On Tue, May 20, 2014 at 7:28 AM, Sven Van Caekenberghe <[email protected]> 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 <[email protected]> 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 > > -- > > View this message in context: > http://forum.world.st/Aconcagua-Hashing-Bug-tp4759622.html > > Sent from the Pharo Smalltalk Developers mailing list archive at > Nabble.com. > > > > >
