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diff --git a/src/include/xlist.h b/src/include/xlist.h
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+// -*- mode:C++; tab-width:8; c-basic-offset:2; indent-tabs-mode:t -*-
+// vim: ts=8 sw=2 smarttab
+/*
+ * Ceph - scalable distributed file system
+ *
+ * Copyright (C) 2004-2006 Sage Weil <sage@newdream.net>
+ *
+ * This is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License version 2.1, as published by the Free Software
+ * Foundation. See file COPYING.
+ *
+ */
+
+#ifndef CEPH_XLIST_H
+#define CEPH_XLIST_H
+
+#include <iterator>
+#include <cstdlib>
+#include <ostream>
+
+#include "include/ceph_assert.h"
+
+template<typename T>
+class xlist {
+public:
+ class item {
+ public:
+ item(T i) : _item(i) {}
+ ~item() {
+ ceph_assert(!is_on_list());
+ }
+
+ item(const item& other) = delete;
+ item(item&& other) = delete;
+ const item& operator= (const item& right) = delete;
+ item& operator= (item&& right) = delete;
+
+ xlist* get_list() { return _list; }
+ bool is_on_list() const { return _list ? true:false; }
+ bool remove_myself() {
+ if (_list) {
+ _list->remove(this);
+ ceph_assert(_list == 0);
+ return true;
+ } else
+ return false;
+ }
+ void move_to_front() {
+ ceph_assert(_list);
+ _list->push_front(this);
+ }
+ void move_to_back() {
+ ceph_assert(_list);
+ _list->push_back(this);
+ }
+
+ private:
+ friend xlist;
+ T _item;
+ item *_prev = nullptr, *_next = nullptr;
+ xlist *_list = nullptr;
+ };
+
+ typedef item* value_type;
+ typedef item* const_reference;
+
+private:
+ item *_front, *_back;
+ size_t _size;
+
+public:
+ xlist(const xlist& other) {
+ _front = other._front;
+ _back = other._back;
+ _size = other._size;
+ }
+
+ xlist() : _front(0), _back(0), _size(0) {}
+ ~xlist() {
+ ceph_assert(_size == 0);
+ ceph_assert(_front == 0);
+ ceph_assert(_back == 0);
+ }
+
+ size_t size() const {
+ ceph_assert((bool)_front == (bool)_size);
+ return _size;
+ }
+ bool empty() const {
+ ceph_assert((bool)_front == (bool)_size);
+ return _front == 0;
+ }
+
+ void clear() {
+ while (_front)
+ remove(_front);
+ ceph_assert((bool)_front == (bool)_size);
+ }
+
+ void push_front(item *i) {
+ if (i->_list)
+ i->_list->remove(i);
+
+ i->_list = this;
+ i->_next = _front;
+ i->_prev = 0;
+ if (_front)
+ _front->_prev = i;
+ else
+ _back = i;
+ _front = i;
+ _size++;
+ }
+ void push_back(item *i) {
+ if (i->_list)
+ i->_list->remove(i);
+
+ i->_list = this;
+ i->_next = 0;
+ i->_prev = _back;
+ if (_back)
+ _back->_next = i;
+ else
+ _front = i;
+ _back = i;
+ _size++;
+ }
+ void remove(item *i) {
+ ceph_assert(i->_list == this);
+
+ if (i->_prev)
+ i->_prev->_next = i->_next;
+ else
+ _front = i->_next;
+ if (i->_next)
+ i->_next->_prev = i->_prev;
+ else
+ _back = i->_prev;
+ _size--;
+
+ i->_list = 0;
+ i->_next = i->_prev = 0;
+ ceph_assert((bool)_front == (bool)_size);
+ }
+
+ T front() { return static_cast<T>(_front->_item); }
+ const T front() const { return static_cast<const T>(_front->_item); }
+
+ T back() { return static_cast<T>(_back->_item); }
+ const T back() const { return static_cast<const T>(_back->_item); }
+
+ void pop_front() {
+ ceph_assert(!empty());
+ remove(_front);
+ }
+ void pop_back() {
+ ceph_assert(!empty());
+ remove(_back);
+ }
+
+ class iterator: std::iterator<std::forward_iterator_tag, T> {
+ private:
+ item *cur;
+ public:
+ iterator(item *i = 0) : cur(i) {}
+ T operator*() { return static_cast<T>(cur->_item); }
+ iterator& operator++() {
+ ceph_assert(cur);
+ ceph_assert(cur->_list);
+ cur = cur->_next;
+ return *this;
+ }
+ bool end() const { return cur == 0; }
+ bool operator==(const iterator& rhs) const {
+ return cur == rhs.cur;
+ }
+ bool operator!=(const iterator& rhs) const {
+ return cur != rhs.cur;
+ }
+ };
+
+ iterator begin() { return iterator(_front); }
+ iterator end() { return iterator(NULL); }
+
+ class const_iterator: std::iterator<std::forward_iterator_tag, T> {
+ private:
+ item *cur;
+ public:
+ const_iterator(item *i = 0) : cur(i) {}
+ const T operator*() { return static_cast<const T>(cur->_item); }
+ const_iterator& operator++() {
+ ceph_assert(cur);
+ ceph_assert(cur->_list);
+ cur = cur->_next;
+ return *this;
+ }
+ bool end() const { return cur == 0; }
+ bool operator==(const_iterator& rhs) const {
+ return cur == rhs.cur;
+ }
+ bool operator!=(const_iterator& rhs) const {
+ return cur != rhs.cur;
+ }
+ };
+
+ const_iterator begin() const { return const_iterator(_front); }
+ const_iterator end() const { return const_iterator(NULL); }
+
+ friend std::ostream &operator<<(std::ostream &oss, const xlist<T> &list) {
+ bool first = true;
+ for (const auto &item : list) {
+ if (!first) {
+ oss << ", ";
+ }
+ oss << *item; /* item should be a pointer */
+ first = false;
+ }
+ return oss;
+ }
+};
+
+
+#endif