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Stream#permutations could produce incorrect value of Iterator[Stream] #5377

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scabug opened this issue Jan 14, 2012 · 3 comments
Closed

Stream#permutations could produce incorrect value of Iterator[Stream] #5377

scabug opened this issue Jan 14, 2012 · 3 comments
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@scabug scabug commented Jan 14, 2012

I wrote following program to check behavior of Stream#permutations() and it compiled with scalac.

{code:title=StreamPermutationBug.scala|borderStyle=solid}
object StreamPermutationBug {
def main(args: Array[String]) {
val list = List(1, 2)
println("from: " + list)

// expected result
list.permutations.map(_.toList).foreach(println)

println("---")

println("from: " + list.toStream)
// unexpected result
list.toStream.permutations.map(_.toList).foreach(println)

println("---")

val range = Range(1, 3)
println("from: " + range) 
// unexpected result
range.toStream.permutations.map(_.toList).foreach(println) 

}
}
{code}

However, the output of this program is unexpected:

{panel:title=Expected Result| borderStyle=dashed| borderColor=#ccc| titleBGColor=#F7D6C1| bgColor=#FFFFCE}
from: List(1, 2)
List(1, 2)
List(2, 1)

from: Stream(1, ?)
List(1, 2)
List(2, 1)

from: Range(1, 2)
List(1, 2)
List(2, 1)
{panel}

{panel:title=Actual Result(unexpected)| borderStyle=dashed| borderColor=#ccc| titleBGColor=#F7D6C1| bgColor=#FFFFCE}
from: List(1, 2)
List(1, 2) // expected
List(2, 1)

from: Stream(1, ?)
List(1, 1) // unexpected!
List(2, 1)

from: Range(1, 2)
List(1, 1) // unexpected!
List(2, 1)
{panel}

It seems that application of "permutations" method to Stream instances could produce incorrect value of Iterator of Stream.
This problem is reproducable in REPL, too.

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@scabug scabug commented Jan 14, 2012

Imported From: https://issues.scala-lang.org/browse/SI-5377?orig=1
Reporter: @kmizu
Affected Versions: 2.9.1

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@scabug scabug commented Jan 24, 2012

@axel22 said:
Another stream-related source of fun - the permutations iterator constructs the collection by adding the current elements of the buffer into a builder. If the builder is lazy, it will add the elements from the buffer into the collection only after the elements in the buffer get swapped around.

@scabug scabug closed this Jan 24, 2012
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@scabug scabug commented Jan 26, 2012

@kmizu said:
Note that the fix on this issue (I wrote) was marged into master branch already:

commit 26afbf85bf17e3839f03cb9e2d981cdccdd2ee69
Author: Kota Mizushima mizukota@gmail.com
Date: Mon Jan 16 23:20:07 2012 +0900

Fixed a bug SI-5377 (https://issues.scala-lang.org/browse/SI-5377).

Added test cases for SI-5377. 
@scabug scabug added this to the 2.10.0 milestone Apr 7, 2017
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