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/* Seive of Eratosthenes
Copyright (C) 1994 Free Software Foundation
This program is free software; you can redistribute it and/or
modify it under the terms of the GNU General Public License as
published by the Free Software Foundation; either version 2, or (at
your option) any later version.
This program is distributed in the hope that it will be useful, but
WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
General Public License for more details.
You should have received a copy of the GNU General Public License
along with this program; if not, write to the Free Software
Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA. */
#include <stdlib.h>
#include <string.h>
/* Array of prime numbers */
int *primes;
/* Allocated size of array `primes'. */
int primessize;
/* Number of primes recorded in array `primes'. */
int nprimes;
/* Initialize primes */
void
initprimes ()
{
primessize = 2;
nprimes = 2;
primes = malloc (sizeof (int) * 2);
primes[0] = 2;
primes[1] = 3;
}
/* Return the next prime greater than or equal to n. */
int
nextprime (int n)
{
int p;
int low, high;
int *iscomp;
int nints;
int lastprime = primes[nprimes - 1];
int i, j;
if (n < primes[0])
return primes[0];
while (n > primes[nprimes - 1])
{
nints = lastprime * lastprime;
iscomp = alloca (sizeof (int) * nints);
bzero (iscomp, sizeof (int) * nints);
for (i = 0; i < nprimes; i++)
for (j = primes[i] * 2; j < nints; j += primes[i])
iscomp[j] = 1;
for (i = lastprime; i < nints; i++)
{
if (nprimes == primessize)
{
primes = realloc (primes, primessize * sizeof (int) * 2);
primessize *= 2;
}
if (!iscomp[i])
primes[nprimes++] = i;
}
}
/* Binary search */
low = 0;
high = nprimes - 1;
p = high / 2;
/* This works because nprimes is always at least 2. */
while (primes[p - 1] >= n || primes[p] < n)
{
if (n > primes[p])
low = p;
else
high = p;
p = ((high - low) / 2) + low;
}
return primes[p];
}
main ()
{
int i;
initprimes();
for (i = 0; i < 100; i++)
printf ("%d\t%d\n", i, nextprime(i));
}
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