int operator++()
{
return find_next_prime();
}
unsigned long
get_nearest_prime(unsigned long near)
{
PRIME_CTX ctx;
unsigned long x, y, div, prime;
int is_prime;
if(init_primes(&ctx))
return 0;
prime = 0;
if(near < ctx.base_primes[ctx.prime_count -1]) {
x=0;
while(ctx.base_primes[x] < near) {
x++;
}
if(x==0) {
prime = ctx.base_primes[x];
} else if(abs(near - ctx.base_primes[x]) > abs(near - ctx.base_primes[x-1])) {
prime = ctx.base_primes[x-1];
} else {
prime = ctx.base_primes[x];
}
}
if(prime) {
free_primes(&ctx);
return prime;
}
div=0;
if(!(near % 2)) {
near++;
}
/* dealing w/ upper limit of unsinged longs. */
if(near==0xffffffff) {
near-=2;
}
if(sqr(ctx.base_primes[ctx.prime_count -1]) < near) {
while(sqr(ctx.base_primes[ctx.prime_count -1]) < near && ctx.base_primes[ctx.prime_count -1] != 65521) {
if(find_next_prime(&ctx)) {
free_primes(&ctx);
return 0;
}
}
div = ctx.prime_count -1;
} else {
x=0;
while(sqr(ctx.base_primes[x]) < near) {
x++;
}
div=x;
}
for(x=0, is_prime=1; x< div && is_prime; x++) {
if(near % ctx.base_primes[x]==0) {
is_prime=0;
}
}
if(is_prime) {
return near;
}
y=0;
prime=0;
while(prime==0) {
is_prime=1;
y+=2;
/*again, dealing w/ upper limit of unsigned longs. */
if(near < 0xffffffff -y) {
for(x=0; x <= div && is_prime; x++) {
if((near +y) % ctx.base_primes[x]==0) {
is_prime=0;
}
}
if(is_prime) {
prime = near +y;
}
}
if(prime==0) {
is_prime=1;
for(x=0; x <= div && is_prime; x++) {
if((near -y) % ctx.base_primes[x]==0) {
is_prime=0;
}
}
if(is_prime) {
prime = near -y;
}
}
}
free_primes(&ctx);
return prime;
}
