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/* Prime number generation
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>
/* Return the next prime greater than or equal to N. */
int
nextprime (int n)
{
static int *q;
static int k = 2;
static int l = 2;
int p;
int *m;
int i, j;
/* You are not expected to understand this. */
if (!q)
{
/* Init */
q = malloc (sizeof (int) * 2);
q[0] = 2;
q[1] = 3;
}
if (n <= q[0])
return q[0];
while (n > q[l - 1])
{
/* Grow */
/* Alloc */
p = q[l-1] * q[l-1];
m = alloca (sizeof (int) * p);
bzero (m, sizeof (int) * p);
/* Sieve */
for (i = 0; i < l; i++)
for (j = q[i] * 2; j < p; j += q[i])
m[j] = 1;
/* Copy */
for (i = q[l-1] + 1; i < p; i++)
{
if (l == k)
{
q = realloc (q, k * sizeof (int) * 2);
k *= 2;
}
if (!m[i])
q[l++] = i;
}
}
/* Search */
i = 0;
j = l - 1;
p = j / 2;
while (q[p - 1] >= n || q[p] < n)
{
if (n > q[p])
i = p + 1;
else
j = p - 1;
p = ((j - i) / 2) + i;
}
return q[p];
}
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