Integrated contributed non-recursive implementation of permutations,

combinations, subsets, by EastSun.  Also gave mutable Seqs an
in-place transform method like the one Map has.  And couldn't
resist slightly reformulating a few set methods, because how can we
settle for "forall(that.contains)" when we could have "this forall that".
(Which is also what normal people hear when we talk about sets.)

Closes #4060, #3644, review by moors.

git-svn-id: http://lampsvn.epfl.ch/svn-repos/scala/scala/trunk@24042 5e8d7ff9-d8ef-0310-90f0-a4852d11357a

src/library/scala/collection/SeqLike.scala

@@ -372,18 +372,127 @@ trait SeqLike[+A, +Repr] extends IterableLike[A, Repr] { self =>
    *  @return   An Iterator which traverses the distinct permutations of this $coll.
    *  @example  `"abb".permutations = Iterator(abb, bab, bba)`
    */
-  def permutations: Iterator[Repr] = {
-    val seen = mutable.HashSet[A]()
-    val xs = thisCollection.toIndexedSeq
+  def permutations: Iterator[Repr] = 
+    if (isEmpty) Iterator(repr)
+    else new PermutationsItr
   
-    if (xs.isEmpty) Iterator.empty
-    else if (xs.tail.isEmpty) Iterator(repr)
-    else xs.indices collect {
-      case idx if !seen(xs(idx))  =>
-        seen += xs(idx)
-        val rest = (xs take idx) ++ (xs drop (idx + 1))
-        rest.permutations map (newBuilder += xs(idx) ++= _ result)
-    } reduceLeft (_ ++ _)
+  /** Iterates over combinations.
+   *
+   *  @return   An Iterator which traverses the possible n-element combinations of this $coll.
+   *  @example  `"abbbc".combinations(2) = Iterator(ab, ac, bb, bc)`
+   */
+  def combinations(n: Int): Iterator[Repr] =
+    if (n < 0 || n > size) Iterator.empty
+    else new CombinationsItr(n)
+  
+  private class PermutationsItr extends Iterator[Repr] {
+    private[this] val (elms, idxs) = init()
+    private var _hasNext = true
+    
+    def hasNext = _hasNext
+    def next: Repr = {
+      if (!hasNext)
+        Iterator.empty.next
+
+      val result = (self.newBuilder ++= elms).result
+      var i = idxs.length - 2
+      while(i >= 0 && idxs(i) >= idxs(i+1))
+        i -= 1
+        
+      if (i < 0)
+        _hasNext = false
+      else {
+        var j = idxs.length - 1
+        while(idxs(j) <= idxs(i)) j -= 1
+          swap(i,j)
+
+        val len = (idxs.length - i) / 2
+        var k = 1
+        while (k <= len) {
+          swap(i+k, idxs.length - k)
+          k += 1
+        }
+      }
+      result
+    }
+    private def swap(i: Int, j: Int) {
+      var tmpI = idxs(i)
+      idxs(i) = idxs(j)
+      idxs(j) = tmpI
+      var tmpE = elms(i)
+      elms(i) = elms(j)
+      elms(j) = tmpE
+    }
+    
+    private[this] def init() = {
+      val m = mutable.HashMap[A, Int]()
+      val (es, is) = thisCollection map (e => (e, m.getOrElseUpdate(e, m.size))) sortBy (_._2) unzip
+
+      (es.toBuffer, is.toArray)
+    }
+  }
+    
+  private class CombinationsItr(n: Int) extends Iterator[Repr] {
+    // generating all nums such that:
+    // (1) nums(0) + .. + nums(length-1) = n
+    // (2) 0 <= nums(i) <= cnts(i), where 0 <= i <= cnts.length-1
+    private val (elms, cnts, nums) = init()
+    private val offs = cnts.scanLeft(0)(_ + _)
+    private var _hasNext = true
+
+    def hasNext = _hasNext
+    def next: Repr = {
+      if (!hasNext)
+        Iterator.empty.next
+      
+      /** Calculate this result. */
+      val buf = self.newBuilder
+      for(k <- 0 until nums.length; j <- 0 until nums(k))
+        buf += elms(offs(k)+j)
+      val res = buf.result
+      
+      /** Prepare for the next call to next. */
+      var idx = nums.length - 1
+      while (idx >= 0 && nums(idx) == cnts(idx))
+        idx -= 1
+
+      idx = nums.lastIndexWhere(_ > 0, idx - 1)
+      
+      if (idx < 0)
+        _hasNext = false
+      else {
+        var sum = nums.slice(idx + 1, nums.length).sum + 1
+        nums(idx) -= 1
+        for (k <- (idx+1) until nums.length) {
+          nums(k) = sum min cnts(k)
+          sum -= nums(k)
+        }
+      }
+      
+      res
+    }
+    
+    /** Rearrange seq to newSeq a0a0..a0a1..a1...ak..ak such that
+     *  seq.count(_ == aj) == cnts(j)
+     *
+     *  @return     (newSeq,cnts,nums)
+     */
+    private def init(): (IndexedSeq[A], Array[Int], Array[Int]) = {
+      val m = mutable.HashMap[A, Int]()
+
+      // e => (e, weight(e))
+      val (es, is) = thisCollection map (e => (e, m.getOrElseUpdate(e, m.size))) sortBy (_._2) unzip
+      val cs = new Array[Int](m.size)
+      is foreach (i => cs(i) += 1)
+      val ns = new Array[Int](cs.length)
+      
+      var r = n
+      0 until ns.length foreach { k =>
+        ns(k) = r min cs(k)
+        r -= ns(k)
+      }
+      (es.toIndexedSeq, cs, ns)
+    }
   }
 
   /** Returns new $coll wih elements in reversed order.

src/library/scala/collection/SetLike.scala

@@ -141,7 +141,7 @@ self =>
    *  @param elem the element to test for membership.
    *  @return  `true` if `elem` is contained in this set, `false` otherwise.
    */
-  def apply(elem: A): Boolean = contains(elem)
+  def apply(elem: A): Boolean = this contains elem
 
   /** Computes the intersection between this set and another set.
    *
@@ -149,7 +149,7 @@ self =>
    *  @return  a new set consisting of all elements that are both in this
    *  set and in the given set `that`. 
    */
-  def intersect(that: Set[A]): This = filter(that.contains)
+  def intersect(that: Set[A]): This = this filter that
 
   /** Computes the intersection between this set and another set.
    *
@@ -158,7 +158,7 @@ self =>
    *  @return  a new set consisting of all elements that are both in this
    *  set and in the given set `that`. 
    */
-  def &(that: Set[A]): This = intersect(that)
+  def &(that: Set[A]): This = this intersect that
 
  /**  This method is an alias for `intersect`. 
    *  It computes an intersection with set `that`.
@@ -166,7 +166,8 @@ self =>
    *
    *  @param that the set to intersect with
    */
-  @deprecated("use & instead") def ** (that: Set[A]): This = intersect(that)
+  @deprecated("use & instead")
+  def ** (that: Set[A]): This = &(that)
   
   /** Computes the union between of set and another set.
    *
@@ -174,7 +175,7 @@ self =>
    *  @return  a new set consisting of all elements that are in this
    *  set or in the given set `that`. 
    */
-  def union(that: Set[A]): This = this.++(that)
+  def union(that: Set[A]): This = this ++ that
 
   /** Computes the union between this set and another set.
    *
@@ -183,7 +184,7 @@ self =>
    *  @return  a new set consisting of all elements that are in this
    *  set or in the given set `that`. 
    */
-  def | (that: Set[A]): This = union(that)
+  def | (that: Set[A]): This = this union that
 
   /** Computes the difference of this set and another set.
    *
@@ -191,7 +192,7 @@ self =>
    *  @return     a set containing those elements of this
    *              set that are not also contained in the given set `that`.
    */
-  def diff(that: Set[A]): This = --(that)
+  def diff(that: Set[A]): This = this -- that
 
   /** The difference of this set and another set.
    *
@@ -200,7 +201,7 @@ self =>
    *  @return     a set containing those elements of this
    *              set that are not also contained in the given set `that`.
    */
-  def &~(that: Set[A]): This = diff(that)
+  def &~(that: Set[A]): This = this diff that
 
   /** Tests whether this set is a subset of another set.
    *
@@ -208,7 +209,74 @@ self =>
    *  @return     `true` if this set is a subset of `that`, i.e. if
    *              every element of this set is also an element of `that`.
    */
-  def subsetOf(that: Set[A]): Boolean = forall(that.contains)
+  def subsetOf(that: Set[A]) = this forall that
+  
+  /** An iterator over all subsets of this set of the given size.
+   *
+   *  @param len  the size of the subsets.
+   *  @return     the iterator.
+   */
+  def subsets(len: Int): Iterator[This] = {
+    if (len < 0 || len > size) throw new IllegalArgumentException(len.toString)
+    else new SubsetsItr(self.toIndexedSeq, len)
+  }
+
+  /** An iterator over all subsets of this set.
+   *
+   *  @return     the iterator.
+   */
+  def subsets: Iterator[This] = new Iterator[This] {
+    private val elms = self.toIndexedSeq
+    private var len = 0
+    private var itr: Iterator[This] = Iterator.empty
+
+    def hasNext = len <= elms.size || itr.hasNext
+    def next = {
+      if (!itr.hasNext) {
+        if (len > elms.size) Iterator.empty.next
+        else {
+          itr = new SubsetsItr(elms, len)
+          len += 1
+        }
+      }
+      
+      itr.next
+    }
+  }
+    
+  /** An Iterator include all subsets containing exactly len elements.
+   *  If the elements in 'This' type is ordered, then the subsets will also be in the same order.
+   *  ListSet(1,2,3).subsets => {1},{2},{3},{1,2},{1,3},{2,3},{1,2,3}}
+   *
+   *  @author Eastsun
+   *  @date 2010.12.6
+   */
+  private class SubsetsItr(elms: IndexedSeq[A], len: Int) extends Iterator[This] {
+    private val idxs = Array.range(0, len+1)
+    private var _hasNext = true
+    idxs(len) = elms.size
+
+    def hasNext = _hasNext
+    def next: This = {
+      if (!hasNext) Iterator.empty.next
+
+      val buf = self.newBuilder
+      idxs.slice(0, len) foreach (idx => buf += elms(idx))
+      val result = buf.result
+
+      var i = len - 1
+      while (i >= 0 && idxs(i) == idxs(i+1)-1) i -= 1
+
+      if (i < 0) _hasNext = false
+      else {
+        idxs(i) += 1
+        for (j <- (i+1) until len)
+          idxs(j) = idxs(j-1) + 1
+      }
+      
+      result
+    }
+  }
 
   /** Defines the prefix of this object's `toString` representation.
    *  @return  a string representation which starts the result of `toString` applied to this set.

src/library/scala/collection/immutable/BitSet.scala

@@ -79,8 +79,6 @@ object BitSet extends BitSetFactory[BitSet] {
     else new BitSetN(elems)
   }
 
-  private val hashSeed = "BitSet".hashCode
-
   class BitSet1(val elems: Long) extends BitSet {
     protected def nwords = 1
     protected def word(idx: Int) = if (idx == 0) elems else 0L

src/library/scala/collection/immutable/IntMap.scala

@@ -55,14 +55,14 @@ object IntMap {
     // develops.  Case objects and custom equality don't mix without
     // careful handling.
     override def equals(that : Any) = that match {
-      case (that : AnyRef) if (this eq that) => true;
-      case (that : IntMap[_]) => false; // The only empty IntMaps are eq Nil
-      case that => super.equals(that);
+      case _: this.type => true
+      case _: IntMap[_] => false // The only empty IntMaps are eq Nil
+      case _            => super.equals(that)
     }
-  };
+  }
 
   private[immutable] case class Tip[+T](key : Int, value : T) extends IntMap[T]{
-    def withValue[S](s : S) = 
+    def withValue[S](s: S) =
       if (s.asInstanceOf[AnyRef] eq value.asInstanceOf[AnyRef]) this.asInstanceOf[IntMap.Tip[S]];
       else IntMap.Tip(key, s);
   }

src/library/scala/collection/mutable/SeqLike.scala

@@ -28,4 +28,19 @@ trait SeqLike[A, +This <: SeqLike[A, This] with Seq[A]]
    *  @throws   IndexOutofBoundsException if the index is not valid.
    */
   def update(idx: Int, elem: A)
+  
+  /** Applies a transformation function to all values contained in this sequence.
+   *  The transformation function produces new values from existing elements.
+   * 
+   * @param f  the transformation to apply
+   * @return   the sequence itself.
+   */
+  def transform(f: A => A): this.type = {
+    var i = 0
+    iterator foreach { el =>
+      update(i, f(el))
+      i += 1
+    }
+    this
+  }
 }

test/files/run/transform.scala

@@ -0,0 +1,8 @@
+object Test {
+  val x = 1 to 10 toBuffer
+  
+  def main(args: Array[String]): Unit = {
+    x transform (_ * 2)
+    assert(x.sum == (1 to 10).sum * 2)
+  }
+}