1+ // https://en.wikipedia.org/wiki/Doubly_linked_list
2+ 3+ function Node ( data ) {
4+ this . data = data
5+ this . next = null
6+ this . prev = null
7+ }
8+ 9+ function DoublyLinkedList ( ) {
10+ this . length = 0
11+ this . head = null
12+ this . tail = null
13+ }
14+ 15+ DoublyLinkedList . prototype . add = function ( value ) {
16+ let node = new Node ( value )
17+ 18+ node . next = this . head
19+ if ( this . head ) {
20+ this . head . prev = node
21+ } else {
22+ this . tail = node
23+ }
24+ this . length ++
25+ this . head = node
26+ node . prev = null
27+ }
28+ 29+ DoublyLinkedList . prototype . remove = function ( position ) {
30+ let toRemove = this . findAt ( position )
31+ 32+ if ( ! toRemove ) return null
33+ 34+ if ( toRemove === this . head ) {
35+ this . head = toRemove . next
36+ if ( ! this . head ) {
37+ this . tail = null
38+ } else {
39+ this . head . prev = null
40+ }
41+ } else if ( toRemove === this . tail ) {
42+ toRemove . prev . next = null
43+ this . tail = toRemove . prev
44+ } else {
45+ toRemove . prev . next = toRemove . next
46+ toRemove . next . prev = toRemove . prev
47+ }
48+ 49+ this . length --
50+ return toRemove
51+ }
52+ 53+ DoublyLinkedList . prototype . findAt = function ( position ) {
54+ if ( position > - 1 && position < this . length ) {
55+ let current = this . head
56+ 57+ for ( let i = 0 ; i ++ < position ; current = current . next ) ;
58+ 59+ return current
60+ }
61+ return null
62+ }
63+ 64+ const test = require ( 'tape' )
65+ 66+ test ( 'Doubly linked list add' , assert => {
67+ let dl = new DoublyLinkedList ( )
68+ dl . add ( 'Hello world!' )
69+ assert . deepEqual ( dl . head . data , 'Hello world!' )
70+ assert . deepEqual ( dl . length , 1 )
71+ 72+ dl . add ( 'Can you see me?' )
73+ assert . deepEqual ( dl . length , 2 )
74+ assert . deepEqual ( dl . tail . data , 'Hello world!' )
75+ assert . end ( )
76+ } )
77+ 78+ test ( 'Doubly linked list findAt' , assert => {
79+ let dl = new DoublyLinkedList ( )
80+ dl . add ( 'Hello world!' )
81+ dl . add ( 'Can you see me?' )
82+ dl . add ( 'Now you don\'t' )
83+ assert . deepEqual ( dl . findAt ( 0 ) . data , 'Now you don\'t' )
84+ assert . deepEqual ( dl . findAt ( 2 ) . data , 'Hello world!' )
85+ assert . deepEqual ( dl . findAt ( 3 ) , null )
86+ assert . end ( )
87+ } )
88+ 89+ test ( 'Doubly linked list remove' , assert => {
90+ let dl = new DoublyLinkedList ( )
91+ dl . add ( 'Hello world!' )
92+ dl . add ( 'Can you see me?' )
93+ dl . add ( 'Now you don\'t' )
94+ let removed = dl . remove ( 2 )
95+ assert . deepEqual ( dl . tail . data , 'Can you see me?' )
96+ assert . deepEqual ( removed . data , 'Hello world!' )
97+ assert . deepEqual ( 2 , dl . length )
98+ dl . remove ( 1 )
99+ assert . deepEqual ( 1 , dl . length )
100+ removed = dl . remove ( 0 )
101+ assert . deepEqual ( removed . data , 'Now you don\'t' )
102+ assert . end ( )
103+ } )
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