2d midpoint displacement

midpointDisplacement

/* http://www.gameprogrammer.com/fractal.html */
// Also, as expected, shifting is waaay faster than division, even in JS :) http://jsperf.com/division-vs-shift unfortunately it also truncates to the mantissa of floating points
// http://jsfiddle.net/digitalicarus/YUDgH/24/
// TODO: abs option, obj args, exception reason
var calcTerrain = function(conf) {
    var height = conf.height,
        swing = conf.swing,
        h = conf.decay || conf.smooth || conf.smoothness,
        i = conf.degree,
        lb = (height-swing < 0) ? conf.lowerBound : null,
        ub = (swing>height) ? conf.upperBound : null;
    
    function CTE(msg) {
        this.name="CalcTerrainException";
        this.message = msg;   
        this.toString = function() {
            return this.name+": "+this.message;
        };        
    };
    
    if(typeof(i) != "number" || i<1)      {
        throw new CTE("degree must be an integer greater than 1");
    }
    if(typeof(swing) != "number" || swing<1)      {
        throw new CTE("swing must be an integer greater than 1");
    }
    if(typeof(height) != "number" || height<0) {
        throw new CTE("height must be an integer greater than 0");
    }
    if(typeof(h) != "number" || h<0 || h>1)      {
        throw new CTE("smoothness must be a floating point between 0 and 1 non-inclusive");
    }
    
    var decay = (Math.pow(2,-h));

    function gen(start,mid,end,swing,i) {       
        // displace midpoint      zero center swing
        //                        0 -> x : -x/2 -> x/2
        mid = mid + Math.random()*swing-(swing/2);
        mid = (lb && mid < lb) ? lb : mid;
        mid = (ub && mid > ub) ? ub : mid;                
        if(i==1) {
            i--;  
            return [mid>>0];                    
        }
        if(i>1) {
            var midleft = (mid+start)/2;
            var midright = (end+mid)/2;
            swing = swing * decay;
            i--;
            /*
            start     startright
            startleft mid
            |         endleft   endright
            |         |         |
            .    .    .    .    .
                 |         |
                 midleft   midright
            */
            return gen(start,midleft,mid,swing,i)
                   .concat([mid>>0])
                   .concat(gen(mid,midright,end,swing,i));
        }
    }
    return gen(height,height,height,swing,i);
};