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Merge pull request #2126 from gsjaardema/patch-49

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Avoid infinite loop for finding large prime values
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Ward Fisher 2021-10-14 16:31:32 -06:00 committed by GitHub
commit 9116700eaf
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1 changed files with 49 additions and 0 deletions
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49
libdispatch/nchashmap.c
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@ -377,6 +377,46 @@ NC_hashmapdeactivate(NC_hashmap* map, uintptr_t data)
/**************************************************/
/* Prime table */
/* Function that returns true if n is prime else returns false */
/* This will currently fail if `n > 180503 * 180503` */
static int isPrime(size_t n)
{
if (n <= 1) return 0;
if (n <= 3) return 1;
for (size_t i=1; i < NC_nprimes - 1; i++) {
size_t prime = NC_primes[i];
if (n % prime == 0) {
return 0;
}
if (prime * prime > n) {
break;
}
}
return 1;
}
/* Function to return the smallest prime number greater than N */
static int nextPrime(size_t val)
{
if (val <= 1)
return 2;
size_t prime = val;
if (prime % 2 == 0) { /* Make sure `prime` is odd */
prime--;
}
/* Loop continuously until isPrime returns */
/* true for a number greater than n */
for (;;) {
prime += 2;
if (isPrime(prime))
break;
}
return prime;
}
/*
Binary search prime table for first prime just greater than or
equal to val
@ -395,6 +435,15 @@ findPrimeGreaterThan(size_t val)
return 0; /* Too big */
v = (unsigned int)val;
if (v > NC_primes[n - 2]) {
/*
If we have a value greater than the largest value in the
NC_primes table, then search for a prime instead of just
doing a simple lookup.
*/
return nextPrime(val);
}
for(;;) {
if(L >= R) break;