Generate a CSV of OEIS A075085

#include <stdio.h>
#include <stdbool.h>
#include <stdlib.h>
#include <stdint.h>
#include <math.h>

/* Generate a CSV of the first MAX (see below) terms of OEIS A075085 */
/* March 2024 - oeisfan(at)phodd.net */

typedef  unsigned long long  Integer;
typedef  unsigned long       Int;
typedef  unsigned short int  usi;

#define FALSE 0
#define TRUE  1

usi jump[8] = {4,2,4,2,4,6,2,6}; /* pattern of primes, mod 30 */

Integer isPrime(Integer x) {

   Int p, sqrtx;
   register usi ji;
   static Integer prev = 0;
   static usi isPrimePrev = FALSE;

   if (x==prev) return isPrimePrev;
   prev = x;

   if (x<2) return isPrimePrev=FALSE;
   if (x==2   ||x==3   ||x==5   ||x==7   ) return isPrimePrev=TRUE;
   if (x %2==0||x %3==0||x %5==0||x %7==0) return isPrimePrev=FALSE;

   sqrtx = sqrt(x);
   for (p = 7, ji = 7; p <= sqrtx; p += jump[ji=(ji+1)&7]) { /* generate a candidate */
     if (x % p == 0) return isPrimePrev=FALSE;
   }

   return isPrimePrev=TRUE;
}

Integer nextPrime(Integer x){
  if (x<2) return 2;
  if (x==2) return 3;
  for (x+=1+(x%2); !isPrime(x); x+=2);
  return x;
}

#define MAX 80000000 /* 80 million */


int main() {
  /* original gnu-bc code; nextprime() not supplied */
  /* scale=0;s=a[n=1]=p=2;print 1,",";2;for(n=2;n<=1000000;++n){
       p=nextprime(p);f=1;
       for(a=p-s%p;f;a+=p){f=0;for(m=1;m<n;m++)if(a[m]==a){f=1;break};if(!f)break};
       a[n]=a;s+=a;print n,",";a};-1
  */

  Integer prevsum, sum, m, n, prime, a_candidate;
  uint64_t *v;
  uint64_t vec_pow;
  Integer vec_index;
  bool found;

  /* Use a bit vector to keep track of which values have been found in the
     sequence. Does not store in order of finding. Does not need to.
  */
  Integer vmax = 26*MAX;
    /* 64*MAX or greater would be better.
       26*MAX skirts compilation errors and happens to work for MAX=80 million
    */
  Integer vvmax = vmax / sizeof(uint64_t);
  v = calloc(1 + vvmax, sizeof(uint64_t)); for(n = 0; n <= vvmax; n++)v[n]=0;
  if(v == NULL){
    fprintf(stderr, "Not enough memory to allocate space!\n");
    return 1;
  }

  sum = prime = 2;
  v[vec_index = 0] = vec_pow = (uint64_t)1 << prime;
  printf("1,2\n"); fflush(stdout);

  for(n = 2; n <= MAX; ++n){
    prime = nextPrime(prime);
    found = TRUE;
    for(a_candidate = prime - sum%prime; found; a_candidate += prime) {
      vec_index = a_candidate >> 6;
      vec_pow = (uint64_t)1 << (a_candidate & 63);
      if(vec_index > vmax) {
        fprintf(stderr, "Not enough space allocated to store flag!\n");
        return 1;
      }
      found = (
        (0 != (v[vec_index] & vec_pow)) ? TRUE : FALSE
      );
      if(!found)break;
    }
    v[vec_index] |= vec_pow;
    prevsum = sum;
    sum += a_candidate;
    if(prevsum > sum){
      fprintf(stderr, "Sum overflow detected!\n");
      return 2;
    }
    printf("%llu,%llu\n", n, a_candidate); fflush(stdout);
  }
  return 0;
}