(function() {

// Returns boolean indicating if the swap was successful.
// Requires that the two signal graphs have exactly the same
// structure.
ElmRuntime.swap = function(from, to) {
    function similar(nodeOld,nodeNew) {
        var idOkay = nodeOld.id === nodeNew.id;
        var lengthOkay = nodeOld.kids.length === nodeNew.kids.length;
        return idOkay && lengthOkay;
    }
    function swap(nodeOld,nodeNew) {
        nodeNew.value = nodeOld.value;
        return true;
    }
    var canSwap = depthFirstTraversals(similar, from.inputs, to.inputs);
    if (canSwap) { depthFirstTraversals(swap, from.inputs, to.inputs); }
    from.node.parentNode.replaceChild(to.node, from.node);
    return canSwap;
}

// Returns false if the node operation f ever fails.
function depthFirstTraversals(f, queueOld, queueNew) {
    if (queueOld.length !== queueNew.length) return false;
    queueOld = queueOld.slice(0);
    queueNew = queueNew.slice(0);

    var seen = [];
    while (queueOld.length > 0 && queueNew.length > 0) {
        var nodeOld = queueOld.pop();
        var nodeNew = queueNew.pop();
        if (seen.indexOf(nodeOld.id) < 0) {
            if (!f(nodeOld, nodeNew)) return false;
            queueOld = queueOld.concat(nodeOld.kids);
            queueNew = queueNew.concat(nodeNew.kids);
            seen.push(nodeOld.id);
        }
    }
    return true;
}

}());