// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// Package list implements a doubly linked list.
//
// To iterate over a list (where l is a *List):
//	for e := l.Front(); e != nil; e = e.Next() {
//		// do something with e.Value
//	}
//
package list

// Element is an element of a linked list.
type Element struct {
	// Next and previous pointers in the doubly-linked list of elements.
	// To simplify the implementation, internally a list l is implemented
	// as a ring, such that &l.root is both the next element of the last
	// list element (l.Back()) and the previous element of the first list
	// element (l.Front()).
	next *Element		// 后继
	prev *Element		// 前驱

	// The list to which this element belongs.
	list *List			// 当前元素属于哪个List，具体作用？？？

	// The value stored with this element.
	Value interface{}		// 数据的值，直接用interface来代替，表明任何类型
}

// Next returns the next list element or nil.
// 返回下一个元素
func (e *Element) Next() *Element {
	// 当前的list成员不为空
	// 下一个Elemnt的地址不是list的root地址
	if p := e.next; e.list != nil && p != &e.list.root {
		return p
	}
	return nil
}

// Prev returns the previous list element or nil.
// 返回前一个元素
func (e *Element) Prev() *Element {
	// 上一个Element的地址不是list的root地址
	// 当前的list成员不为空
	if p := e.prev; e.list != nil && p != &e.list.root {
		return p
	}
	return nil
}

// List represents a doubly linked list.
// The zero value for List is an empty list ready to use.
type List struct {

	// root，挂一个链表，即元素链表，类似链表头，不存放数据
	root Element // sentinel list element, only &root, root.prev, and root.next are used

	// 链表元素的个数
	len  int     // current list length excluding (this) sentinel element
}

// Init initializes or clears list l.
func (l *List) Init() *List {
	// 初始化root为本身
	// list的长度为0
	l.root.next = &l.root
	l.root.prev = &l.root
	l.len = 0
	return l
}

// New returns an initialized list.
// 产生一个新的链表并初始化
func New() *List { return new(List).Init() }

// Len returns the number of elements of list l.
// The complexity is O(1).
// 烦很链表元素个数，不需要遍历
func (l *List) Len() int { return l.len }

// Front returns the first element of list l or nil if the list is empty.
// 返回第一个元素
func (l *List) Front() *Element {
	// 链表无元素，则返回nil
	if l.len == 0 {
		return nil
	}
	// 链表有元素，返回第一个元素
	return l.root.next
}

// Back returns the last element of list l or nil if the list is empty.
// 返回最后一个元素，从中可以知道是循环链表
func (l *List) Back() *Element {
	if l.len == 0 {
		return nil
	}
	return l.root.prev
}

// lazyInit lazily initializes a zero List value.
// 无元素则初始化，有元素则不处理
func (l *List) lazyInit() {
	if l.root.next == nil {
		l.Init()
	}
}

// insert inserts e after at, increments l.len, and returns e.
// 在元素at之后插入e，返回e
func (l *List) insert(e, at *Element) *Element {
	// at X => at e X
	// e的next变成X
	// e的prev改成at
	// at的next从X改成e
	// X的prev从at改成e
	// 数量自增
	// list成员改成当前链表，表示属于此链表
	e.prev = at
	e.next = at.next
	e.prev.next = e
	e.next.prev = e
	e.list = l
	l.len++
	return e
}

// insertValue is a convenience wrapper for insert(∈{Value: v}, at).
// 封装insert接口，为了传递一个值
func (l *List) insertValue(v interface{}, at *Element) *Element {
	return l.insert(∈{Value: v}, at)
}

// remove removes e from its list, decrements l.len, and returns e.
// 将元素从list中删除，并返回删除元素的指针
func (l *List) remove(e *Element) *Element {
	// 删除元素
	// 假设：P e N
	// P为e的前驱，N为e的后继
	// P的后继改成N
	// N的前驱改成P
	// 注意：要删除的前驱和后继指针必须变成niil，
	// 如果不置为空，就意味着，P和N的内存永远释放不掉，即使后面的P和N从链表中删掉
	// 个数自动减1
	e.prev.next = e.next
	e.next.prev = e.prev
	e.next = nil // avoid memory leaks
	e.prev = nil // avoid memory leaks
	e.list = nil
	l.len--
	return e
}

// move moves e to next to at and returns e.
// 将元素e移动到at后面
func (l *List) move(e, at *Element) *Element {
	if e == at {
		return e
	}
	// 假设元素e和at分别是以下情况：
	// eP e eN
	// atP at atN

	// 操作流程：
	// 删除e
	// 将e放到at和atN中间，即最终的结果为：
	// aP eN, at e atN
	e.prev.next = e.next
	e.next.prev = e.prev

	e.prev = at
	e.next = at.next
	e.prev.next = e
	e.next.prev = e

	return e
}

// Remove removes e from l if e is an element of list l.
// It returns the element value e.Value.
// The element must not be nil.
// 删除元素，必须对元素的list元素校验，必须要属于当前链表
func (l *List) Remove(e *Element) interface{} {
	if e.list == l {
		// if e.list == l, l must have been initialized when e was inserted
		// in l or l == nil (e is a zero Element) and l.remove will crash
		l.remove(e)
	}
	return e.Value
}

// PushFront inserts a new element e with value v at the front of list l and returns e.
// 在链表头部插入，即root之后
func (l *List) PushFront(v interface{}) *Element {
	l.lazyInit()
	return l.insertValue(v, &l.root)
}

// PushBack inserts a new element e with value v at the back of list l and returns e.
// 在链表尾部插入元素
func (l *List) PushBack(v interface{}) *Element {
	l.lazyInit()
	return l.insertValue(v, l.root.prev)
}

// InsertBefore inserts a new element e with value v immediately before mark and returns e.
// If mark is not an element of l, the list is not modified.
// The mark must not be nil.
// 在mark之前插入元素
func (l *List) InsertBefore(v interface{}, mark *Element) *Element {
	if mark.list != l {
		return nil
	}
	// see comment in List.Remove about initialization of l
	return l.insertValue(v, mark.prev)
}

// InsertAfter inserts a new element e with value v immediately after mark and returns e.
// If mark is not an element of l, the list is not modified.
// The mark must not be nil.
// 在mark之后插入v
func (l *List) InsertAfter(v interface{}, mark *Element) *Element {
	if mark.list != l {
		return nil
	}
	// see comment in List.Remove about initialization of l
	return l.insertValue(v, mark)
}

// MoveToFront moves element e to the front of list l.
// If e is not an element of l, the list is not modified.
// The element must not be nil.
// 将元素放到链表头，也就是root之后
func (l *List) MoveToFront(e *Element) {
	if e.list != l || l.root.next == e {
		return
	}
	// see comment in List.Remove about initialization of l
	l.move(e, &l.root)
}

// MoveToBack moves element e to the back of list l.
// If e is not an element of l, the list is not modified.
// The element must not be nil.
// 将e放到链表尾部
func (l *List) MoveToBack(e *Element) {
	if e.list != l || l.root.prev == e {
		return
	}
	// see comment in List.Remove about initialization of l
	l.move(e, l.root.prev)
}

// MoveBefore moves element e to its new position before mark.
// If e or mark is not an element of l, or e == mark, the list is not modified.
// The element and mark must not be nil.
// 将e放到mark前
func (l *List) MoveBefore(e, mark *Element) {
	if e.list != l || e == mark || mark.list != l {
		return
	}
	l.move(e, mark.prev)
}

// MoveAfter moves element e to its new position after mark.
// If e or mark is not an element of l, or e == mark, the list is not modified.
// The element and mark must not be nil.
// 将e放到mark后
func (l *List) MoveAfter(e, mark *Element) {
	if e.list != l || e == mark || mark.list != l {
		return
	}
	l.move(e, mark)
}

// PushBackList inserts a copy of another list at the back of list l.
// The lists l and other may be the same. They must not be nil.
// 将链表other放到l的后面
func (l *List) PushBackList(other *List) {
	l.lazyInit()
	for i, e := other.Len(), other.Front(); i > 0; i, e = i-1, e.Next() {
		l.insertValue(e.Value, l.root.prev)
	}
}

// PushFrontList inserts a copy of another list at the front of list l.
// The lists l and other may be the same. They must not be nil.
// 将other放到l的前面
func (l *List) PushFrontList(other *List) {
	l.lazyInit()
	for i, e := other.Len(), other.Back(); i > 0; i, e = i-1, e.Prev() {
		l.insertValue(e.Value, &l.root)
	}
}