/**
 * jQuery.crSpline v0.0.1
 * http://github.com/MmmCurry/jquery.crSpline
 *
 * Supports animation along Catmull-Rom splines based on a series of waypoints.
 * Usage: See demo.js, demo.html
 * 
 * Copyright 2010, M. Ian Graham
 * MIT License
 *
 */

(function($){

        $.crSpline = {};

        // Catmull-Rom interpolation between p0 and p1 for previous point p_1 and later point p2
        // http://en.wikipedia.org/wiki/Cubic_Hermite_spline#Catmull.E2.80.93Rom_spline
        var interpolate = function (t, p_1, p0, p1, p2) {
                return Math.floor((t * ((2 - t) * t - 1) * p_1 +
                        (t * t * (3 * t - 5) + 2) * p0 +
                        t * ((4 - 3 * t) * t + 1) * p1 +
                        (t - 1) * t * t * p2
                        ) / 2);
        };

        // Extend this p1,p2 sequence linearly to a new p3
        var generateExtension = function (p1, p2) {
                return [
                        p2[0] + (p2[0] - p1[0]),
                        p2[1] + (p2[1] - p1[1])
                        ];

        };

        // Return an animation object based on a sequence of points
        // pointList must be an array of [x,y] pairs
        $.crSpline.buildSequence = function(pointList) {
                var res = {};
                var seq = [];
                var numSegments;

                if (pointList.length < 2) {
                        throw "crSpline.buildSequence requires at least two points";
                }

                // Generate the first p_1 so the caller doesn't need to provide it
                seq.push(generateExtension(pointList[1], pointList[0]));

                // Throw provided points on the list
                for (var i = 0; i < pointList.length; i++) {
                        seq.push(pointList[i]);
                }

                // Generate the last p2 so the caller doesn't need to provide it
                seq.push(generateExtension(seq[seq.length-2], seq[seq.length-1]));

                numSegments = seq.length - 3;

                res.getPos = function (t) {
                        // XXX For now, assume all segments take equal time
                        var segNum = Math.floor(t * numSegments);
                        if (segNum === numSegments) {
                                return {
                                        left: seq[seq.length-2][0],
                                        top: seq[seq.length-2][1]
                                        };
                        }
                        var microT = (t - segNum/numSegments) * numSegments;
                        var result = {
                                left: interpolate(microT,
                                                seq[segNum][0],
                                                seq[segNum+1][0],
                                                seq[segNum+2][0],
                                                seq[segNum+3][0]) + "px",
                                top: interpolate(microT,
                                                seq[segNum][1],
                                                seq[segNum+1][1],
                                                seq[segNum+2][1],
                                                seq[segNum+3][1]) + "px"
                                };
                        return result;
                };
                return res;
        };

        $.fx.step.crSpline = function (fx) {
                var css = fx.end.getPos(fx.pos);
                for (var i in css) {
                        fx.elem.style[i] = css[i];
                }
        };

})(jQuery);