Skip to content
This repository
Fetching contributors…

Octocat-spinner-32-eaf2f5

Cannot retrieve contributors at this time

file 1130 lines (1033 sloc) 40.588 kb
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 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500 501 502 503 504 505 506 507 508 509 510 511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527 528 529 530 531 532 533 534 535 536 537 538 539 540 541 542 543 544 545 546 547 548 549 550 551 552 553 554 555 556 557 558 559 560 561 562 563 564 565 566 567 568 569 570 571 572 573 574 575 576 577 578 579 580 581 582 583 584 585 586 587 588 589 590 591 592 593 594 595 596 597 598 599 600 601 602 603 604 605 606 607 608 609 610 611 612 613 614 615 616 617 618 619 620 621 622 623 624 625 626 627 628 629 630 631 632 633 634 635 636 637 638 639 640 641 642 643 644 645 646 647 648 649 650 651 652 653 654 655 656 657 658 659 660 661 662 663 664 665 666 667 668 669 670 671 672 673 674 675 676 677 678 679 680 681 682 683 684 685 686 687 688 689 690 691 692 693 694 695 696 697 698 699 700 701 702 703 704 705 706 707 708 709 710 711 712 713 714 715 716 717 718 719 720 721 722 723 724 725 726 727 728 729 730 731 732 733 734 735 736 737 738 739 740 741 742 743 744 745 746 747 748 749 750 751 752 753 754 755 756 757 758 759 760 761 762 763 764 765 766 767 768 769 770 771 772 773 774 775 776 777 778 779 780 781 782 783 784 785 786 787 788 789 790 791 792 793 794 795 796 797 798 799 800 801 802 803 804 805 806 807 808 809 810 811 812 813 814 815 816 817 818 819 820 821 822 823 824 825 826 827 828 829 830 831 832 833 834 835 836 837 838 839 840 841 842 843 844 845 846 847 848 849 850 851 852 853 854 855 856 857 858 859 860 861 862 863 864 865 866 867 868 869 870 871 872 873 874 875 876 877 878 879 880 881 882 883 884 885 886 887 888 889 890 891 892 893 894 895 896 897 898 899 900 901 902 903 904 905 906 907 908 909 910 911 912 913 914 915 916 917 918 919 920 921 922 923 924 925 926 927 928 929 930 931 932 933 934 935 936 937 938 939 940 941 942 943 944 945 946 947 948 949 950 951 952 953 954 955 956 957 958 959 960 961 962 963 964 965 966 967 968 969 970 971 972 973 974 975 976 977 978 979 980 981 982 983 984 985 986 987 988 989 990 991 992 993 994 995 996 997 998 999 1000 1001 1002 1003 1004 1005 1006 1007 1008 1009 1010 1011 1012 1013 1014 1015 1016 1017 1018 1019 1020 1021 1022 1023 1024 1025 1026 1027 1028 1029 1030 1031 1032 1033 1034 1035 1036 1037 1038 1039 1040 1041 1042 1043 1044 1045 1046 1047 1048 1049 1050 1051 1052 1053 1054 1055 1056 1057 1058 1059 1060 1061 1062 1063 1064 1065 1066 1067 1068 1069 1070 1071 1072 1073 1074 1075 1076 1077 1078 1079 1080 1081 1082 1083 1084 1085 1086 1087 1088 1089 1090 1091 1092 1093 1094 1095 1096 1097 1098 1099 1100 1101 1102 1103 1104 1105 1106 1107 1108 1109 1110 1111 1112 1113 1114 1115 1116 1117 1118 1119 1120 1121 1122 1123 1124 1125 1126 1127 1128 1129
/* __ *\
** ________ ___ / / ___ Scala API **
** / __/ __// _ | / / / _ | (c) 2003-2011, LAMP/EPFL **
** __\ \/ /__/ __ |/ /__/ __ | http://scala-lang.org/ **
** /____/\___/_/ |_/____/_/ | | **
** |/ **
\* */

package scala.collection
package immutable

import generic._
import mutable.{Builder, StringBuilder, LazyBuilder, ListBuffer}
import scala.annotation.tailrec
import Stream.cons
import language.implicitConversions

/** The class `Stream` implements lazy lists where elements
* are only evaluated when they are needed. Here is an example:
*
* {{{
* import scala.math.BigInt
* object Main extends App {
*
* val fibs: Stream[BigInt] = BigInt(0) #:: BigInt(1) #:: fibs.zip(fibs.tail).map { n => n._1 + n._2 }
*
* fibs take 5 foreach println
* }
*
* // prints
* //
* // 0
* // 1
* // 1
* // 2
* // 3
* }}}
*
* The `Stream` class also employs memoization such that previously computed
* values are converted from `Stream` elements to concrete values of type `A`.
* To illustrate, we will alter body of the `fibs` value above and take some
* more values:
*
* {{{
* import scala.math.BigInt
* object Main extends App {
*
* val fibs: Stream[BigInt] = BigInt(0) #:: BigInt(1) #:: fibs.zip(
* fibs.tail).map(n => {
* println("Adding %d and %d".format(n._1, n._2))
* n._1 + n._2
* })
*
* fibs take 5 foreach println
* fibs take 6 foreach println
* }
*
* // prints
* //
* // 0
* // 1
* // Adding 0 and 1
* // 1
* // Adding 1 and 1
* // 2
* // Adding 1 and 2
* // 3
*
* // And then prints
* //
* // 0
* // 1
* // 1
* // 2
* // 3
* // Adding 2 and 3
* // 5
* }}}
*
* There are a number of subtle points to the above example.
*
* - The definition of `fibs` is a `val` not a method. The memoization of the
* `Stream` requires us to have somewhere to store the information and a `val`
* allows us to do that.
*
* - While the `Stream` is actually being modified during access, this does not
* change the notion of its immutability. Once the values are memoized they do
* not change and values that have yet to be memoized still "exist", they
* simply haven't been realized yet.
*
* - One must be cautious of memoization; you can very quickly eat up large
* amounts of memory if you're not careful. The reason for this is that the
* memoization of the `Stream` creates a structure much like
* [[scala.collection.immutable.List]]. So long as something is holding on to
* the head, the head holds on to the tail, and so it continues recursively.
* If, on the other hand, there is nothing holding on to the head (e.g. we used
* `def` to define the `Stream`) then once it is no longer being used directly,
* it disappears.
* {{{
* // For example, let's build the natural numbers and do some silly iteration
* // over them.
*
* // We'll start with a silly iteration
* def loop(s: String, i: Int, iter: Iterator[Int]): Unit = {
* // Stop after 200,000
* if (i < 200001) {
* if (i % 50000 == 0) println(s + i)
* loop(s, iter.next, iter)
* }
* }
*
* // Our first Stream definition will be a val definition
* val stream1: Stream[Int] = {
* def loop(v: Int): Stream[Int] = v #:: loop(v + 1)
* loop(0)
* }
*
* // Because stream1 is a val, everything that the iterator produces is held
* // by virtue of the fact that the head of the Stream is held in stream1
* val it1 = stream1.iterator
* loop("Iterator1: ", it1.next, it1)
*
* // We can redefine this Stream such that all we have is the Iterator left
* // and allow the Stream to be garbage collected as required. Using a def
* // to provide the Stream ensures that no val is holding onto the head as
* // is the case with stream1
* def stream2: Stream[Int] = {
* def loop(v: Int): Stream[Int] = v #:: loop(v + 1)
* loop(0)
* }
* val it2 = stream2.iterator
* loop("Iterator2: ", it2.next, it2)
*
* // And, of course, we don't actually need a Stream at all for such a simple
* // problem. There's no reason to use a Stream if you don't actually need
* // one.
* val it3 = new Iterator[Int] {
* var i = -1
* def hasNext = true
* def next(): Int = { i += 1; i }
* }
* loop("Iterator3: ", it3.next, it3)
* }}}
*
* - The fact that `tail` works at all is of interest. In the definition of
* `fibs` we have an initial `(0, 1, Stream(...))` so `tail` is deterministic.
* If we deinfed `fibs` such that only `0` were concretely known then the act
* of determining `tail` would require the evaluation of `tail` which would
* cause an infinite recursion and stack overflow. If we define a definition
* where the tail is not initially computable then we're going to have an
* infinite recursion:
* {{{
* // The first time we try to access the tail we're going to need more
* // information which will require us to recurse, which will require us to
* // recurse, which...
* val sov: Stream[Vector[Int]] = Vector(0) #:: sov.zip(sov.tail).map { n => n._1 ++ n._2 }
* }}}
*
* The definition of `fibs` above creates a larger number of objects than
* necessary depending on how you might want to implement it. The following
* implementation provides a more "cost effective" implementation due to the
* fact that it has a more direct route to the numbers themselves:
*
* {{{
* lazy val fib: Stream[Int] = {
* def loop(h: Int, n: Int): Stream[Int] = h #:: loop(n, h + n)
* loop(1, 1)
* }
* }}}
*
* @tparam A the type of the elements contained in this stream.
*
* @author Martin Odersky, Matthias Zenger
* @version 1.1 08/08/03
* @since 2.8
* @see [[http://docs.scala-lang.org/overviews/collections/concrete-immutable-collection-classes.html#streams "Scala's Collection Library overview"]]
* section on `Streams` for more information.

* @define naturalsEx def naturalsFrom(i: Int): Stream[Int] = i #:: naturalsFrom(i + 1)
* @define Coll Stream
* @define coll stream
* @define orderDependent
* @define orderDependentFold
*/
abstract class Stream[+A] extends AbstractSeq[A]
                             with LinearSeq[A]
                             with GenericTraversableTemplate[A, Stream]
                             with LinearSeqOptimized[A, Stream[A]] {
self =>
  override def companion: GenericCompanion[Stream] = Stream

  import scala.collection.{Traversable, Iterable, Seq, IndexedSeq}

  /** Indicates whether or not the `Stream` is empty.
*
* @return `true` if the `Stream` is empty and `false` otherwise.
*/
  def isEmpty: Boolean

  /** Gives constant time access to the first element of this `Stream`. Using
* the `fibs` example from earlier:
*
* {{{
* println(fibs head)
* // prints
* // 0
* }}}
*
* @return The first element of the `Stream`.
* @throws Predef.NoSuchElementException if the stream is empty.
*/
  def head: A

  /** A stream consisting of the remaining elements of this stream after the
* first one.
*
* Note that this method does not force evaluation of the `Stream` but merely
* returns the lazy result.
*
* @return The tail of the `Stream`.
* @throws Predef.UnsupportedOperationException if the stream is empty.
*/
  def tail: Stream[A]

  /** Is the tail of this stream defined? */
  protected def tailDefined: Boolean

  // Implementation of abstract method in Traversable

  // New methods in Stream

  /** The stream resulting from the concatenation of this stream with the argument stream.
* @param rest The stream that gets appended to this stream
* @return The stream containing elements of this stream and the traversable object.
*/
  def append[B >: A](rest: => TraversableOnce[B]): Stream[B] =
    if (isEmpty) rest.toStream else cons(head, tail append rest)

  /** Forces evaluation of the whole stream and returns it.
*
* @note Often we use `Stream`s to represent an infinite set or series. If
* that's the case for your particular `Stream` then this function will never
* return and will probably crash the VM with an `OutOfMemory` exception.
*
* @return The fully realized `Stream`.
*/
  def force: Stream[A] = {
    var these = this
    while (!these.isEmpty) these = these.tail
    this
  }

  /** Prints elements of this stream one by one, separated by commas. */
  def print() { print(", ") }

  /** Prints elements of this stream one by one, separated by `sep`.
* @param sep The separator string printed between consecutive elements.
*/
  def print(sep: String) {
    def loop(these: Stream[A], start: String) {
      Console.print(start)
      if (these.isEmpty) Console.print("empty")
      else {
        Console.print(these.head)
        loop(these.tail, sep)
      }
    }
    loop(this, "")
  }

  /** Returns the length of this `Stream`.
*
* @note In order to compute the length of the `Stream`, it must first be
* fully realized, which could cause the complete evaluation of an infinite
* series, assuming that's what your `Stream` represents.
*
* @return The length of this `Stream`.
*/
  override def length: Int = {
    var len = 0
    var left = this
    while (!left.isEmpty) {
      len += 1
      left = left.tail
    }
    len
  }

  /** It's an imperfect world, but at least we can bottle up the
* imperfection in a capsule.
*/
  @inline private def asThat[That](x: AnyRef): That = x.asInstanceOf[That]
  @inline private def asStream[B](x: AnyRef): Stream[B] = x.asInstanceOf[Stream[B]]
  @inline private def isStreamBuilder[B, That](bf: CanBuildFrom[Stream[A], B, That]) =
    bf(repr).isInstanceOf[Stream.StreamBuilder[_]]

  // Overridden methods from Traversable

  override def toStream: Stream[A] = this

  override def hasDefiniteSize = {
    def loop(s: Stream[A]): Boolean = s.isEmpty || s.tailDefined && loop(s.tail)
    loop(this)
  }

  /** Create a new stream which contains all elements of this stream followed by
* all elements of Traversable `that`.
*
* @note It's subtle why this works. We know that if the target type of the
* [[scala.collection.mutable.Builder]] `That` is either a `Stream`, or one of
* its supertypes, or undefined, then `StreamBuilder` will be chosen for the
* implicit. We recognize that fact and optimize to get more laziness.
*
* @note This method doesn't cause the `Stream` to be fully realized but it
* should be noted that using the `++` operator from another collection type
* could cause infinite realization of a `Stream`. For example, referring to
* the definition of `fibs` in the preamble, the following would never return:
* `List(BigInt(12)) ++ fibs`.
*
* @tparam B The element type of the returned collection.'''That'''
* @param that The [[scala.collection.GenTraversableOnce]] the be contatenated
* to this `Stream`.
* @return A new collection containing the result of concatenating `this` with
* `that`.
*/
  override def ++[B >: A, That](that: GenTraversableOnce[B])(implicit bf: CanBuildFrom[Stream[A], B, That]): That =
    // we assume there is no other builder factory on streams and therefore know that That = Stream[A]
    if (isStreamBuilder(bf)) asThat(
      if (isEmpty) that.toStream
      else cons(head, asStream[A](tail ++ that))
    )
    else super.++(that)(bf)

  override def +:[B >: A, That](elem: B)(implicit bf: CanBuildFrom[Stream[A], B, That]): That =
    if (isStreamBuilder(bf)) asThat(cons(elem, this))
    else super.+:(elem)(bf)

  /**
* Create a new stream which contains all intermediate results of applying the
* operator to subsequent elements left to right. `scanLeft` is analogous to
* `foldLeft`.
*
* @note This works because the target type of the
* [[scala.collection.mutable.Builder]] `That` is a `Stream`.
*
* @param z The initial value for the scan.
* @param op A function that will apply operations to successive values in the
* `Stream` against previous accumulated results.
* @return A new collection containing the modifications from the application
* of `op`.
*/
  override final def scanLeft[B, That](z: B)(op: (B, A) => B)(implicit bf: CanBuildFrom[Stream[A], B, That]): That =
    if (isStreamBuilder(bf)) asThat(
      if (isEmpty) Stream(z)
      else cons(z, asStream[B](tail.scanLeft(op(z, head))(op)))
    )
    else super.scanLeft(z)(op)(bf)

  /** Returns the stream resulting from applying the given function `f` to each
* element of this stream. This returns a lazy `Stream` such that it does not
* need to be fully realized.
*
* @example {{{
* $naturalsEx
* naturalsFrom(1).map(_ + 10) take 5 mkString(", ")
* // produces: "11, 12, 13, 14, 15"
* }}}
*
* @tparam B The element type of the returned collection '''That'''.
* @param f function to apply to each element.
* @return `f(a,,0,,), ..., f(a,,n,,)` if this sequence is `a,,0,,, ..., a,,n,,`.
*/
  override final def map[B, That](f: A => B)(implicit bf: CanBuildFrom[Stream[A], B, That]): That = {
    if (isStreamBuilder(bf)) asThat(
      if (isEmpty) Stream.Empty
      else cons(f(head), asStream[B](tail map f))
    )
    else super.map(f)(bf)
  }

  override final def collect[B, That](pf: PartialFunction[A, B])(implicit bf: CanBuildFrom[Stream[A], B, That]): That = {
    if (!isStreamBuilder(bf)) super.collect(pf)(bf)
    else {
      // this implementation avoids:
      // 1) stackoverflows (could be achieved with tailrec, too)
      // 2) out of memory errors for big streams (`this` reference can be eliminated from the stack)
      var rest: Stream[A] = this
      while (rest.nonEmpty && !pf.isDefinedAt(rest.head)) rest = rest.tail

      // without the call to the companion object, a thunk is created for the tail of the new stream,
      // and the closure of the thunk will reference `this`
      if (rest.isEmpty) Stream.Empty.asInstanceOf[That]
      else Stream.collectedTail(rest, pf, bf).asInstanceOf[That]
    }
  }

  /** Applies the given function `f` to each element of this stream, then
* concatenates the results. As with `map` this function does not need to
* realize the entire `Stream` but continues to keep it as a lazy `Stream`.
*
* @example {{{
* // Let's create a Stream of Vectors, each of which contains the
* // collection of Fibonacci numbers up to the current value. We
* // can then 'flatMap' that Stream.
*
* val fibVec: Stream[Vector[Int]] = Vector(0) #:: Vector(0, 1) #:: fibVec.zip(fibVec.tail).map(n => {
* n._2 ++ Vector(n._1.last + n._2.last)
* })
*
* fibVec take 5 foreach println
* // prints
* // Vector(0)
* // Vector(0, 1)
* // Vector(0, 1, 1)
* // Vector(0, 1, 1, 2)
* // Vector(0, 1, 1, 2, 3)
*
* // If we now want to `flatMap` across that stream by adding 10
* // we can see what the series turns into:
*
* fibVec.flatMap(_.map(_ + 10)) take 15 mkString(", ")
* // produces: 10, 10, 11, 10, 11, 11, 10, 11, 11, 12, 10, 11, 11, 12, 13
* }}}
*
* @tparam B The element type of the returned collection '''That'''.
* @param f the function to apply on each element.
* @return `f(a,,0,,) ::: ... ::: f(a,,n,,)` if
* this stream is `[a,,0,,, ..., a,,n,,]`.
*/
  override final def flatMap[B, That](f: A => GenTraversableOnce[B])(implicit bf: CanBuildFrom[Stream[A], B, That]): That =
    // we assume there is no other builder factory on streams and therefore know that That = Stream[B]
    // optimisations are not for speed, but for functionality
    // see tickets #153, #498, #2147, and corresponding tests in run/ (as well as run/stream_flatmap_odds.scala)
    if (isStreamBuilder(bf)) asThat(
      if (isEmpty) Stream.Empty
      else {
        // establish !prefix.isEmpty || nonEmptyPrefix.isEmpty
        var nonEmptyPrefix = this
        var prefix = f(nonEmptyPrefix.head).toStream
        while (!nonEmptyPrefix.isEmpty && prefix.isEmpty) {
          nonEmptyPrefix = nonEmptyPrefix.tail
          if(!nonEmptyPrefix.isEmpty)
            prefix = f(nonEmptyPrefix.head).toStream
        }

        if (nonEmptyPrefix.isEmpty) Stream.empty
        else prefix append asStream[B](nonEmptyPrefix.tail flatMap f)
      }
    )
    else super.flatMap(f)(bf)

  /** Returns all the elements of this `Stream` that satisfy the predicate `p`
* in a new `Stream` - i.e. it is still a lazy data structure. The order of
* the elements is preserved
*
* @param p the predicate used to filter the stream.
* @return the elements of this stream satisfying `p`.
*
* @example {{{
* $naturalsEx
* naturalsFrom(1) 10 } filter { _ % 5 == 0 } take 10 mkString(", ")
* // produces
* }}}
*/
  override def filter(p: A => Boolean): Stream[A] = {
    // optimization: drop leading prefix of elems for which f returns false
    // var rest = this dropWhile (!p(_)) - forget DRY principle - GC can't collect otherwise
    var rest = this
    while (!rest.isEmpty && !p(rest.head)) rest = rest.tail
    // private utility func to avoid `this` on stack (would be needed for the lazy arg)
    if (rest.nonEmpty) Stream.filteredTail(rest, p)
    else Stream.Empty
  }

  override final def withFilter(p: A => Boolean): StreamWithFilter = new StreamWithFilter(p)

  /** A lazier implementation of WithFilter than TraversableLike's.
*/
  final class StreamWithFilter(p: A => Boolean) extends WithFilter(p) {

    override def map[B, That](f: A => B)(implicit bf: CanBuildFrom[Stream[A], B, That]): That = {
      def tailMap = asStream[B](tail withFilter p map f)
      if (isStreamBuilder(bf)) asThat(
        if (isEmpty) Stream.Empty
        else if (p(head)) cons(f(head), tailMap)
        else tailMap
      )
      else super.map(f)(bf)
    }

    override def flatMap[B, That](f: A => GenTraversableOnce[B])(implicit bf: CanBuildFrom[Stream[A], B, That]): That = {
      def tailFlatMap = asStream[B](tail withFilter p flatMap f)
      if (isStreamBuilder(bf)) asThat(
        if (isEmpty) Stream.Empty
        else if (p(head)) f(head).toStream append tailFlatMap
        else tailFlatMap
      )
      else super.flatMap(f)(bf)
    }

    override def foreach[B](f: A => B) =
      for (x <- self)
        if (p(x)) f(x)

    override def withFilter(q: A => Boolean): StreamWithFilter =
      new StreamWithFilter(x => p(x) && q(x))
  }

  /** A lazier Iterator than LinearSeqLike's. */
  override def iterator: Iterator[A] = new StreamIterator(self)

  /** Apply the given function `f` to each element of this linear sequence
* (while respecting the order of the elements).
*
* @param f The treatment to apply to each element.
* @note Overridden here as final to trigger tail-call optimization, which
* replaces 'this' with 'tail' at each iteration. This is absolutely
* necessary for allowing the GC to collect the underlying stream as elements
* are consumed.
* @note This function will force the realization of the entire stream
* unless the `f` throws an exception.
*/
  @tailrec
  override final def foreach[B](f: A => B) {
    if (!this.isEmpty) {
      f(head)
      tail.foreach(f)
    }
  }

  /** Stream specialization of foldLeft which allows GC to collect along the
* way.
*
* @tparam B The type of value being accumulated.
* @param z The initial value seeded into the function `op`.
* @param op The operation to perform on successive elements of the `Stream`.
* @return The accumulated value from successive applications of `op`.
*/
  @tailrec
  override final def foldLeft[B](z: B)(op: (B, A) => B): B = {
    if (this.isEmpty) z
    else tail.foldLeft(op(z, head))(op)
  }

  /** Stream specialization of reduceLeft which allows GC to collect
* along the way.
*
* @tparam B The type of value being accumulated.
* @param f The operation to perform on successive elements of the `Stream`.
* @return The accumulated value from successive applications of `f`.
*/
  override final def reduceLeft[B >: A](f: (B, A) => B): B = {
    if (this.isEmpty) throw new UnsupportedOperationException("empty.reduceLeft")
    else {
      var reducedRes: B = this.head
      var left = this.tail
      while (!left.isEmpty) {
        reducedRes = f(reducedRes, left.head)
        left = left.tail
      }
      reducedRes
    }
  }

  /** Returns all the elements of this stream that satisfy the predicate `p`
* returning of [[scala.Tuple2]] of `Stream`s obeying the partition predicate
* `p`. The order of the elements is preserved.
*
* @param p the predicate used to filter the stream.
* @return the elements of this stream satisfying `p`.
*
* @example {{{
* $naturalsEx
* val parts = naturalsFrom(1) partition { _ % 2 == 0 }
* parts._1 take 10 mkString ", "
* // produces: "2, 4, 6, 8, 10, 12, 14, 16, 18, 20"
* parts._2 take 10 mkString ", "
* // produces: "1, 3, 5, 7, 9, 11, 13, 15, 17, 19"
* }}}
*
*/
  override def partition(p: A => Boolean): (Stream[A], Stream[A]) = (filter(p(_)), filterNot(p(_)))

  /** Returns a stream formed from this stream and the specified stream `that`
* by associating each element of the former with the element at the same
* position in the latter.
*
* If one of the two streams is longer than the other, its remaining elements
* are ignored.
*
* The return type of this function may not be obvious. The lazy aspect of
* the returned value is different than that of `partition`. In `partition`
* we get back a [[scala.Tuple2]] of two lazy `Stream`s whereas here we get
* back a single lazy `Stream` of [[scala.Tuple2]]s where the
* [[scala.Tuple2]]'s type signature is `(A1, B)`.
*
* @tparam A1 The type of the first parameter of the zipped tuple
* @tparam B The type of the second parameter of the zipped tuple
* @tparam That The type of the returned `Stream`.
* @return `Stream({a,,0,,,b,,0,,}, ...,
* {a,,min(m,n),,,b,,min(m,n),,)}` when
* `Stream(a,,0,,, ..., a,,m,,)
* zip Stream(b,,0,,, ..., b,,n,,)` is invoked.
*
* @example {{{
* $naturalsEx
* naturalsFrom(1) zip naturalsFrom(2) zip take 5 foreach println
* // prints
* // (1,2)
* // (2,3)
* // (3,4)
* // (4,5)
* // (5,6)
* }}}
*/
  override final def zip[A1 >: A, B, That](that: collection.GenIterable[B])(implicit bf: CanBuildFrom[Stream[A], (A1, B), That]): That =
    // we assume there is no other builder factory on streams and therefore know that That = Stream[(A1, B)]
    if (isStreamBuilder(bf)) asThat(
      if (this.isEmpty || that.isEmpty) Stream.Empty
      else cons((this.head, that.head), asStream[(A1, B)](this.tail zip that.tail))
    )
    else super.zip(that)(bf)

  /** Zips this iterable with its indices. `s.zipWithIndex` is equivalent to `s
* zip s.indices`.
*
* This method is much like `zip` in that it returns a single lazy `Stream` of
* [[scala.Tuple2]].
*
* @tparam A1 The type of the first element of the [[scala.Tuple2]] in the
* resulting stream.
* @tparam That The type of the resulting `Stream`.
* @return `Stream({a,,0,,,0}, ..., {a,,n,,,n)}`
*
* @example {{{
* $naturalsEx
* (naturalsFrom(1) zipWithIndex) take 5 foreach println
* // prints
* // (1,0)
* // (2,1)
* // (3,2)
* // (4,3)
* // (5,4)
* }}}
*/
  override def zipWithIndex[A1 >: A, That](implicit bf: CanBuildFrom[Stream[A], (A1, Int), That]): That =
    this.zip[A1, Int, That](Stream.from(0))

  /** Write all defined elements of this iterable into given string builder.
* The written text begins with the string `start` and is finished by the string
* `end`. Inside, the string representations of defined elements (w.r.t.
* the method `toString()`) are separated by the string `sep`. The method will
* not force evaluation of undefined elements. A tail of such elements will be
* represented by a `"?"` instead.
*
* @param b The [[collection.mutable.StringBuilder]] factory to which we need
* to add the string elements.
* @param start The prefix of the resulting string (e.g. "Stream(")
* @param sep The separator between elements of the resulting string (e.g. ",")
* @param end The end of the resulting string (e.g. ")")
* @return The original [[collection.mutable.StringBuilder]] containing the
* resulting string.
*/
  override def addString(b: StringBuilder, start: String, sep: String, end: String): StringBuilder = {
    def loop(pre: String, these: Stream[A]) {
      if (these.isEmpty) b append end
      else {
        b append pre append these.head
        if (these.tailDefined) loop(sep, these.tail)
        else b append sep append "?" append end
      }
    }
    b append start
    loop("", this)
    b
  }

  override def mkString(sep: String): String = mkString("", sep, "")
  override def mkString: String = mkString("")
  override def mkString(start: String, sep: String, end: String): String = {
    this.force
    super.mkString(start, sep, end)
  }
  override def toString = super.mkString(stringPrefix + "(", ", ", ")")

  override def splitAt(n: Int): (Stream[A], Stream[A]) = (take(n), drop(n))

  /** Returns the `n` first elements of this `Stream` as another `Stream`, or
* else the whole `Stream`, if it has less than `n` elements.
*
* The result of `take` is, again, a `Stream` meaning that it also does not
* make any needless evaluations of the `Stream` itself, delaying that until
* the usage of the resulting `Stream`.
*
* @param n the number of elements to take.
* @return the `n` first elements of this stream.
*
* @example {{{
* $naturalsEx
* scala> naturalsFrom(5) take 5
* res1: scala.collection.immutable.Stream[Int] = Stream(5, ?)
*
* scala> naturalsFrom(5) take 5 mkString ", "
* // produces: "5, 6, 7, 8, 9"
* }}}
*/
  override def take(n: Int): Stream[A] =
    if (n <= 0 || isEmpty) Stream.empty
    else if (n == 1) cons(head, Stream.empty)
    else cons(head, tail take n-1)

  @tailrec final override def drop(n: Int): Stream[A] =
    if (n <= 0 || isEmpty) this
    else tail drop n-1

  /** A substream starting at index `from` and extending up to (but not including)
* index `until`. This returns a `Stream` that is lazily evaluated.
*
* @param start The index of the first element of the returned subsequence
* @param end The index of the element following the returned subsequence
* @return A new string containing the elements requested from `start` until
* `end`.
*
* @example {{{
* naturalsFrom(0) slice(50, 60) mkString ", "
* // produces: "50, 51, 52, 53, 54, 55, 56, 57, 58, 59"
* }}}
*/
  override def slice(from: Int, until: Int): Stream[A] = {
    val lo = from max 0
    if (until <= lo || isEmpty) Stream.empty
    else this drop lo take (until - lo)
  }

  /** The stream without its last element.
*
* @return A new `Stream` containing everything but the last element. If your
* `Stream` represents an infinite series, this method will not return.
*
* @throws `Predef.UnsupportedOperationException` if the stream is empty.
*/
  override def init: Stream[A] =
    if (isEmpty) super.init
    else if (tail.isEmpty) Stream.Empty
    else cons(head, tail.init)

  /** Returns the rightmost `n` elements from this iterable.
*
* @note Take serious caution here. If the `Stream` represents an infinite
* series then this function ''will not return''. The right most elements of
* an infinite series takes an infinite amount of time to produce.
*
* @param n the number of elements to take
* @return The last `n` elements from this `Stream`.
*/
  override def takeRight(n: Int): Stream[A] = {
    var these: Stream[A] = this
    var lead = this drop n
    while (!lead.isEmpty) {
      these = these.tail
      lead = lead.tail
    }
    these
  }

  // there's nothing we can do about dropRight, so we just keep the definition
  // in LinearSeq

  /** Returns the longest prefix of this `Stream` whose elements satisfy the
* predicate `p`.
*
* @param p the test predicate.
* @return A new `Stream` representing the values that satisfy the predicate
* `p`.
*
* @example {{{
+ naturalsFrom(0) takeWhile { _ < 5 } mkString ", "
* produces: "0, 1, 2, 3, 4"
* }}}
*/
  override def takeWhile(p: A => Boolean): Stream[A] =
    if (!isEmpty && p(head)) cons(head, tail takeWhile p)
    else Stream.Empty

  /** Returns the a `Stream` representing the longest suffix of this iterable
* whose first element does not satisfy the predicate `p`.
*
* @note This method realizes the entire `Stream` beyond the truth value of
* the predicate `p`.
*
* @param p the test predicate.
* @return A new `Stream` representing the results of applying `p` to the
* oringal `Stream`.
*
* @example {{{
* // Assume we have a Stream that takes the first 20 natural numbers
* def naturalsLt50(i: Int): Stream[Int] = i #:: { if (i < 20) naturalsLt50(i * + 1) else Stream.Empty }
* naturalsLt50(0) dropWhile { _ < 10 }
* // produces: "10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20"
* }}}
*/
  override def dropWhile(p: A => Boolean): Stream[A] = {
    var these: Stream[A] = this
    while (!these.isEmpty && p(these.head)) these = these.tail
    these
  }

  /** Builds a new stream from this stream in which any duplicates (wrt to ==)
* have been removed. Among duplicate elements, only the first one is
* retained in the resulting `Stream`.
*
* @return A new `Stream` representing the result of applying distinctness to
* the original `Stream`.
* @example {{{
* // Creates a Stream where every element is duplicated
* def naturalsFrom(i: Int): Stream[Int] = i #:: { i #:: naturalsFrom(i + 1) }
* naturalsFrom(1) take 6 mkString ", "
* // produces: "1, 1, 2, 2, 3, 3"
* (naturalsFrom(1) distinct) take 6 mkString ", "
* // produces: "1, 2, 3, 4, 5, 6"
* }}}
*/
  override def distinct: Stream[A] =
    if (isEmpty) this
    else cons(head, tail.filter(head != _).distinct)

  /** Returns a new sequence of given length containing the elements of this
* sequence followed by zero or more occurrences of given elements.
*
* @tparam B The type of the value to pad with.
* @tparam That The type contained within the resulting `Stream`.
* @param len The number of elements to pad into the `Stream`.
* @param elem The value of the type `B` to use for padding.
* @return A new `Stream` representing the collection with values padding off
* to the end. If your `Stream` represents an infinite series, this method will
* not return.
* @example {{{
* def naturalsFrom(i: Int): Stream[Int] = i #:: { if (i < 5) naturalsFrom(i + 1) else Stream.Empty }
* naturalsFrom(1) padTo(10, 0) foreach println
* // prints
* // 1
* // 2
* // 3
* // 4
* // 5
* // 0
* // 0
* // 0
* // 0
* // 0
* }}}
*/
  override def padTo[B >: A, That](len: Int, elem: B)(implicit bf: CanBuildFrom[Stream[A], B, That]): That = {
    def loop(len: Int, these: Stream[A]): Stream[B] =
      if (these.isEmpty) Stream.fill(len)(elem)
      else cons(these.head, loop(len - 1, these.tail))

    if (isStreamBuilder(bf)) asThat(loop(len, this))
    else super.padTo(len, elem)(bf)
  }

  /** A list consisting of all elements of this list in reverse order.
*
* @note This function must realize the entire `Stream` in order to perform
* this operation so if your `Stream` represents an infinite sequence then
* this function will never return.
*
* @return A new `Stream` containing the representing of the original `Stream`
* in reverse order.
*
* @example {{{
* def naturalsFrom(i: Int): Stream[Int] = i #:: { if (i < 5) naturalsFrom(i + 1) else Stream.Empty }
* (naturalsFrom(1) reverse) foreach println
* // prints
* // 5
* // 4
* // 3
* // 2
* // 1
* }}}
*/
  override def reverse: Stream[A] = {
    var result: Stream[A] = Stream.Empty
    var these = this
    while (!these.isEmpty) {
      val r = Stream.consWrapper(result).#::(these.head)
      r.tail // force it!
      result = r
      these = these.tail
    }
    result
  }

  /** Evaluates and concatenates all elements within the `Stream` into a new
* flattened `Stream`.
*
* @tparam B The type of the elements of the resulting `Stream`.
* @return A new `Stream` of type `B` of the flattened elements of `this`
* `Stream`.
* @example {{{
* val sov: Stream[Vector[Int]] = Vector(0) #:: Vector(0, 0) #:: sov.zip(sov.tail).map { n => n._1 ++ n._2 }
* sov flatten take 10 mkString ", "
* // produces: "0, 0, 0, 0, 0, 0, 0, 0, 0, 0"
* }}}
*/
  override def flatten[B](implicit asTraversable: A => /*<:<!!!*/ GenTraversableOnce[B]): Stream[B] = {
    def flatten1(t: Traversable[B]): Stream[B] =
      if (!t.isEmpty)
        cons(t.head, flatten1(t.tail))
      else
        tail.flatten

    if (isEmpty) Stream.empty
    else flatten1(asTraversable(head).seq.toTraversable)
  }

  override def view = new StreamView[A, Stream[A]] {
    protected lazy val underlying = self.repr
    override def iterator = self.iterator
    override def length = self.length
    override def apply(idx: Int) = self.apply(idx)
  }

  /** Defines the prefix of this object's `toString` representation as `Stream`.
*/
  override def stringPrefix = "Stream"

}

/** A specialized, extra-lazy implementation of a stream iterator, so it can
* iterate as lazily as it traverses the tail.
*/
final class StreamIterator[+A] private() extends AbstractIterator[A] with Iterator[A] {
  def this(self: Stream[A]) {
    this()
    these = new LazyCell(self)
  }

  // A call-by-need cell.
  class LazyCell(st: => Stream[A]) {
    lazy val v = st
  }

  private var these: LazyCell = _

  def hasNext: Boolean = these.v.nonEmpty
  def next(): A =
    if (isEmpty) Iterator.empty.next
    else {
      val cur = these.v
      val result = cur.head
      these = new LazyCell(cur.tail)
      result
    }
  override def toStream = {
    val result = these.v
    these = new LazyCell(Stream.empty)
    result
  }
  override def toList = toStream.toList
}

/**
* The object `Stream` provides helper functions to manipulate streams.
*
* @author Martin Odersky, Matthias Zenger
* @version 1.1 08/08/03
* @since 2.8
*/
object Stream extends SeqFactory[Stream] {

  /** The factory for streams.
* @note Methods such as map/flatMap will not invoke the `Builder` factory,
* but will return a new stream directly, to preserve laziness.
* The new stream is then cast to the factory's result type.
* This means that every CanBuildFrom that takes a
* Stream as its From type parameter must yield a stream as its result parameter.
* If that assumption is broken, cast errors might result.
*/
  class StreamCanBuildFrom[A] extends GenericCanBuildFrom[A]

  implicit def canBuildFrom[A]: CanBuildFrom[Coll, A, Stream[A]] = new StreamCanBuildFrom[A]

  /** Creates a new builder for a stream */
  def newBuilder[A]: Builder[A, Stream[A]] = new StreamBuilder[A]

  import scala.collection.{Iterable, Seq, IndexedSeq}

  /** A builder for streams
* @note This builder is lazy only in the sense that it does not go downs the spine
* of traversables that are added as a whole. If more laziness can be achieved,
* this builder should be bypassed.
*/
  class StreamBuilder[A] extends scala.collection.mutable.LazyBuilder[A, Stream[A]] {
    def result: Stream[A] = parts.toStream flatMap (_.toStream)
  }

  object Empty extends Stream[Nothing] with Serializable {
    override def isEmpty = true
    override def head = throw new NoSuchElementException("head of empty stream")
    override def tail = throw new UnsupportedOperationException("tail of empty stream")
    def tailDefined = false
  }

  /** The empty stream */
  override def empty[A]: Stream[A] = Empty

  /** A stream consisting of given elements */
  override def apply[A](xs: A*): Stream[A] = xs.toStream

  /** A wrapper class that adds `#::` for cons and `#:::` for concat as operations
* to streams.
*/
  class ConsWrapper[A](tl: => Stream[A]) {
    def #::(hd: A): Stream[A] = cons(hd, tl)
    def #:::(prefix: Stream[A]): Stream[A] = prefix append tl
  }

  /** A wrapper method that adds `#::` for cons and `#::: for concat as operations
* to streams.
*/
  implicit def consWrapper[A](stream: => Stream[A]): ConsWrapper[A] =
    new ConsWrapper[A](stream)

  /** An extractor that allows to pattern match streams with `#::`.
*/
  object #:: {
    def unapply[A](xs: Stream[A]): Option[(A, Stream[A])] =
      if (xs.isEmpty) None
      else Some((xs.head, xs.tail))
  }

  /** An alternative way of building and matching Streams using Stream.cons(hd, tl).
*/
  object cons {

    /** A stream consisting of a given first element and remaining elements
* @param hd The first element of the result stream
* @param tl The remaining elements of the result stream
*/
    def apply[A](hd: A, tl: => Stream[A]) = new Cons(hd, tl)

    /** Maps a stream to its head and tail */
    def unapply[A](xs: Stream[A]): Option[(A, Stream[A])] = #::.unapply(xs)
  }

  /** A lazy cons cell, from which streams are built. */
  @SerialVersionUID(-602202424901551803L)
  final class Cons[+A](hd: A, tl: => Stream[A]) extends Stream[A] with Serializable {
    override def isEmpty = false
    override def head = hd
    @volatile private[this] var tlVal: Stream[A] = _
    def tailDefined: Boolean = tlVal ne null
    override def tail: Stream[A] = {
      if (!tailDefined)
        synchronized {
          if (!tailDefined) tlVal = tl
        }

      tlVal
    }
  }

  /** An infinite stream that repeatedly applies a given function to a start value.
*
* @param start the start value of the stream
* @param f the function that's repeatedly applied
* @return the stream returning the infinite sequence of values `start, f(start), f(f(start)), ...`
*/
  def iterate[A](start: A)(f: A => A): Stream[A] = cons(start, iterate(f(start))(f))

  override def iterate[A](start: A, len: Int)(f: A => A): Stream[A] =
    iterate(start)(f) take len

  /**
* Create an infinite stream starting at `start` and incrementing by
* step `step`.
*
* @param start the start value of the stream
* @param step the increment value of the stream
* @return the stream starting at value `start`.
*/
  def from(start: Int, step: Int): Stream[Int] =
    cons(start, from(start+step, step))

  /**
* Create an infinite stream starting at `start` and incrementing by `1`.
*
* @param start the start value of the stream
* @return the stream starting at value `start`.
*/
  def from(start: Int): Stream[Int] = from(start, 1)

  /**
* Create an infinite stream containing the given element expression (which
* is computed for each occurrence).
*
* @param elem the element composing the resulting stream
* @return the stream containing an infinite number of elem
*/
  def continually[A](elem: => A): Stream[A] = cons(elem, continually(elem))

  override def fill[A](n: Int)(elem: => A): Stream[A] =
    if (n <= 0) Empty else cons(elem, fill(n-1)(elem))

  override def tabulate[A](n: Int)(f: Int => A): Stream[A] = {
    def loop(i: Int): Stream[A] =
      if (i >= n) Empty else cons(f(i), loop(i+1))
    loop(0)
  }

  override def range[T: Integral](start: T, end: T, step: T): Stream[T] = {
    val num = implicitly[Integral[T]]
    import num._

    if (if (step < zero) start <= end else end <= start) Empty
    else cons(start, range(start + step, end, step))
  }

  private[immutable] def filteredTail[A](stream: Stream[A], p: A => Boolean) = {
    cons(stream.head, stream.tail filter p)
  }

  private[immutable] def collectedTail[A, B, That](stream: Stream[A], pf: PartialFunction[A, B], bf: CanBuildFrom[Stream[A], B, That]) = {
    cons(pf(stream.head), stream.tail.collect(pf)(bf).asInstanceOf[Stream[B]])
  }
}


Something went wrong with that request. Please try again.