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From: markspace <-@.>
Newsgroups: comp.lang.java.programmer
Subject: Re: Using Enumerated Types as Array Indexes
Date: Thu, 18 Aug 2011 14:48:48 -0700
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On 8/18/2011 1:49 PM, Andreas Leitgeb wrote:
> markspace<-@>  wrote:
>> On 8/18/2011 11:37 AM, Andreas Leitgeb wrote:
>>> Apart from that, it is a dirty hack to avoid conditionals.
>>
>> One thing to keep in mind here I think is the relative execution time
>> for each operation: [...]
>>
>> So preferring modulus to ?: or Math.abs() might be a misoptimization.
>
> Math.abs() does an entirely wrong job for the original context.
>
> Now, let's say  N is the array's length, and n the result
> from the latest increment:
>
>    ( n % N + N ) % N
> versus
>    ( n % N<  0 ) ? n % N + N : n % N
> or with an helper var r:
>    ( ( r = n % N )<  0 ) ? r + N : r
>
> PS: I didn't previously say *why* I avoided conditionals.


Yeah, your requirements are unspecified.  I was thinking:

arr[ Math.abs( n % N ) ]

Would at least prevent overflow, although I don't think it would provide 
a strictly incrementing index for arr.

Or you could just strip off the sign bit:

arr[ (n&0x7FFFFFFF) % N ]

That gets rid of those pesky negative numbers right quick.  Though I'm 
still not convinced that this is "better."