From: Lou Vanek <vanek@...>
Date: 2006-01-06T11:34:34+09:00
Subject: Re: [SUMMARY] Numeric Maze (#60)

Then I guess I just love implementation details.
;)

 >ruby -e "puts (2**29).class"
Fixnum
 >time ruby -e '1_000_000.times {  2**29 }'

real    0m25.788s
user    0m25.686s
sys     0m0.031s
 >time ruby -e '1_000_000.times {  2.0**29 }'

real    0m1.233s
user    0m1.234s
sys     0m0.000s
 >ruby -e "puts (2.0**29).class"
Float
 >time ruby -e '1_000_000.times {  1<<29 }'

real    0m0.559s
user    0m0.577s
sys     0m0.000s
 >ruby -e "puts (1<<29).class"
Fixnum
 >time ruby -e '1_000_000.times {  2*2*2*2*2*2*2*2*2*2*2*2*2*2*2*2*2*2*2*2*2*2*2*2*2*2*2*2*2 }'

real    0m5.970s
user    0m5.983s
sys     0m0.015s
 >ruby -e "puts (2*2*2*2*2*2*2*2*2*2*2*2*2*2*2*2*2*2*2*2*2*2*2*2*2*2*2*2*2).class"
Fixnum


Simon Kr�ger wrote:
> Lou Vanek wrote:
> 
>> The shift operator comes in handy and can save a lot of time.
>> These are equivalent operations:
>>
>>  >time ruby -e '1_000_000.times { ||  2**31 }'
>>
>> real    0m33.225s
>> user    0m32.452s
>> sys     0m0.015s
>>  >time ruby -e '1_000_000.times { ||  1<<31 }'
>>
>> real    0m3.469s
>> user    0m3.421s
>> sys     0m0.000s
>>  >time ruby -e '1_000_000.times { || 
>> 2*2*2*2*2*2*2*2*2*2*2*2*2*2*2*2*2*2*2*2*2*2*2*2*2*2*2*2*2*2*2 }'
>>
>> real    0m13.839s
>> user    0m13.656s
>> sys     0m0.015s
>>
>>
>> Kero wrote:
> 
> 
> This is an implementation detail,
> 
> the ** operator of Fixnum calls rb_big_pow converting self to a Bignum 
> regardless of the value. '<<' checks the size of the result and uses 
> real bitshifting if possible.
> 
> While this was interresting to me it's not the explanation of the effect 
> above because the value 2**31 is always a bignum, but the implementation 
> of the ** operator in Bignum isn't very fast (while shifting the 
> internal representation of the bignum is faster)
> 
> To prove my point that this isn't realy because bitshifts are faster try 
> this:
> 
> 1_000_000.times { ||  2.0**30 }
> 
> At least on my machine this is faster than any of your three examples 
> above. (if you choose smaller values '<<' will be the fastest because it 
> doesn't convert to bignums as stated above)
> 
> cheers
> 
> Simon