module.exports = function(environment) {
    var Dimension = require("../tree/dimension"),
        Color = require("../tree/color"),
        Expression = require("../tree/expression"),
        Quoted = require("../tree/quoted"),
        URL = require("../tree/url"),
        functionRegistry = require("./function-registry");

    functionRegistry.add("svg-gradient", function(direction) {

        var stops,
            gradientDirectionSvg,
            gradientType = "linear",
            rectangleDimension = 'x="0" y="0" width="1" height="1"',
            renderEnv = {compress: false},
            returner,
            directionValue = direction.toCSS(renderEnv),
			i, color, position, positionValue, alpha;

        function throwArgumentDescriptor() {
            throw { type: "Argument",
					message: "svg-gradient expects direction, start_color [start_position], [color position,]...," +
							" end_color [end_position] or direction, color list" };
        }

        if (arguments.length == 2) {
            if (arguments[1].value.length < 2) {
                throwArgumentDescriptor();
            }
            stops = arguments[1].value;
        } else if (arguments.length < 3) {
            throwArgumentDescriptor();
        } else {
            stops = Array.prototype.slice.call(arguments, 1);
        }

        switch (directionValue) {
            case "to bottom":
                gradientDirectionSvg = 'x1="0%" y1="0%" x2="0%" y2="100%"';
                break;
            case "to right":
                gradientDirectionSvg = 'x1="0%" y1="0%" x2="100%" y2="0%"';
                break;
            case "to bottom right":
                gradientDirectionSvg = 'x1="0%" y1="0%" x2="100%" y2="100%"';
                break;
            case "to top right":
                gradientDirectionSvg = 'x1="0%" y1="100%" x2="100%" y2="0%"';
                break;
            case "ellipse":
            case "ellipse at center":
                gradientType = "radial";
                gradientDirectionSvg = 'cx="50%" cy="50%" r="75%"';
                rectangleDimension = 'x="-50" y="-50" width="101" height="101"';
                break;
            default:
                throw { type: "Argument", message: "svg-gradient direction must be 'to bottom', 'to right'," +
                    " 'to bottom right', 'to top right' or 'ellipse at center'" };
        }
        returner = '<?xml version="1.0" ?>' +
            '<svg xmlns="http://www.w3.org/2000/svg" version="1.1" width="100%" height="100%" viewBox="0 0 1 1" preserveAspectRatio="none">' +
            '<' + gradientType + 'Gradient id="gradient" gradientUnits="userSpaceOnUse" ' + gradientDirectionSvg + '>';

        for (i = 0; i < stops.length; i+= 1) {
            if (stops[i] instanceof Expression) {
                color = stops[i].value[0];
                position = stops[i].value[1];
            } else {
                color = stops[i];
                position = undefined;
            }

            if (!(color instanceof Color) || (!((i === 0 || i + 1 === stops.length) && position === undefined) && !(position instanceof Dimension))) {
                throwArgumentDescriptor();
            }
            positionValue = position ? position.toCSS(renderEnv) : i === 0 ? "0%" : "100%";
            alpha = color.alpha;
            returner += '<stop offset="' + positionValue + '" stop-color="' + color.toRGB() + '"' + (alpha < 1 ? ' stop-opacity="' + alpha + '"' : '') + '/>';
        }
        returner += '</' + gradientType + 'Gradient>' +
            '<rect ' + rectangleDimension + ' fill="url(#gradient)" /></svg>';

        returner = encodeURIComponent(returner);

        returner = "data:image/svg+xml," + returner;
        return new URL(new Quoted("'" + returner + "'", returner, false, this.index, this.currentFileInfo), this.index, this.currentFileInfo);
    });
};