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From: red floyd <no.spam.here@its.invalid>
Newsgroups: comp.lang.c++
Subject: Re: OT: Re: Sieve of Erastosthenes optimized to the max
Date: Thu, 22 Aug 2024 17:00:42 -0700
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On 8/22/2024 1:56 PM, Vir Campestris wrote:
> On 16/08/2024 18:35, Bonita Montero wrote:
>> But basically I don't think it is a good idea to skip numbers exept
>> multiples of two. With the three you save a sixth of memory, with
>> the five you save a 15-th and at the end you get about 20% less
>> storage (1 / (2 * 3) + 1 / (2 * 3 * 5) + 1 / (2 * 3 * 5 * 7) ...)
>> for a lot of computation. That's the point where I dropped this
>> idea and I think this extra computation is higher than the time
>> for the saved memory loads.
> 
> I've been running some experiments.
> 
> Skipping evens only is nice and simple on computation; that's good.
> 
> Skipping something else requires table lookups. I've knocked up some 
> code that uses the correct table for skipping primes (but not for mask 
> bit selection) and run it for 2*3*5, 2*3*5*7, and 2*3*5*7*11.
>
You can skip multiples of three, by starting with 7, and then
alternately adding 4 then 2.

e.g.

incr = 2;
for (val = 7 ; val < MAX_PRIME_TO_CHECK ; val += incr)
{
     check_for_prime(val);
     incr = 6 - incr;
}