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Standard library for the Scala Programming Language
/* __ *\
** ________ ___ / / ___ Scala API **
** / __/ __// _ | / / / _ | (c) 2003-2013, 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
*/
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)
}
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
}
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)
}
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)
}
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)
@inline override final
def foreach[B](f: A => B) {
var these = this
while (!these.isEmpty) {
f(these.head)
these = these.tail
}
}
@deprecated("use `distinct` instead", "2.8.0")
def removeDuplicates: List[A] = distinct
}
/** 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
}
}
private def oldWriteObject(out: ObjectOutputStream) {
var xs: List[B] = this
while (!xs.isEmpty) { out.writeObject(xs.head); xs = xs.tail }
out.writeObject(ListSerializeEnd)
}
}
/** $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
/** Create a sorted list with element values `v,,>n+1,, = step(v,,n,,)`
* where `v,,0,, = start` and elements are in the range between `start`
* (inclusive) and `end` (exclusive).
*
* @param start the start value of the list
* @param end the end value of the list
* @param step the increment function of the list, which given `v,,n,,`,
* computes `v,,n+1,,`. Must be monotonically increasing
* or decreasing.
* @return the sorted list of all integers in range `[start;end)`.
*/
@deprecated("use `iterate` instead", "2.8.0")
def range(start: Int, end: Int, step: Int => Int): List[Int] = {
val up = step(start) > start
val down = step(start) < start
val b = new ListBuffer[Int]
var i = start
while ((!up || i < end) && (!down || i > end)) {
b += i
val next = step(i)
if (i == next)
throw new IllegalArgumentException("the step function did not make any progress on "+ i)
i = next
}
b.toList
}
/** Create a list containing several copies of an element.
*
* @param n the length of the resulting list
* @param elem the element composing the resulting list
* @return a list composed of `n` elements all equal to `elem`
*/
@deprecated("use `fill` instead", "2.8.0")
def make[A](n: Int, elem: A): List[A] = {
val b = new ListBuffer[A]
var i = 0
while (i < n) {
b += elem
i += 1
}
b.toList
}
/** Concatenate all the elements of a given list of lists.
*
* @param xss the list of lists that are to be concatenated
* @return the concatenation of all the lists
*/
@deprecated("use `xss.flatten` instead of `List.flatten(xss)`", "2.8.0")
def flatten[A](xss: List[List[A]]): List[A] = {
val b = new ListBuffer[A]
for (xs <- xss) {
var xc = xs
while (!xc.isEmpty) {
b += xc.head
xc = xc.tail
}
}
b.toList
}
/** Transforms a list of pairs into a pair of lists.
*
* @param xs the list of pairs to unzip
* @return a pair of lists.
*/
@deprecated("use `xs.unzip` instead of `List.unzip(xs)`", "2.8.0")
def unzip[A,B](xs: List[(A,B)]): (List[A], List[B]) = {
val b1 = new ListBuffer[A]
val b2 = new ListBuffer[B]
var xc = xs
while (!xc.isEmpty) {
b1 += xc.head._1
b2 += xc.head._2
xc = xc.tail
}
(b1.toList, b2.toList)
}
/** Transforms an iterable of pairs into a pair of lists.
*
* @param xs the iterable of pairs to unzip
* @return a pair of lists.
*/
@deprecated("use `xs.unzip` instead of `List.unzip(xs)`", "2.8.0")
def unzip[A,B](xs: Iterable[(A,B)]): (List[A], List[B]) =
xs.foldRight[(List[A], List[B])]((Nil, Nil)) {
case ((x, y), (xs, ys)) => (x :: xs, y :: ys)
}
/**
* Returns the `Left` values in the given `Iterable` of `Either`s.
*/
@deprecated("use `xs collect { case Left(x: A) => x }` instead of `List.lefts(xs)`", "2.8.0")
def lefts[A, B](es: Iterable[Either[A, B]]) =
es.foldRight[List[A]](Nil)((e, as) => e match {
case Left(a) => a :: as
case Right(_) => as
})
/**
* Returns the `Right` values in the given `Iterable` of `Either`s.
*/
@deprecated("use `xs collect { case Right(x: B) => x }` instead of `List.rights(xs)`", "2.8.0")
def rights[A, B](es: Iterable[Either[A, B]]) =
es.foldRight[List[B]](Nil)((e, bs) => e match {
case Left(_) => bs
case Right(b) => b :: bs
})
/** Transforms an Iterable of Eithers into a pair of lists.
*
* @param es the iterable of Eithers to separate
* @return a pair of lists.
*/
@deprecated("use `(for (Left(x) <- es) yield x, for (Right(x) <- es) yield x)` instead", "2.8.0")
def separate[A,B](es: Iterable[Either[A, B]]): (List[A], List[B]) =
es.foldRight[(List[A], List[B])]((Nil, Nil)) {
case (Left(a), (lefts, rights)) => (a :: lefts, rights)
case (Right(b), (lefts, rights)) => (lefts, b :: rights)
}
/** Converts an iterator to a list.
*
* @param it the iterator to convert
* @return a list that contains the elements returned by successive
* calls to `it.next`
*/
@deprecated("use `it.toList` instead of `List.toList(it)`", "2.8.0")
def fromIterator[A](it: Iterator[A]): List[A] = it.toList
/** Converts an array into a list.
*
* @param arr the array to convert
* @return a list that contains the same elements than `arr`
* in the same order
*/
@deprecated("use `array.toList` instead of `List.fromArray(array)`", "2.8.0")
def fromArray[A](arr: Array[A]): List[A] = fromArray(arr, 0, arr.length)
/** Converts a range of an array into a list.
*
* @param arr the array to convert
* @param start the first index to consider
* @param len the length of the range to convert
* @return a list that contains the same elements than `arr`
* in the same order
*/
@deprecated("use `array.view(start, end).toList` instead of `List.fromArray(array, start, end)`", "2.8.0")
def fromArray[A](arr: Array[A], start: Int, len: Int): List[A] = {
var res: List[A] = Nil
var i = start + len
while (i > start) {
i -= 1
res = arr(i) :: res
}
res
}
/** Returns the list resulting from applying the given function `f`
* to corresponding elements of the argument lists.
*
* @param f function to apply to each pair of elements.
* @return `[f(a,,0,,,b,,0,,), ..., f(a,,n,,,b,,n,,)]` if the lists are
* `[a,,0,,, ..., a,,k,,]`, `[b,,0,,, ..., b,,l,,]` and
* `n = min(k,l)`
*/
@deprecated("use `(xs, ys).zipped.map(f)` instead of `List.map2(xs, ys)(f)`", "2.8.0")
def map2[A,B,C](xs: List[A], ys: List[B])(f: (A, B) => C): List[C] = {
val b = new ListBuffer[C]
var xc = xs
var yc = ys
while (!xc.isEmpty && !yc.isEmpty) {
b += f(xc.head, yc.head)
xc = xc.tail
yc = yc.tail
}
b.toList
}
/** Tests whether the given predicate `p` holds
* for all corresponding elements of the argument lists.
*
* @param f function to apply to each pair of elements.
* @return `(p(a0,b0) &&
* ... && p(an,bn))]`
* if the lists are `[a0, ..., ak]`;
* `[b0, ..., bl]`
* and `n = min(k,l)`
*/
@deprecated("use `(xs, ys).zipped.forall(f)` instead of `List.forall2(xs, ys)(f)`", "2.8.0")
def forall2[A,B](xs: List[A], ys: List[B])(f: (A, B) => Boolean): Boolean = {
var xc = xs
var yc = ys
while (!xc.isEmpty && !yc.isEmpty) {
if (!f(xc.head, yc.head)) return false
xc = xc.tail
yc = yc.tail
}
true
}
/** Tests whether the given predicate `p` holds
* for some corresponding elements of the argument lists.
*
* @param f function to apply to each pair of elements.
* @return `n != 0 && (p(a0,b0) ||
* ... || p(an,bn))]` if the lists are
* `[a0, ..., ak]`,
* `[b0, ..., bl]` and
* `n = min(k,l)`
*/
@deprecated("use `(xs, ys).zipped.exists(f)` instead of `List.exists2(xs, ys)(f)`", "2.8.0")
def exists2[A,B](xs: List[A], ys: List[B])(f: (A, B) => Boolean): Boolean = {
var xc = xs
var yc = ys
while (!xc.isEmpty && !yc.isEmpty) {
if (f(xc.head, yc.head)) return true
xc = xc.tail
yc = yc.tail
}
false
}
/** Transposes a list of lists.
* pre: All element lists have the same length.
*
* @param xss the list of lists
* @return the transposed list of lists
*/
@deprecated("use `xss.transpose` instead of `List.transpose(xss)`", "2.8.0")
def transpose[A](xss: List[List[A]]): List[List[A]] = {
val buf = new ListBuffer[List[A]]
var yss = xss
while (!yss.head.isEmpty) {
buf += (yss map (_.head))
yss = (yss map (_.tail))
}
buf.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