/**
 * Provides constrained xy positioning support for Widgets, through an extension.
 *
 * It builds on top of the widget-position module, to provide constrained positioning support.
 *
 * @module widget-position-constrain
 */
var CONSTRAIN = "constrain",
    CONSTRAIN_XYCHANGE = "constrain|xyChange",
    CONSTRAIN_CHANGE = "constrainChange",

    PREVENT_OVERLAP = "preventOverlap",
    ALIGN = "align",

    EMPTY_STR = "",

    BINDUI = "bindUI",

    XY = "xy",
    X_COORD = "x",
    Y_COORD = "y",

    Node = Y.Node,

    VIEWPORT_REGION = "viewportRegion",
    REGION = "region",

    PREVENT_OVERLAP_MAP;

/**
 * A widget extension, which can be used to add constrained xy positioning support to the base Widget class,
 * through the <a href="Base.html#method_build">Base.build</a> method. This extension requires that
 * the WidgetPosition extension be added to the Widget (before WidgetPositionConstrain, if part of the same
 * extension list passed to Base.build).
 *
 * @class WidgetPositionConstrain
 * @param {Object} User configuration object
 */
function PositionConstrain(config) {}

/**
 * Static property used to define the default attribute
 * configuration introduced by WidgetPositionConstrain.
 *
 * @property ATTRS
 * @type Object
 * @static
 */
PositionConstrain.ATTRS = {

    /**
     * @attribute constrain
     * @type boolean | Node
     * @default null
     * @description The node to constrain the widget's bounding box to, when setting xy. Can also be
     * set to true, to constrain to the viewport.
     */
    constrain : {
        value: null,
        setter: "_setConstrain"
    },

    /**
     * @attribute preventOverlap
     * @type boolean
     * @description If set to true, and WidgetPositionAlign is also added to the Widget,
     * constrained positioning will attempt to prevent the widget's bounding box from overlapping
     * the element to which it has been aligned, by flipping the orientation of the alignment
     * for corner based alignments
     */
    preventOverlap : {
        value:false
    }
};

/**
 * @property _PREVENT_OVERLAP
 * @static
 * @protected
 * @type Object
 * @description The set of positions for which to prevent
 * overlap.
 */
PREVENT_OVERLAP_MAP = PositionConstrain._PREVENT_OVERLAP = {
    x: {
        "tltr": 1,
        "blbr": 1,
        "brbl": 1,
        "trtl": 1
    },
    y : {
        "trbr": 1,
        "tlbl": 1,
        "bltl": 1,
        "brtr": 1
    }
};

PositionConstrain.prototype = {

    initializer : function() {
        if (!this._posNode) {
            Y.error("WidgetPosition needs to be added to the Widget, before WidgetPositionConstrain is added");
        }
        Y.after(this._bindUIPosConstrained, this, BINDUI);
    },

    /**
     * Calculates the constrained positions for the XY positions provided, using
     * the provided node argument is passed in. If no node value is passed in, the value of
     * the "constrain" attribute is used.
     *
     * @method getConstrainedXY
     * @param {Array} xy The xy values to constrain
     * @param {Node | boolean} node Optional. The node to constrain to, or true for the viewport
     * @return {Array} The constrained xy values
     */
    getConstrainedXY : function(xy, node) {
        node = node || this.get(CONSTRAIN);

        var constrainingRegion = this._getRegion((node === true) ? null : node),
            nodeRegion = this._posNode.get(REGION);

        return [
            this._constrain(xy[0], X_COORD, nodeRegion, constrainingRegion),
            this._constrain(xy[1], Y_COORD, nodeRegion, constrainingRegion)
        ];
    },

    /**
     * Constrains the widget's bounding box to a node (or the viewport). If xy or node are not
     * passed in, the current position and the value of "constrain" will be used respectively.
     *
     * The widget's position will be changed to the constrained position.
     *
     * @method constrain
     * @param {Array} xy Optional. The xy values to constrain
     * @param {Node | boolean} node Optional. The node to constrain to, or true for the viewport
     */
    constrain : function(xy, node) {
        var currentXY,
            constrainedXY,
            constraint = node || this.get(CONSTRAIN);

        if (constraint) {
            currentXY = xy || this.get(XY);
            constrainedXY = this.getConstrainedXY(currentXY, constraint);

            if (constrainedXY[0] !== currentXY[0] || constrainedXY[1] !== currentXY[1]) {
                this.set(XY, constrainedXY, { constrained:true });
            }
        }
    },

    /**
     * The setter implementation for the "constrain" attribute.
     *
     * @method _setConstrain
     * @protected
     * @param {Node | boolean} val The attribute value
     */
    _setConstrain : function(val) {
        return (val === true) ? val : Node.one(val);
    },

    /**
     * The method which performs the actual constrain calculations for a given axis ("x" or "y") based
     * on the regions provided.
     *
     * @method _constrain
     * @protected
     *
     * @param {Number} val The value to constrain
     * @param {String} axis The axis to use for constrainment
     * @param {Region} nodeRegion The region of the node to constrain
     * @param {Region} constrainingRegion The region of the node (or viewport) to constrain to
     *
     * @return {Number} The constrained value
     */
    _constrain: function(val, axis, nodeRegion, constrainingRegion) {
        if (constrainingRegion) {

            if (this.get(PREVENT_OVERLAP)) {
                val = this._preventOverlap(val, axis, nodeRegion, constrainingRegion);
            }

            var x = (axis == X_COORD),

                regionSize    = (x) ? constrainingRegion.width : constrainingRegion.height,
                nodeSize      = (x) ? nodeRegion.width : nodeRegion.height,
                minConstraint = (x) ? constrainingRegion.left : constrainingRegion.top,
                maxConstraint = (x) ? constrainingRegion.right - nodeSize : constrainingRegion.bottom - nodeSize;

            if (val < minConstraint || val > maxConstraint) {
                if (nodeSize < regionSize) {
                    if (val < minConstraint) {
                        val = minConstraint;
                    } else if (val > maxConstraint) {
                        val = maxConstraint;
                    }
                } else {
                    val = minConstraint;
                }
            }
        }

        return val;
    },

    /**
     * The method which performs the preventOverlap calculations for a given axis ("x" or "y") based
     * on the value and regions provided.
     *
     * @method _preventOverlap
     * @protected
     *
     * @param {Number} val The value being constrain
     * @param {String} axis The axis to being constrained
     * @param {Region} nodeRegion The region of the node being constrained
     * @param {Region} constrainingRegion The region of the node (or viewport) we need to constrain to
     *
     * @return {Number} The constrained value
     */
    _preventOverlap : function(val, axis, nodeRegion, constrainingRegion) {

        var align = this.get(ALIGN),
            x = (axis === X_COORD),
            nodeSize,
            alignRegion,
            nearEdge,
            farEdge,
            spaceOnNearSide,
            spaceOnFarSide;

        if (align && align.points && PREVENT_OVERLAP_MAP[axis][align.points.join(EMPTY_STR)]) {

            alignRegion = this._getRegion(align.node);

            if (alignRegion) {
                nodeSize        = (x) ? nodeRegion.width : nodeRegion.height;
                nearEdge        = (x) ? alignRegion.left : alignRegion.top;
                farEdge         = (x) ? alignRegion.right : alignRegion.bottom;
                spaceOnNearSide = (x) ? alignRegion.left - constrainingRegion.left : alignRegion.top - constrainingRegion.top;
                spaceOnFarSide  = (x) ? constrainingRegion.right - alignRegion.right : constrainingRegion.bottom - alignRegion.bottom;
            }

            if (val > nearEdge) {
                if (spaceOnFarSide < nodeSize && spaceOnNearSide > nodeSize) {
                    val = nearEdge - nodeSize;
                }
            } else {
                if (spaceOnNearSide < nodeSize && spaceOnFarSide > nodeSize) {
                    val = farEdge;
                }
            }
        }

        return val;
    },

    /**
     * Binds event listeners responsible for updating the UI state in response to
     * Widget constrained positioning related state changes.
     * <p>
     * This method is invoked after bindUI is invoked for the Widget class
     * using YUI's aop infrastructure.
     * </p>
     *
     * @method _bindUIPosConstrained
     * @protected
     */
    _bindUIPosConstrained : function() {
        this.after(CONSTRAIN_CHANGE, this._afterConstrainChange);
        this._enableConstraints(this.get(CONSTRAIN));
    },

    /**
     * After change listener for the "constrain" attribute, responsible
     * for updating the UI, in response to attribute changes.
     *
     * @method _afterConstrainChange
     * @protected
     * @param {EventFacade} e The event facade
     */
    _afterConstrainChange : function(e) {
        this._enableConstraints(e.newVal);
    },

    /**
     * Updates the UI if enabling constraints, and sets up the xyChange event listeners
     * to constrain whenever the widget is moved. Disabling constraints removes the listeners.
     *
     * @method _enableConstraints
     * @private
     * @param {boolean} enable Enable or disable constraints
     */
    _enableConstraints : function(enable) {
        if (enable) {
            this.constrain();
            this._cxyHandle = this._cxyHandle || this.on(CONSTRAIN_XYCHANGE, this._constrainOnXYChange);
        } else if (this._cxyHandle) {
            this._cxyHandle.detach();
            this._cxyHandle = null;
        }
    },

    /**
     * The on change listener for the "xy" attribute. Modifies the event facade's
     * newVal property with the constrained XY value.
     *
     * @method _constrainOnXYChange
     * @protected
     * @param {EventFacade} e The event facade for the attribute change
     */
    _constrainOnXYChange : function(e) {
        if (!e.constrained) {
            e.newVal = this.getConstrainedXY(e.newVal);
        }
    },

    /**
     * Utility method to normalize region retrieval from a node instance,
     * or the viewport, if no node is provided.
     *
     * @method _getRegion
     * @private
     * @param {Node} node Optional.
     */
    _getRegion : function(node) {
        var region;
        if (!node) {
            region = this._posNode.get(VIEWPORT_REGION);
        } else {
            node = Node.one(node);
            if (node) {
                region = node.get(REGION);
            }
        }
        return region;
    }
};

Y.WidgetPositionConstrain = PositionConstrain;
