<br><br><div class="gmail_quote">On Mon, May 28, 2012 at 11:13 PM, Frank Shearar <span dir="ltr"><<a href="mailto:frank.shearar@gmail.com" target="_blank">frank.shearar@gmail.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
<div><div>On 28 May 2012 17:43, Colin Putney <<a href="mailto:colin@wiresong.com" target="_blank">colin@wiresong.com</a>> wrote:<br>
><br>
> On 2012-05-28, at 7:59 AM, Frank Shearar wrote:<br>
><br>
>>> That would be a nice property, but hash functions are not injective. If they<br>
>>> were, then either the codomain would be too large (not SmallInteger in our<br>
>>> case, which makes hashing impractical) or there were no need for the use of<br>
>>> hashing at all, since there were no collisions.<br>
>><br>
>> Sure, collisions mean that you can have a ~= b and yet a hash = b<br>
>> hash. Nevertheless, CompiledMethod define: #hash as: [^ 1] satisfies<br>
>> the above test, so on its own it's insufficient. I wouldn't ask for<br>
>> testing CM x CM - {CompiledMethods whose hashes collide} !<br>
><br>
> 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.<br>
<br>
</div></div>I just want a test demonstrating that, even if it's just for carefully<br>
constructed CompiledMethods, sometime cmA hash ~= cmB hash when cmA ~=<br>
cmB. At the moment the test thoroughly demonstrates half of the hash's<br>
behaviour - when things are =, their hashes are =.<br>
<br>
I agree that demonstrating the collision rate of the hash function is<br>
beyond the scope of a unit test.<br></blockquote><div><br></div><div>How about adding a count that e.g. demands that the number of distinct hashes is better than half the number of distinct CompiledMethods. e.g.</div><div>
<br></div><div><div>testHash</div><div><span style="white-space:pre-wrap"> </span>| ai |</div><div><span style="white-space:pre-wrap"> </span>ai := CompiledMethod allInstances.</div><div>
<span style="white-space:pre-wrap"> </span>ai do:</div><div><span style="white-space:pre-wrap"> </span>[:a|</div><div><span style="white-space:pre-wrap"> </span>ai do:</div>
<div><span style="white-space:pre-wrap"> </span>[:b|</div><div><span style="white-space:pre-wrap"> </span>a = b ifTrue: [self assert: a hash = b hash]]].</div></div><div><span style="white-space:pre-wrap"> </span>self assert: (ai collect: [:cm| cm hash]) asSet size * 2 >= ai asSet size</div>
</div>-- <br>best,<div>Eliot</div><br>