package {
    import flash.display.Graphics;
    import flash.display.Sprite;
    import flash.events.Event;
    import flash.filters.*;

[SWF(width="400", height="400", backgroundColor="0x000000")]
    public class BoidDemo extends Sprite {
        private const NUMBOIDS:int = 70;
        private var boids:Array = new Array();
        private var sprites:Array = new Array();
        
        public function BoidDemo() {
            addEventListener(Event.ENTER_FRAME, onEnterFrame);
            var i:int;
            
            for (i = 0; i < NUMBOIDS; i++) {
                var b:Boid = new Boid();
                b.px = 400 * Math.random();
                b.py = 400 * Math.random();
                b.vx = 30 * (Math.random() - 0.5);
                b.vy = 30 * (Math.random() - 0.5);
                boids[i] = b;
                
                sprites[i] = new Sprite();
                var g:Graphics = sprites[i].graphics;
                g.lineStyle(1, 0xffffff);
                g.moveTo(4, 0); g.lineTo(-3, -3);
                g.lineTo(-3, 3); g.lineTo(4, 0); g.lineTo(-8, 0);
                sprites[i].x = b.px;
                sprites[i].y = b.py;
            }
            
            for (i = 0; i < NUMBOIDS; i++)
                this.addChild(sprites[i]);

//this.filters = [new GlowFilter(0xff5555, 1.0, 4, 4)];
        }
        
        public function onEnterFrame(ev:Event) : void { frame(); }
        
        public function frame() : void {
            for (var i:int = 0; i < NUMBOIDS; i++) {
                var b:Boid = boids[i];
                b.force(boids);
                b.update();
                sprites[i].x = b.px;
                sprites[i].y = b.py;
                sprites[i].rotation = Math.atan2(b.vy, b.vx) * 180 / Math.PI;
            }
        }
    }
}

class Boid {
    public var px:Number, py:Number;
    public var vx:Number, vy:Number;
    private var ax:Number, ay:Number;
    public function Boid() {}
    
    public function force(boids:Array) : void {
        // find the nearlest neighbor
        var nearlest:Boid = null;
        var dx:Number, dy:Number;
        var dist2:Number;
        var mindist2:Number = Number.MAX_VALUE;
        var i:String;
        var b:Boid;
        var count:int = 0;
        var cx:Number = 0cy:Number = 0;
        for (i in boids) {
            b = boids[i];
            if (b == this)
                continue;
            dx = b.px - px; dy = b.py - py;
            dist2 = dx * dx + dy * dy;
            if (dist2 < mindist2) {
                mindist2 = dist2;
                nearlest = b;
            }
            if (dist2 < 1500) {
                cx += b.px; cy += b.py;
                count++;
            }
        }
        if (nearlest == null)
            return;
            
        ax = ay = 0;
        var npx:Number = nearlest.px;
        var npy:Number = nearlest.py;
        var nvx:Number = nearlest.vx;
        var nvy:Number = nearlest.vy;            
        dx = (npx - px); dy = (npy - py);
        dist2 = dx * dx + dy * dy;
        if (dist2 > 1500)
            return;
        
        // Separation
        var dist:Number = Math.sqrt(dist2);
        ax += dx / dist * (dist - 25) * 0.5;
        ay += dy / dist * (dist - 25) * 0.5;
            
        // Alignment
        ax += (nvx - vx) * 0.2; ay += (nvy - vy) * 0.2;
        
        // Cohesion
        dx = (cx/count - px); dy = (cy/count - py);
        ax += dx * 0.1; ay += dy * 0.1;
        ax += 10 * (Math.random() - 0.5); ay += 10 * (Math.random() - 0.5);

// boundary
        if (px < 50)       ax += (50 - px) * 0.05;
        else if (px > 350) ax += (350 - px) * 0.05;
        if (py < 50)       ay += (50 - py) * 0.05;
        else if (py > 350) ay += (350 - py) * 0.05;
    }
    
    public function update() : void
    {
        px += vx * (1.0/12); py += vy * (1.0/12);
        vx += ax * (1.0/12); vy += ay * (1.0/12);
        
        // speed limit
        var v:Number = Math.sqrt(vx * vx + vy * vy);
        if (v > 17) {
            vx = vx / v * 17; vy = vy / v * 17;
        } else if (v < 8) {
            vx = vx / v * 8; vy = vy / v * 8;
        }
    }
}