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Scala hash code of some Double objects is different when it is seen as Double and when it is seen as Any #5640

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scabug opened this Issue Apr 2, 2012 · 9 comments

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scabug commented Apr 2, 2012

The same object d below has different scala hash code
when it is seen as static type Double and when it is seen as static type Any .

def testHashHash {
  val d = (BigInt(1) << 64).toDouble
  val a: Any = d
  println("d.hashCode="+d.hashCode)
  println("d.##="+d.##)
  println("a.##="+a.##)
  assert(d.## == a.##)
}

This is because BoxesRunTime.hashFromDouble(java.lang.Double n) and ScalaRunTime.hash(dv: Double) don't match.

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scabug commented Apr 2, 2012

Imported From: https://issues.scala-lang.org/browse/SI-5640?orig=1
Reporter: @nadezhin
Affected Versions: 2.9.1, 2.10.0
Other Milestones: 2.10.0

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scabug commented May 8, 2012

@magarciaEPFL said:
Minimized further:

scala> val d = (BigInt(1) << 64).toDouble
d: Double = 1.8446744073709552E19

scala> val a: Any = d
a: Any = 1.8446744073709552E19

scala> scala.Double.box(d).hashCode()
res0: Int = 1139802112

scala> scala.runtime.ScalaRunTime.hash(d)
res1: Int = 1602224128

scala> scala.runtime.ScalaRunTime.hash(a)
res2: Int = 1139802112
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scabug commented May 8, 2012

@dgruntz said:
Actually, a and d have the same hashCode. It is ## which returns different results than hashCode for boxed numeric types (and for null) as documented.

scala> val d = (BigInt(1) << 64).toDouble
d: Double = 1.8446744073709552E19

scala> val a: Any = d
a: Any = 1.8446744073709552E19

scala> d.hashCode
res22: Int = 1139802112

scala> a.hashCode
res23: Int = 1139802112

So if this behavior is changed, do not forget to update the docs of ## as well.

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scabug commented May 8, 2012

@paulp said (edited on May 8, 2012 10:50:50 PM UTC):

## isn't supposed to give different answers based on the static type of the argument.
That (x: Any).## != (x: Foo).## is definitely a bug.  This is already recognized in the doc for ##.
"if two value type instances compare as true, then ## will produce the same hash value for each of them."
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scabug commented May 8, 2012

@paulp said:
Oh yeah, I forgot how irreconcilable this situation is. What I said is only true up to about 16 bits of width, then it becomes impossible.

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scabug commented May 8, 2012

@paulp said:
58bb2d1bd2

@scabug scabug closed this May 8, 2012

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scabug commented May 9, 2012

@dgruntz said:
runtime.BoxesRunTime.hashFromDouble(d) still returns the wrong value. Should that also be fixed, or is this method no longer used anywhere (in the context of ##)?
I claim that hashFromDouble should also check whether the argument can be represented correctly within a float and then return that hashCode.

scala> runtime.BoxesRunTime.hashFromNumber(d)
res16: Int = 1139802112

scala> runtime.BoxesRunTime.hashFromNumber(d.toFloat)
res17: Int = 1602224128

scala> d == d.toFloat
res18: Boolean = true
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scabug commented May 9, 2012

@paulp said:
Sorry, I'm doing things in a bit of a rush. I see there are redundant implementations in BoxesRunTime and ScalaRunTime, don't know why. If you are up for sending a pull request I'd be thrilled.

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scabug commented May 9, 2012

@dgruntz said:
really? here it is: b5919100e7

@scabug scabug added the minimized label Apr 7, 2017

@scabug scabug added this to the 2.10.0-M3 milestone Apr 7, 2017

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