Re: [Pharo-project] IdentitySet but using #hash rather than #identityHash ?
On 16.12.2011 00:40, Henrik Sperre Johansen wrote:
On 15.12.2011 23:35, Mariano Martinez Peck wrote:
Well...it is much slower :( it seems that the cost of (aKey identityHash + ( aKey mareaClass identityHash bitShift: 12) + (aKey basicSize bitShift: 24) is bigger than the colisions. Anyway, thanks for the nice thread. I learned.
Cheers
Well, first of all, that always creates a largeInteger, since identityHash is already shifted by 22... You'd want to be using basicIdentityHash. 2nd off, basicSize is useless for non-variable classes, which I guess is the common case. 3rd, putting class hash in the high bits won't really help much if you're serializing many instances of the same class, as they will all occupy a sequential hash range.
So if I were you, I'd try with something like obj basicIdentityHash << 12 + (obj class basicIdentityHash) before discarding it outright due to computation cost.
Cheers, Henry
Note: This is not portable to Squeak. They kept the old version of identityHash (renamed to basicIdentityHash in Pharo), and introduced scaledIdentityHash (doing the same as identityHash in Pharo), due to backwards compatability concerns for users explicitly relying on #identityHash returning a value in range [0, 4095]. Cheers, Henry
On Fri, 16 Dec 2011, Henrik Sperre Johansen wrote:
On 16.12.2011 00:40, Henrik Sperre Johansen wrote:
On 15.12.2011 23:35, Mariano Martinez Peck wrote:
Well...it is much slower :( it seems that the cost of (aKey identityHash + ( aKey mareaClass identityHash bitShift: 12) + (aKey basicSize bitShift: 24) is bigger than the colisions. Anyway, thanks for the nice thread. I learned.
Cheers
Well, first of all, that always creates a largeInteger, since identityHash is already shifted by 22... You'd want to be using basicIdentityHash. 2nd off, basicSize is useless for non-variable classes, which I guess is the common case. 3rd, putting class hash in the high bits won't really help much if you're serializing many instances of the same class, as they will all occupy a sequential hash range.
So if I were you, I'd try with something like obj basicIdentityHash << 12 + (obj class basicIdentityHash) before discarding it outright due to computation cost.
Cheers, Henry
Note: This is not portable to Squeak. They kept the old version of identityHash (renamed to basicIdentityHash in Pharo), and introduced scaledIdentityHash (doing the same as identityHash in Pharo), due to backwards compatability concerns for users explicitly relying on #identityHash returning a value in range [0, 4095].
I saw no benefit (and I still don't see any) of changing the behavior of #identityHash and using #basicIdentityHash, so I didn't integrate Martin's suggestion. It's just a side effect that "old code" still works in Squeak. Levente
Cheers, Henry
participants (2)
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Henrik Sperre Johansen -
Levente Uzonyi