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Make case class hashCode take class into account #2159

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Merged
merged 4 commits into from
Apr 3, 2017
Merged

Make case class hashCode take class into account #2159

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f801c83
Make case class hashCode take class into account
odersky Mar 30, 2017
6aba35f
Optimize hashCode computation
odersky Mar 31, 2017
aa2522c
Use full name of case classes for hashing
odersky Mar 31, 2017
710e700
Merge branch 'master' into fix-hashcode
felixmulder Apr 1, 2017
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5 changes: 3 additions & 2 deletions compiler/src/dotty/tools/dotc/transform/SyntheticMethods.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -144,20 +144,21 @@ class SyntheticMethods(thisTransformer: DenotTransformer) {

/** The class
*
* package p
* case class C(x: T, y: T)
*
* gets the hashCode method:
*
* def hashCode: Int = {
* <synthetic> var acc: Int = 0xcafebabe;
* <synthetic> var acc: Int = "p.C".hashCode // constant folded
* acc = Statics.mix(acc, x);
* acc = Statics.mix(acc, Statics.this.anyHash(y));
* Statics.finalizeHash(acc, 2)
* }
*/
def caseHashCodeBody(implicit ctx: Context): Tree = {
val acc = ctx.newSymbol(ctx.owner, "acc".toTermName, Mutable | Synthetic, defn.IntType, coord = ctx.owner.pos)
val accDef = ValDef(acc, Literal(Constant(0xcafebabe)))
val accDef = ValDef(acc, Literal(Constant(clazz.fullName.toString.hashCode)))
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scalac uses productPrefix (which is user definable), rather than the class name.

What is the definition of fullName for non-member classes?

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scalac uses productPrefix (which is user definable), rather than the class name.

What's the motivation behind this?
Product prefix isn't required to even return the same value, so one can break hash-code by returning different values there.

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@retronym retronym Apr 3, 2017 •
edited
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Neither are the case accessors (they could be vars).

I guess a motivation would be to have more stable hash codes for a local case class so that it wasn't volatile wrt fresh name suffixes. Perhaps this implementation acheives the same by inlining fullName, rather than calling this.getClass.toString).

I'm just pointing out the deviation.

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AFAIK, The full name here won't include any fresh name suffices. They will be only added after Flatten. Two local classes with the same name will get the same constant.

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Neither are the case accessors (they could be vars).

I see now, though, that a mutable productPrefix would be worse that a var accessor, as equals does not check that this.productPrefix == that.productPrefix.

val mixes = for (accessor <- accessors.toList) yield
Assign(ref(acc), ref(defn.staticsMethod("mix")).appliedTo(ref(acc), hashImpl(accessor)))
val finish = ref(defn.staticsMethod("finalizeHash")).appliedTo(ref(acc), Literal(Constant(accessors.size)))
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8 changes: 4 additions & 4 deletions tests/run/caseClassHash.check
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@@ -1,9 +1,9 @@
Foo(true,-1,-1,d,-5,-10,500.0,500.0,List(),5.0)
Foo(true,-1,-1,d,-5,-10,500.0,500.0,List(),5)
1383698062
1383698062
205963949
205963949
true
## method 1: 1383698062
## method 2: 1383698062
## method 1: 205963949
## method 2: 205963949
Murmur 1: 1383698062
Murmur 2: 1383698062