use a linked list for filter runs. (#5206)

* Changed the filterrunprefixes to use LinkedList

* Testing for Linked List

* Finishing touches to LinkedList

* Minor corrections to link-list coding style

* Corrected for sneaky bug in linkedList

core/modules/utils/linked-list.js

@@ -0,0 +1,118 @@
+/*\
+module-type: utils
+title: $:/core/modules/utils/linkedlist.js
+type: application/javascript
+
+This is a doubly-linked indexed list intended for manipulation, particularly
+pushTop, which it does with significantly better performance than an array.
+
+\*/
+(function(){
+
+function LinkedList() {
+	this.clear();
+};
+
+LinkedList.prototype.clear = function() {
+	this.index = Object.create(null);
+	// LinkedList performs the duty of both the head and tail node
+	this.next = this;
+	this.prev = this;
+	this.length = 0;
+};
+
+LinkedList.prototype.remove = function(value) {
+	if($tw.utils.isArray(value)) {
+		for(var t=0; t<value.length; t++) {
+			this._removeOne(value[t]);
+		}
+	} else {
+		this._removeOne(value);
+	}
+};
+
+LinkedList.prototype._removeOne = function(value) {
+	var node = this.index[value];
+	if(node) {
+		node.prev.next = node.next;
+		node.next.prev = node.prev;
+		this.length -= 1;
+		// Point index to the next instance of the same value, maybe nothing.
+		this.index[value] = node.copy;
+	}
+	return node;
+};
+
+LinkedList.prototype._linkToEnd = function(node) {
+	// Sticks the given node onto the end of the list.
+	this.prev.next = node;
+	node.prev = this.prev;
+	this.prev = node;
+	node.next = this;
+	this.length += 1;
+};
+
+LinkedList.prototype.push = function(/* values */) {
+	for(var i = 0; i < arguments.length; i++) {
+		var value = arguments[i];
+		var node = {value: value};
+		var preexistingNode = this.index[value];
+		this._linkToEnd(node);
+		if(preexistingNode) {
+			// We want to keep pointing to the first instance, but we want
+			// to have that instance (or chain of instances) point to the
+			// new one.
+			while (preexistingNode.copy) {
+				preexistingNode = preexistingNode.copy;
+			}
+			preexistingNode.copy = node;
+		} else {
+			this.index[value] = node;
+		}
+	}
+};
+
+LinkedList.prototype.pushTop = function(value) {
+	if($tw.utils.isArray(value)) {
+		for(var t=0; t<value.length; t++) {
+			this._removeOne(value[t]);
+		}
+		this.push.apply(this, value);
+	} else {
+		var node = this._removeOne(value);
+		if(!node) {
+			node = {value: value};
+			this.index[value] = node;
+		} else {
+			// Put this node at the end of the copy chain.
+			var preexistingNode = node;
+			while(preexistingNode.copy) {
+				preexistingNode = preexistingNode.copy;
+			}
+			// The order of these three statements is important,
+			// because sometimes preexistingNode == node.
+			preexistingNode.copy = node;
+			this.index[value] = node.copy;
+			node.copy = undefined;
+		}
+		this._linkToEnd(node);
+	}
+};
+
+LinkedList.prototype.each = function(callback) {
+	for(var ptr = this.next; ptr !== this; ptr = ptr.next) {
+		callback(ptr.value);
+	}
+};
+
+LinkedList.prototype.toArray = function() {
+	var output = [];
+	for(var ptr = this.next; ptr !== this; ptr = ptr.next) {
+		output.push(ptr.value);
+	}
+	return output;
+};
+
+exports.LinkedList = LinkedList;
+
+})();