1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
|
/* __ *\
** ________ ___ / / ___ Scala API **
** / __/ __// _ | / / / _ | (c) 2003-2011, LAMP/EPFL **
** __\ \/ /__/ __ |/ /__/ __ | http://scala-lang.org/ **
** /____/\___/_/ |_/____/_/ | | **
** |/ **
\* */
package scala.collection
package immutable
import generic._
import mutable.{Builder, ListBuffer}
import scala.annotation.tailrec
import java.io._
/** A class for immutable linked lists representing ordered collections
* of elements of type.
*
* This class comes with two implementing case classes `scala.Nil`
* and `scala.::` that implement the abstract members `isEmpty`,
* `head` and `tail`.
*
* This class is optimal for last-in-first-out (LIFO), stack-like access patterns. If you need another access
* pattern, for example, random access or FIFO, consider using a collection more suited to this than `List`.
*
* @example {{{
* // Make a list via the companion object factory
* val days = List("Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday")
*
* // Make a list element-by-element
* val when = "AM" :: "PM" :: List()
*
* // Pattern match
* days match {
* case firstDay :: otherDays =>
* println("The first day of the week is: " + firstDay)
* case List() =>
* println("There don't seem to be any week days.")
* }
* }}}
*
* ==Performance==
* '''Time:''' `List` has `O(1)` prepend and head/tail access. Most other operations are `O(n)` on the number of elements in the list.
* This includes the index-based lookup of elements, `length`, `append` and `reverse`.
*
* '''Space:''' `List` implements '''structural sharing''' of the tail list. This means that many operations are either
* zero- or constant-memory cost.
* {{{
* val mainList = List(3, 2, 1)
* val with4 = 4 :: mainList // re-uses mainList, costs one :: instance
* val with42 = 42 :: mainList // also re-uses mainList, cost one :: instance
* val shorter = mainList.tail // costs nothing as it uses the same 2::1::Nil instances as mainList
* }}}
*
* @author Martin Odersky and others
* @version 2.8
* @since 1.0
* @see [[http://docs.scala-lang.org/overviews/collections/concrete-immutable-collection-classes.html#lists "Scala's Collection Library overview"]]
* section on `Lists` for more information.
*
* @define coll list
* @define Coll `List`
* @define thatinfo the class of the returned collection. In the standard library configuration,
* `That` is always `List[B]` because an implicit of type `CanBuildFrom[List, B, That]`
* is defined in object `List`.
* @define bfinfo an implicit value of class `CanBuildFrom` which determines the
* result class `That` from the current representation type `Repr`
* and the new element type `B`. This is usually the `canBuildFrom` value
* defined in object `List`.
* @define orderDependent
* @define orderDependentFold
* @define mayNotTerminateInf
* @define willNotTerminateInf
*/
sealed abstract class List[+A] extends AbstractSeq[A]
with LinearSeq[A]
with Product
with GenericTraversableTemplate[A, List]
with LinearSeqOptimized[A, List[A]] {
override def companion: GenericCompanion[List] = List
import scala.collection.{Iterable, Traversable, Seq, IndexedSeq}
def isEmpty: Boolean
def head: A
def tail: List[A]
// New methods in List
/** Adds an element at the beginning of this list.
* @param x the element to prepend.
* @return a list which contains `x` as first element and
* which continues with this list.
*
* @usecase def ::(x: A): List[A]
* @inheritdoc
*
* Example:
* {{{1 :: List(2, 3) = List(2, 3).::(1) = List(1, 2, 3)}}}
*/
def ::[B >: A] (x: B): List[B] =
new scala.collection.immutable.::(x, this)
/** Adds the elements of a given list in front of this list.
* @param prefix The list elements to prepend.
* @return a list resulting from the concatenation of the given
* list `prefix` and this list.
*
* @usecase def :::(prefix: List[A]): List[A]
* @inheritdoc
*
* Example:
* {{{List(1, 2) ::: List(3, 4) = List(3, 4).:::(List(1, 2)) = List(1, 2, 3, 4)}}}
*/
def :::[B >: A](prefix: List[B]): List[B] =
if (isEmpty) prefix
else if (prefix.isEmpty) this
else (new ListBuffer[B] ++= prefix).prependToList(this)
/** Adds the elements of a given list in reverse order in front of this list.
* `xs reverse_::: ys` is equivalent to
* `xs.reverse ::: ys` but is more efficient.
*
* @param prefix the prefix to reverse and then prepend
* @return the concatenation of the reversed prefix and the current list.
*
* @usecase def reverse_:::(prefix: List[A]): List[A]
* @inheritdoc
*/
def reverse_:::[B >: A](prefix: List[B]): List[B] = {
var these: List[B] = this
var pres = prefix
while (!pres.isEmpty) {
these = pres.head :: these
pres = pres.tail
}
these
}
/** Builds a new list by applying a function to all elements of this list.
* Like `xs map f`, but returns `xs` unchanged if function
* `f` maps all elements to themselves (as determined by `eq`).
*
* @param f the function to apply to each element.
* @tparam B the element type of the returned collection.
* @return a list resulting from applying the given function
* `f` to each element of this list and collecting the results.
*
* @usecase def mapConserve(f: A => A): List[A]
* @inheritdoc
*/
@inline final def mapConserve[B >: A <: AnyRef](f: A => B): List[B] = {
@tailrec
def loop(mapped: ListBuffer[B], unchanged: List[A], pending: List[A]): List[B] =
if (pending.isEmpty) {
if (mapped eq null) unchanged
else mapped.prependToList(unchanged)
}
else {
val head0 = pending.head
val head1 = f(head0)
if (head1 eq head0.asInstanceOf[AnyRef])
loop(mapped, unchanged, pending.tail)
else {
val b = if (mapped eq null) new ListBuffer[B] else mapped
var xc = unchanged
while (xc ne pending) {
b += xc.head
xc = xc.tail
}
b += head1
val tail0 = pending.tail
loop(b, tail0, tail0)
}
}
loop(null, this, this)
}
// Overridden methods from IterableLike and SeqLike or overloaded variants of such methods
override def ++[B >: A, That](that: GenTraversableOnce[B])(implicit bf: CanBuildFrom[List[A], B, That]): That = {
val b = bf(this)
if (b.isInstanceOf[ListBuffer[_]]) (this ::: that.seq.toList).asInstanceOf[That]
else super.++(that)
}
override def +:[B >: A, That](elem: B)(implicit bf: CanBuildFrom[List[A], B, That]): That = bf match {
case _: List.GenericCanBuildFrom[_] => (elem :: this).asInstanceOf[That]
case _ => super.+:(elem)(bf)
}
override def toList: List[A] = this
override def take(n: Int): List[A] = {
val b = new ListBuffer[A]
var i = 0
var these = this
while (!these.isEmpty && i < n) {
i += 1
b += these.head
these = these.tail
}
if (these.isEmpty) this
else b.toList
}
override def drop(n: Int): List[A] = {
var these = this
var count = n
while (!these.isEmpty && count > 0) {
these = these.tail
count -= 1
}
these
}
/**
* @example {{{
* // Given a list
* val letters = List('a','b','c','d','e')
*
* // `slice` returns all elements beginning at index `from` and afterwards,
* // up until index `until` (excluding index `until`.)
* letters.slice(1,3) // Returns List('b','c')
* }}}
*/
override def slice(from: Int, until: Int): List[A] = {
val lo = scala.math.max(from, 0)
if (until <= lo || isEmpty) Nil
else this drop lo take (until - lo)
}
override def takeRight(n: Int): List[A] = {
@tailrec
def loop(lead: List[A], lag: List[A]): List[A] = lead match {
case Nil => lag
case _ :: tail => loop(tail, lag.tail)
}
loop(drop(n), this)
}
// dropRight is inherited from LinearSeq
override def splitAt(n: Int): (List[A], List[A]) = {
val b = new ListBuffer[A]
var i = 0
var these = this
while (!these.isEmpty && i < n) {
i += 1
b += these.head
these = these.tail
}
(b.toList, these)
}
@inline final override def takeWhile(p: A => Boolean): List[A] = {
val b = new ListBuffer[A]
var these = this
while (!these.isEmpty && p(these.head)) {
b += these.head
these = these.tail
}
b.toList
}
@inline final override def dropWhile(p: A => Boolean): List[A] = {
@tailrec
def loop(xs: List[A]): List[A] =
if (xs.isEmpty || !p(xs.head)) xs
else loop(xs.tail)
loop(this)
}
@inline final override def span(p: A => Boolean): (List[A], List[A]) = {
val b = new ListBuffer[A]
var these = this
while (!these.isEmpty && p(these.head)) {
b += these.head
these = these.tail
}
(b.toList, these)
}
// Overridden with an implementation identical to the inherited one (at this time)
// solely so it can be finalized and thus inlinable.
@inline final override def foreach[U](f: A => U) {
var these = this
while (!these.isEmpty) {
f(these.head)
these = these.tail
}
}
override def reverse: List[A] = {
var result: List[A] = Nil
var these = this
while (!these.isEmpty) {
result = these.head :: result
these = these.tail
}
result
}
override def stringPrefix = "List"
override def toStream : Stream[A] =
if (isEmpty) Stream.Empty
else new Stream.Cons(head, tail.toStream)
}
/** The empty list.
*
* @author Martin Odersky
* @version 1.0, 15/07/2003
* @since 2.8
*/
@SerialVersionUID(0 - 8256821097970055419L)
case object Nil extends List[Nothing] {
override def isEmpty = true
override def head: Nothing =
throw new NoSuchElementException("head of empty list")
override def tail: List[Nothing] =
throw new UnsupportedOperationException("tail of empty list")
// Removal of equals method here might lead to an infinite recursion similar to IntMap.equals.
override def equals(that: Any) = that match {
case that1: scala.collection.GenSeq[_] => that1.isEmpty
case _ => false
}
}
/** A non empty list characterized by a head and a tail.
* @param hd the first element of the list
* @param tl the list containing the remaining elements of this list after the first one.
* @tparam B the type of the list elements.
* @author Martin Odersky
* @version 1.0, 15/07/2003
* @since 2.8
*/
@SerialVersionUID(0L - 8476791151983527571L)
final case class ::[B](private var hd: B, private[scala] var tl: List[B]) extends List[B] {
override def head : B = hd
override def tail : List[B] = tl
override def isEmpty: Boolean = false
private def writeObject(out: ObjectOutputStream) {
out.writeObject(ListSerializeStart) // needed to differentiate with the legacy `::` serialization
out.writeObject(this.hd)
out.writeObject(this.tl)
}
private def readObject(in: ObjectInputStream) {
val obj = in.readObject()
if (obj == ListSerializeStart) {
this.hd = in.readObject().asInstanceOf[B]
this.tl = in.readObject().asInstanceOf[List[B]]
} else oldReadObject(in, obj)
}
/* The oldReadObject method exists here for compatibility reasons.
* :: objects used to be serialized by serializing all the elements to
* the output stream directly, but this was broken (see SI-5374).
*/
private def oldReadObject(in: ObjectInputStream, firstObject: AnyRef) {
hd = firstObject.asInstanceOf[B]
assert(hd != ListSerializeEnd)
var current: ::[B] = this
while (true) in.readObject match {
case ListSerializeEnd =>
current.tl = Nil
return
case a : Any =>
val list : ::[B] = new ::(a.asInstanceOf[B], Nil)
current.tl = list
current = list
}
}
}
/** $factoryInfo
* @define coll list
* @define Coll `List`
*/
object List extends SeqFactory[List] {
import scala.collection.{Iterable, Seq, IndexedSeq}
/** $genericCanBuildFromInfo */
implicit def canBuildFrom[A]: CanBuildFrom[Coll, A, List[A]] =
ReusableCBF.asInstanceOf[GenericCanBuildFrom[A]]
def newBuilder[A]: Builder[A, List[A]] = new ListBuffer[A]
override def empty[A]: List[A] = Nil
override def apply[A](xs: A*): List[A] = xs.toList
}
/** Only used for list serialization */
@SerialVersionUID(0L - 8287891243975527522L)
private[scala] case object ListSerializeStart
/** Only used for list serialization */
@SerialVersionUID(0L - 8476791151975527571L)
private[scala] case object ListSerializeEnd
|