after claiming his code worked in C but not in C++, here is u/frymimori's code vs shellsort both, in C

evendumber.c

#include <stdio.h>
#include <stdlib.h>
#include <time.h>

void sorterQ(unsigned long a, int * b) {
	unsigned long c = a;
	unsigned long d = 0;
	unsigned long e;
	int f;

	while (a != 1) {
		a--;

		if (b[a] > b[a - 1]) {
			d = a - 1;
			e = d;
			f = b[d];

			while (d != 0) {
				d--;

				if (b[d] < f) {
					f = b[d];
					e = d;
				}
			}

			b[e] = b[a];
			b[a] = f;
		}
	}

	while (d == 0) {
		a = c;

		while (a != 1) {
			a--;

			if (b[a] > b[a - 1]) {
				d = b[a];
				b[a] = b[a - 1];
				b[a - 1] = d;
				d = 1;
			}
		}

		d ^= 1;
	}

	return;
}

void shellsort(int a[], int n) {
    int h;
    for (h = 1; h < n/9; h = 3*h+1);
    while (h > 0) {
        for (int i = h; i < n; i++) {
            int j = i; int v = a[i];
            while (j >= h && a[j - h] > v) {
                a[j] = a[j - h];
                j -= h;
            }
            a[j] = v;
        }
        h /= 3;
    }
}

void checksort(int a[], int n) {
    for (int i = 0; i < n - 1; i++) {
        if (a[i+1] < a[i]) {
            printf("Sort failed.\n");
            return;
        }
    }
    printf("Sorted.\n");
}

int main(int argc, char *argv[]) {
    int n = atoi(argv[1]);
    int *a = malloc(sizeof(int) * n);
    int *b = malloc(sizeof(int) * n);
    for (int i = 0; i < n; i++) {
        int p = rand() % RAND_MAX;
        a[i] = p;
        b[i] = p;
    }
    printf("Sorting: %d items.\n", n);
    clock_t start, end;
    double time;

    start = clock();
    shellsort(b, n);
    end = clock();
    time = ((double)(end - start)) / CLOCKS_PER_SEC;
    printf("shellsort finished in: %f s\n", time);
    checksort(b, n);

    start = clock();
    sorterQ(n, a);
    end = clock();
    time = ((double)(end - start)) / CLOCKS_PER_SEC;
    printf("Redditer sort finished in: %f s\n", time);
    checksort(a, n);

    
    return 0;
}
/*
max@MaxGorenLaptop:/mnt/c/Users/mgoren/langtonsJava$ ./stupider 1000 ; ./stupider 10000 ; ./stupider 100000
Sorting: 1000 items.
shellsort finished in: 0.000085 s    
Sorted.
Redditer sort finished in: 0.001011 s
Sort failed.
Sorting: 10000 items.
shellsort finished in: 0.001316 s    
Sorted.
Redditer sort finished in: 0.144406 s
Sort failed.
Sorting: 100000 items.
shellsort finished in: 0.033668 s
Sorted.
Redditer sort finished in: 19.408300 s
Sort failed.
max@MaxGorenLaptop:/mnt/c/Users/mgoren/langtonsJava$
*/