segment.js

window.int_x = 0;
window.int_y = 0;

var DONT_INTERSECT = 0;
var COLLINEAR = 1;
var DO_INTERSECT = 2;

function same_sign(a, b){ return ( ( a * b ) >= 0 ); }

var Segment = function(x, y, vecx, vecy)
{
    this.x = x;
    this.y = y;
    this.vecx = vecx;
    this.vecy = vecy;

//	var that = this;

    this.draw = function(width, color)
    {
        gfx.beginPath();
        gfx.lineWidth = width;
        gfx.moveTo(this.x, this.y);
        gfx.lineTo(this.x + this.vecx, this.y + this.vecy);
        gfx.strokeStyle = color;
        gfx.stroke();
    }

    this.length = function() {
        var dx = this.vecx;
        var dy = this.vecy;
        return Math.sqrt(dx * dx + dy * dy);
    }

    this.normal = function() {
        var x1 = this.y;
        var y1 = this.x + this.vecx;
        var y2 = this.x;
        var x2 = this.y + this.vecy;
        return new Segment(x1, y1, x2-x1, y2-y1);
    }

    this.center = function() {
        var _x = this.x + this.x + this.vecx;
        var _y = this.y + this.y + this.vecy;
        _x /= 2;
        _y /= 2;
        return new Point(_x, _y);
    }

    this.unit = function() {
        return new Segment(0, 0, this.vecx / this.length(), this.vecy / this.length());
    }

    this.multiply = function(multiplier)
    {
        return new Segment(0, 0, this.vecx*multiplier, this.vecy*multiplier);
    }

    this.project = function( seg_onto )
    {
        var vec = new Vector(this.vecx, this.vecy);
        var onto = new Vector(seg_onto.vecx, seg_onto.vecy);
        var d = onto.dot(onto);
        if (0 < d) {
            var dp = vec.dot(onto);
            var multiplier = dp / d;
            var rx = onto.x * multiplier;
            var ry = onto.y * multiplier;
            return new Point(rx, ry);
        }
        return new Point(0, 0);
    }

    this.intersect = function( segment )
    {
        // a
        var x1 = this.x;
        var y1 = this.y;
        var x2 = this.x + this.vecx;
        var y2 = this.y + this.vecy;

        // b
        var x3 = segment.x;
        var y3 = segment.y;
        var x4 = segment.x + segment.vecx;
        var y4 = segment.y + segment.vecy;

        var a1, a2, b1, b2, c1, c2;
        var r1, r2 , r3, r4;
        var denom, offset, num;

        a1 = y2 - y1;
        b1 = x1 - x2;
        c1 = (x2 * y1) - (x1 * y2);

        r3 = ((a1 * x3) + (b1 * y3) + c1);
        r4 = ((a1 * x4) + (b1 * y4) + c1);

        if ((r3 != 0) && (r4 != 0) && same_sign(r3, r4))
            return DONT_INTERSECT;

        a2 = y4 - y3; // Compute a2, b2, c2
        b2 = x3 - x4;
        c2 = (x4 * y3) - (x3 * y4);
        r1 = (a2 * x1) + (b2 * y1) + c2; // Compute r1 and r2
        r2 = (a2 * x2) + (b2 * y2) + c2;

        if ((r1 != 0) && (r2 != 0) && (same_sign(r1, r2)))
            return DONT_INTERSECT;

        denom = (a1 * b2) - (a2 * b1); //Line segments intersect: compute intersection point.

        if (denom == 0)
            return COLLINEAR;

        if (denom < 0) offset = -denom / 2; else offset = denom / 2;

        num = (b1 * c2) - (b2 * c1);
        if (num < 0) window.int_x = (num - offset) / denom; else window.int_x = (num + offset) / denom;

        num = (a2 * c1) - (a1 * c2);
        if (num < 0)  window.int_y = ( num - offset) / denom; else window.int_y = (num + offset) / denom;

        return DO_INTERSECT;
    }
}

function vec2ang(x,y)
{
    angleInRadians = Math.atan2(y, x);
    angleInDegrees = (angleInRadians / Math.PI) * 180.0;
    return angleInDegrees;
}

function ang2vec(angle)
{
    var radians = angle * (Math.PI / 180.0);
    var x = Math.cos(radians);
    var y = Math.sin(radians);

    var a = new Segment(0, 0, x, y);
    var u = a.normal().unit();

    return [u.vecx, u.vecy];
}