l, null);

    while (cursor.prev !== null) {
        item = cursor.prev;
        cursor.prev = item.prev;

        fn.call(context, item.data, item, this);
    }

    // pop cursor
    releaseCursor(this);
};

List.prototype.forEachRight = List.prototype.eachRight;

List.prototype.reduce = function(fn, initialValue, context) {
    var item;

    if (context === undefined) {
        context = this;
    }

    // push cursor
    var cursor = allocateCursor(this, null, this.head);
    var acc = initialValue;

    while (cursor.next !== null) {
        item = cursor.next;
        cursor.next = item.next;

        acc = fn.call(context, acc, item.data, item, this);
    }

    // pop cursor
    releaseCursor(this);

    return acc;
};

List.prototype.reduceRight = function(fn, initialValue, context) {
    var item;

    if (context === undefined) {
        context = this;
    }

    // push cursor
    var cursor = allocateCursor(this, this.tail, null);
    var acc = initialValue;

    while (cursor.prev !== null) {
        item = cursor.prev;
        cursor.prev = item.prev;

        acc = fn.call(context, acc, item.data, item, this);
    }

    // pop cursor
    releaseCursor(this);

    return acc;
};

List.prototype.nextUntil = function(start, fn, context) {
    if (start === null) {
        return;
    }

    var item;

    if (context === undefined) {
        context = this;
    }

    // push cursor
    var cursor = allocateCursor(this, null, start);

    while (cursor.next !== null) {
        item = cursor.next;
        cursor.next = item.next;

        if (fn.call(context, item.data, item, this)) {
            break;
        }
    }

    // pop cursor
    releaseCursor(this);
};

List.prototype.prevUntil = function(start, fn, context) {
    if (start === null) {
        return;
    }

    var item;

    if (context === undefined) {
        context = this;
    }

    // push cursor
    var cursor = allocateCursor(this, start, null);

    while (cursor.prev !== null) {
        item = cursor.prev;
        cursor.prev = item.prev;

        if (fn.call(context, item.data, item, this)) {
            break;
        }
    }

    // pop cursor
    releaseCursor(this);
};

List.prototype.some = function(fn, context) {
    var cursor = this.head;

    if (context === undefined) {
        context = this;
    }

    while (cursor !== null) {
        if (fn.call(context, cursor.data, cursor, this)) {
            return true;
        }

        cursor = cursor.next;
    }

    return false;
};

List.prototype.map = function(fn, context) {
    var result = new List();
    var cursor = this.head;

    if (context === undefined) {
        context = this;
    }

    while (cursor !== null) {
        result.appendData(fn.call(context, cursor.data, cursor, this));
        cursor = cursor.next;
    }

    return result;
};

List.prototype.filter = function(fn, context) {
    var result = new List();
    var cursor = this.head;

    if (context === undefined) {
        context = this;
    }

    while (cursor !== null) {
        if (fn.call(context, cursor.data, cursor, this)) {
            result.appendData(cursor.data);
        }
        cursor = cursor.next;
    }

    return result;
};

List.prototype.clear = function() {
    this.head = null;
    this.tail = null;
};

List.prototype.copy = function() {
    var result = new List();
    var cursor = this.head;

    while (cursor !== null) {
        result.insert(createItem(cursor.data));
        cursor = cursor.next;
    }

    return result;
};

List.prototype.prepend = function(item) {
    //      head
    //    ^
    // item
    this.updateCursors(null, item, this.head, item);

    // insert to the beginning of the list
    if (this.head !== null) {
        // new item <- first item
        this.head.prev = item;

        // new item -> first item
        item.next = this.head;
    } else {
        // if list has no head, then it also has no tail
        // in this case tail points to the new item
        this.tail = item;
    }

    // head always points to new item
    this.head = item;

    return this;
};

List.prototype.prependData = function(data) {
    return this.prepend(createItem(data));
};

List.prototype.append = function(item) {
    return this.insert(item);
};

List.prototype.appendData = function(data) {
    return this.insert(createItem(data));
};

List.prototype.insert = function(item, before) {
    if (before !== undefined && before !== null) {
        // prev   before
        //      ^
        //     item
        this.updateCursors(before.prev, item, before, item);

        if (before.prev === null) {
            // insert to the beginning of list
            if (this.head !== before) {
                throw new Error('before doesn\'t belong to list');
            }

            // since head points to before therefore list doesn't empty
            // no need to check tail
            this.head = item;
            before.prev = item;
            item.next = before;

            this.updateCursors(null, item);
        } else {

            // insert between two items
            before.prev.next = item;
            item.prev = before.prev;

            before.prev = item;
            item.next = before;
        }
    } else {
        // tail
        //      ^
        //      item
        this.updateCursors(this.tail, item, null, item);

        // insert to the ending of the list
        if (this.tail !== null) {
            // last item -> new item
            this.tail.next = item;

            // last item <- new item
            item.prev = this.tail;
        } else {
            // if list has no tail, then it also has no head
            // in this case head points to new item
            this.head = item;
        }

        // tail always points to new item
        this.tail = item;
    }

    return this;
};

List.prototype.insertData = function(data, before) {
    return this.insert(createItem(data), before);
};

List.prototype.remove = function(item) {
    //      item
    //       ^
    // prev     next
    this.updateCursors(item, item.prev, item, item.next);

    if (item.prev !== null) {
        item.prev.next = item.next;
    } else {
        if (this.head !== item) {
            throw new Error('item doesn\'t belong to list');
        }

        this.head = item.next;
    }

    if (item.next !== null) {
        item.next.prev = item.prev;
    } else {
        if (this.tail !== item) {
            throw new Error('item doesn\'t belong to list');
        }

        this.tail = item.prev;
    }

    item.prev = null;
    item.next = null;

    return item;
};

List.prototype.push = function(data) {
    this.insert(createItem(data));
};

List.prototype.pop = function() {
    if (this.tail !== null) {
        return this.remove(this.tail);
    }
};

List.prototype.unshift = function(data) {
    this.prepend(createItem(data));
};

List.prototype.shift = function() {
    if (this.head !== null) {
        return this.remove(this.head);
    }
};

List.prototype.prependList = function(list) {
    return this.insertList(list, this.head);
};

List.prototype.appendList = function(list) {
    return this.insertList(list);
};

List.prototype.insertList = function(list, before) {
    // ignore empty lists
    if (list.head === null) {
        return this;
    }

    if (before !== undefined && before !== null) {
        this.updateCursors(before.prev, list.tail, before, list.head);

        // insert in the middle of dist list
        if (before.prev !== null) {
            // before.prev <-> list.head
            before.prev.next = list.head;
            list.head.prev = before.prev;
        } else {
            this.head = list.head;
        }

        before.prev = list.tail;
        list.tail.next = before;
    } else {
        this.updateCursors(this.tail, list.tail, null, list.head);

        // insert to end of the list
        if (this.tail !== null) {
            // if destination list has a tail, then it also has a head,
            // but head doesn't change

            // dest tail -> source head
            this.tail.next = list.head;

            // dest tail <- source head
            list.head.prev = this.tail;
        } else {
            // if list has no a tail, then it also has no a head
            // in this case points head to new item
            this.head = list.head;
        }

        // tail always start point to new item
        this.tail = list.tail;
    }

    list.head = null;
    list.tail = null;

    return this;
};

List.prototype.replace = function(oldItem, newItemOrList) {
    if ('head' in newItemOrList) {
        this.insertList(newItemOrList, oldItem);
    } else {
        this.insert(newItemOrList, oldItem);
    }

    this.remove(oldItem);
};

module.exports = List;
