Fast hashtable

[Cecil Ward via Digitalmars-d]

On Wednesday, 1 March 2017 at 12:59:30 UTC, deadalnix wrote:
> On Wednesday, 1 March 2017 at 06:44:34 UTC, Cecil Ward wrote:
>>     const uint power2 = 512; // say, some 1 << n anyway
>>     const uint prime = 509; // some prime just below the 
>> power, some prime > power2/2
>>
>>     static assert( power2 - 1 - prime < prime );
>>
>>     x = x & ( power2 - 1 );
>>     x = ( x >= prime ) ? x - prime : x;
>>
>> which is good news on my x86 with GDC -O3 (only 3 operations, 
>> and sub cmovx ) - all well provided you make sure that you are 
>> getting CMOVx not branches. I could work out the power from 
>> the prime using CTFE given a bit of thought. Maybe CTFE could 
>> even do the reverse?
>>
>> Have I finally gone mad?
>
> The lower slot will be twice as crowded as the higher ones.

Sorry, I think I was unclear, I was suggesting the author should 
use modulo the prime. The power of two is irrelevant, it's just a 
quick(er?) way of computing modulo. Are we on the same wavelength?