On Tue, May 20, 2014 at 7:28 AM, Sven Van Caekenberghe <sven@stfx.eu> wrote:

So ?

1/2 hash ~= 0.5 hash

((1/2) hash = 0.5 hash) true

((1/2) hash ~= 0.5 hash) true

and

((1/3) hash =(1.0/3) hash) false

The implementation (in 3.0) has a special case for denominator is a power of 2:

Fraction>>hash

"Hash is reimplemented because = is implemented.

Care is taken that a Fraction equal to a Float also have an equal hash"

| tmp |

denominator isPowerOfTwo ifTrue: [

"If denominator is a power of two, I can be exactly equal to a Float"

tmp := self asFloat.

tmp isFinite ifTrue: [^tmp hash]].

"Else, I cannot be exactly equal to a Float, use own hash algorithm.

(Assume the fraction is already reduced)"

^numerator hash bitXor: denominator hash

Float>>hash

"Hash is reimplemented because = is implemented. Both words of the float are used; 8 bits are removed from each end to clear most of the exponent regardless of the byte ordering. (The bitAnd:'s ensure that the intermediate results do not become a large integer.) Slower than the original version in the ratios 12:5 to 2:1 depending on values. (DNS, 11 May, 1997)"

(self isFinite and: [self fractionPart = 0.0]) ifTrue: [^self truncated hash].

^ (((self basicAt: 1) bitAnd: 16r00FFFF00) +

�� ((self basicAt: 2) bitAnd: 16r00FFFF00)) bitShift: -8

Mmmh, I am curious to know what the rationale for this is.

Getting into:��

Fraction>>isPowerOfTwo

|reduced|

reduced := self reduced.

^(reduced numerator = 1 and: [reduced denominator isPowerOfTwo])

or: [reduced denominator = 1 and: [reduced numerator isPowerOfTwo]]

calls in >>reduced, which in turn calls gcd: ...

That's quite something to be aware of.

Phil

��

I mean what would you expect, how should this have to work ?

Maybe the hashing specialists have something to say about this.

On 20 May 2014, at 01:38, Sean P. DeNigris <sean@clipperadams.com> wrote:

> (1/2) teaspoons hash ~= 0.5 teaspoons hash
>
> b.t.w. Phexample caught this automatically because it checks hashes when
> comparing for equality... pretty cool
>
>
>
> -----
> Cheers,
> Sean
> --
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>