[druntime] TypeInfo.getHash problems

[H. S. Teoh]

On Thu, Mar 01, 2012 at 12:36:31PM +0400, Dmitry Olshansky wrote:
> On 01.03.2012 1:45, H. S. Teoh wrote:
> >I found the following bugs with the current implementations of
> >getHash in the various TypeInfo classes:
> >
> >1) TypeInfo_Array.getHash doesn't work correctly with class objects.
> >with custom toHash() methods. Or, for that matter, arrays of AA's
> >(which have their own problems--see (3a)). It just does a hash of the
> >binary representation of the array.
> >
> >2) TypeInfo_StaticArray.getHash doesn't suffer from (1), but it does
> >suffer from another problem: it simply sums the hashes from each
> >element, but this is prone to reordering hash collisions: an int[3]
> >with elements [1,2,3] will have the same hash value as [3,2,1] or any
> >reordering thereof.
> >
> >3) TypeInfo_AssociativeArray.getHash is not implemented, so it just
> >defaults to the base class getHash, which just does a hash of the
> >binary representation of the AA. Unfortunately, this is fraught with
> >problems:
> >
> >	a) Two AA's with exactly the same key/value pairs may be binary
> >	unequal, because the underlying size of the hash table may be
> >	different (this happens when you initialize an AA with a literal
> >	vs. manually setting each element).
> >
> >	b) If values are objects that implement toHash(), then the
> >	current implementation will have the wrong behaviour.
> >
> >4) TypeInfo_Class.getHash will call the object's toHash method, if
> >implemented, otherwise it hashes the binary representation of the
> >object. But if the object has members that are references/pointers or
> >other objects/structs that implement getHash, this will be wrong.
[...]
> >To fix (1) may cause noticeable slowdown when the key is an array of
> >basic types, such as dstring (we'd have to compute the hash of each
> >dchar, then compute the hash of the hashes).  Is there an easy way to
> >tell whether or not an array's elements are binary-hashable? If so,
> >we can still use hashing of the binary representation of the array
> >for the simple cases (dstring, wstring, array of structs without
> >nested refs/objects), and save the slower code for more complicated
> >arrays.
> >
> 
> std.traits hasIndirections?

This has to be fixed in druntime, though. Is there a druntime equivalent
to std.traits?


> >For (2), we may be able to incorporate the array index into the hash
> >summation so as to make it resistant to reordering collisions. But it
> >may be that we might as well just compute the hash of the hashes at
> >this point?
> 
> I suspect arrays should be usually hashed as strings, e.g. a simplistic
> hash = seed;
> foreach(x; arr)
>    hash = mix(hash, x);//if x is not integral use x.toHash()
> hash = mix(hash, arr.length);
> 
> where mix(x) is k*x+b, or (x<<k) ^ b or whatever.

Good idea!


> >For (3), we can collect the hashes of the keys (already stored in the
> >bucket) and the hashes of the values, and doing a hash on them. (This
> >seems to be the easiest one to fix?)
> >
> Or hash them as array of pairs, but whatever is faster.

I did a quick implementation of this:

	https://github.com/D-Programming-Language/druntime/pull/171

Basically do a hash of the respective hash values of the key and value,
and sum them over all pairs. The sum is commutative, so it avoids the
problem of two AA's with identical contents but different internal
hashtable sizes causing pair reordering.


> >To fix (4) will require compile-time reflection... but may introduce
> >slowdowns? Is there a way to detect the easy cases (binary-hashable)
> >so that we only incur slowdown on the complicated cases?
> >
> 
> see  point 1.
[...]

It would be good if there was a clean way to do this in druntime.


T

-- 
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