A simple sieve in Phobos?

[bearophile]

There is a efficient Sieve implementation in C++ here:

http://code.activestate.com/recipes/577966-even-faster-prime-generator/?in=lang-cpp

There are of course far faster implementations, but its 
performance is not bad, while being still simple and quite short.

A D implementation (it's not a final version because the global 
enums should be removed and replaced with run-time variables or 
template arguments. And something different from just counting 
must be added):


import std.stdio, std.algorithm, std.typecons;

enum uint MAX_N = 1_000_000_000U;
enum uint RT_MAX_N = 32_000; // square of max prime under this 
limit should exceed MAX_N.
enum uint B_SIZE = 20_000;   // not sure what optimal value for 
this is;
                              // currently must avoid overflow 
when squared.

// mod 30, (odd) primes have remainders 1,7,11,13,17,19,23,29.
// e.g. start with mark[B_SIZE/30]
// and offset[] = {1, 7, 11, ...}
// then mark[i] corresponds to 30 * (i / 8) + b - 1 + offset[i % 
8].
Tuple!(uint, size_t, uint) calcPrimes() pure nothrow {
     // Assumes p, b odd and p * p won't overflow.
     static void crossOut(in uint p, in uint b, bool[] mark)
     pure nothrow {
         uint si = (p - (b % p)) % p;
         if (si & 1)
             si += p;
         if (p ^^ 2 > b)
             si = si.max(p ^^ 2 - b);

         for (uint i = si / 2; i < B_SIZE / 2; i += p)
             mark[i] = true;
     }

     uint pCount = 1; uint lastP = 2;
     // Do something with first prime (2) here.

     uint[] smallP = [2];

     bool[B_SIZE / 2] mark = false;
     foreach (immutable uint i; 1 .. B_SIZE / 2) {
         if (!mark[i]) {
             pCount++; lastP = 2 * i + 1;
             // Do something with lastP here.

             smallP ~= lastP;
             if (lastP ^^ 2 <= B_SIZE)
                 crossOut(2 * i + 1, 1, mark);
         } else
             mark[i] = false;
     }

     for (uint b = 1 + B_SIZE; b < MAX_N; b += B_SIZE) {
         for (uint i = 1; smallP[i] ^^ 2 < b + B_SIZE; ++i)
             crossOut(smallP[i], b, mark);
         foreach (immutable uint i; 0 .. B_SIZE / 2) {
             if (!mark[i]) {
                 pCount++; lastP = 2 * i + b;
                 // Do something with lastP here.

                 if (lastP <= RT_MAX_N)
                     smallP ~= lastP;
             } else
                 mark[i] = false;
         }
     }

     return tuple(pCount, smallP.length, lastP);
}

void main() {
     immutable result = calcPrimes;

     writeln("Found ", result[0], " primes in total.");
     writeln("Recorded ", result[1], " small primes, up to ", 
RT_MAX_N);
     writeln("Last prime found was ", result[2]);
}


Is it a good idea to add a simple but reasonably fast Sieve 
implementation to Phobos? I have needed a prime numbers lazy 
range, and a isPrime() function numerous times. (And as for 
std.numeric.fft, people that need even more performance will use 
code outside Phobos).

Bye,
bearophile