineSegment.prototype.intersect = function(that) {
    var d = (this.x1 - this.x2) * (that.y1 - that.y2) -
            (this.y1 - this.y2) * (that.x1 - that.x2);

    if (eq(d, 0)) {
        // The two lines are parallel or very close.
        return {
            x : NaN,
            y : NaN
        };
    }

    var t1  = this.x1 * this.y2 - this.y1 * this.x2,
        t2  = that.x1 * that.y2 - that.y1 * that.x2,
        x   = (t1 * (that.x1 - that.x2) - t2 * (this.x1 - this.x2)) / d,
        y   = (t1 * (that.y1 - that.y2) - t2 * (this.y1 - this.y2)) / d,
        in1 = (gt(x, Math.min(this.x1, this.x2)) && lt(x, Math.max(this.x1, this.x2)) &&
               gt(y, Math.min(this.y1, this.y2)) && lt(y, Math.max(this.y1, this.y2))),
        in2 = (gt(x, Math.min(that.x1, that.x2)) && lt(x, Math.max(that.x1, that.x2)) &&
               gt(y, Math.min(that.y1, that.y2)) && lt(y, Math.max(that.y1, that.y2)));

    return {
        x   : x,
        y   : y,
        in1 : in1,
        in2 : in2
    };
};

geo.LineSegment.prototype.x = function(y) {
    // x = (C - By) / a;
    if (this.a) {
        return (this.c - this.b * y) / this.a;
    } else {
        // a == 0 -> horizontal line
        return NaN;
    }
};

geo.LineSegment.prototype.y = function(x) {
    // y = (C - Ax) / b;
    if (this.b) {
        return (this.c - this.a * x) / this.b;
    } else {
        // b == 0 -> vertical line
        return NaN;
    }
};

geo.LineSegment.prototype.length = function() {
    return Math.sqrt(
        (this.y2 - this.y1) * (this.y2 - this.y1) +
        (this.x2 - this.x1) * (this.x2 - this.x1));
};

geo.LineSegment.prototype.offset = function(x, y) {
    return new geo.LineSegment(
        this.x1 + x, this.y1 + y,
        this.x2 + x, this.y2 + y);
};

})();