Hi Sven, I am not sure it should replace the standard implementation. Integer>>factorial is essentially there for teaching purpose in my opinion. I am not sure how often factorial is used in practice. The most important for me is not it to be fast, but to be an excellent example of OOP. Your implementation could be next to the standard one though. Cheers, Alexandre On 16 Jun 2011, at 06:38, Sven Van Caekenberghe wrote:
Hi,
On Planet Smalltalk I read about this challenge: http://www.parcplace.net/list/vwnc-archive/1106/msg00080.html the goal being to optimize the current, naive Integer>>#factorial implementation.
I had some old Lisp code hanging around, implementing an algorithm that I once found somewhere, and I ported it to Smalltalk. I am sure there are faster solutions, but this one is twice as fast as the original while still being easy to understand and it has a useful, safe helper method:
Integer>>#svcFactorial "Answer the factorial of the receiver."
self = 0 ifTrue: [ ^ 1 ]. self > 0 ifTrue: [ ^ 1 productUpTo: self ]. self error: 'Not valid for negative integers'
Integer>>#productUpTo: anInteger "Answer the product of all integers from the receiver (non-inclusive) up to anInteger (inclusive)."
| difference split | self assert: (self between: 0 and: anInteger). "the idea is to multiply LargePositiveIntegers of approximately the same size" difference := anInteger - self. difference = 0 ifTrue: [ ^ 1 ]. difference = 1 ifTrue: [ ^ anInteger ]. difference = 2 ifTrue: [ ^ (anInteger - 1) * anInteger ]. difference = 3 ifTrue: [ ^ (anInteger - 2) * (anInteger - 1) * anInteger ]. difference = 4 ifTrue: [ ^ (anInteger - 3) * (anInteger - 2) * (anInteger - 1) * anInteger ]. split := (self + anInteger) bitShift: -1. ^ (self productUpTo: split) * (split productUpTo: anInteger)
Would it be a good idea to replace the current implementation with this one ?
Sven
-- _,.;:~^~:;._,.;:~^~:;._,.;:~^~:;._,.;:~^~:;._,.;: Alexandre Bergel http://www.bergel.eu ^~:;._,.;:~^~:;._,.;:~^~:;._,.;:~^~:;._,.;:~^~:;.