The Trunk: KernelTests-eem.224.mcz

> Eliot Miranda uploaded a new version of KernelTests to project The Trunk:
> http://source.squeak.org/trunk/KernelTests-eem.224.mcz
>
> ==================== Summary ====================
>
> Name: KernelTests-eem.224
> Author: eem
> Time: 27 May 2012, 3:16:16.618 pm
> UUID: 2d674fd0-22e3-4b5c-82ae-af5642c9614f
> Ancestors: KernelTests-nice.223
>
> Add a long test case for CompiledMethod = & hash
>
> =============== Diff against KernelTests-nice.223 ===============
>
> Item was added:
> + LongTestCase subclass: #CompiledMethodComparisonTest
> +       instanceVariableNames: ''
> +       classVariableNames: ''
> +       poolDictionaries: ''
> +       category: 'KernelTests-Methods'!
>
> Item was added:
> + ----- Method: CompiledMethodComparisonTest>>testHash (in category 'tests') -----
> + testHash
> +       | ai |
> +       ai := CompiledMethod allInstances.
> +       ai do:
> +               [:a|
> +               ai do:
> +                       [:b|
> +                       a = b ifTrue: [self assert: a hash = b hash]]]!
>
>

> On Mon, 28 May 2012, Frank Shearar wrote:
>
>> On 27 May 2012 23:16,  <[hidden email]> wrote:
>>>
>>> Eliot Miranda uploaded a new version of KernelTests to project The Trunk:
>>> http://source.squeak.org/trunk/KernelTests-eem.224.mcz
>>>
>>> ==================== Summary ====================
>>>
>>> Name: KernelTests-eem.224
>>> Author: eem
>>> Time: 27 May 2012, 3:16:16.618 pm
>>> UUID: 2d674fd0-22e3-4b5c-82ae-af5642c9614f
>>> Ancestors: KernelTests-nice.223
>>>
>>> Add a long test case for CompiledMethod = & hash
>>>
>>> =============== Diff against KernelTests-nice.223 ===============
>>>
>>> Item was added:
>>> + LongTestCase subclass: #CompiledMethodComparisonTest
>>> +       instanceVariableNames: ''
>>> +       classVariableNames: ''
>>> +       poolDictionaries: ''
>>> +       category: 'KernelTests-Methods'!
>>>
>>> Item was added:
>>> + ----- Method: CompiledMethodComparisonTest>>testHash (in category
>>> 'tests') -----
>>> + testHash
>>> +       | ai |
>>> +       ai := CompiledMethod allInstances.
>>> +       ai do:
>>> +               [:a|
>>> +               ai do:
>>> +                       [:b|
>>> +                       a = b ifTrue: [self assert: a hash = b hash]]]!
>>>
>>>
>>
>> Not necessarily here, but it'd be good to see that (a = b) ifFalse:
>> [self deny: a hash = b hash] holds.
>
>
> That would be a nice property, but hash functions are not injective. If they
> were, then either the codomain would be too large (not SmallInteger in our
> case, which makes hashing impractical) or there were no need for the use of
> hashing at all, since there were no collisions.

>
> On 2012-05-28, at 7:59 AM, Frank Shearar wrote:
>
>>> That would be a nice property, but hash functions are not injective. If they
>>> were, then either the codomain would be too large (not SmallInteger in our
>>> case, which makes hashing impractical) or there were no need for the use of
>>> hashing at all, since there were no collisions.
>>
>> Sure, collisions mean that you can have a ~= b and yet a hash = b
>> hash. Nevertheless, CompiledMethod define: #hash as: [^ 1] satisfies
>> the above test, so on its own it's insufficient. I wouldn't ask for
>> testing CM x CM  - {CompiledMethods whose hashes collide} !
>
> So, essentially, you want a test that ensures that we have a high-quality hash function. That would be nice, but I'm not sure it fits into the structure of a unit test. Porting Andres' hash function tools to Squeak would probably be the best way to do that. Short of that, I'd suggest a simple smoke test - say, asserting that there are few collisions between the methods of TestCase or something like that.