gistfile1.c

struct thingvec {
    thing *data;
    size_t capacity, size;
};

void append(struct thingvec *self, thing t)
{
    if (self->size >= self->capacity) {                 // there's no free slots left
        size_t newcap = self->capacity * 2;             // double in capacity, hits O(N) case every log(N) appends
        size_t ts = sizeof(thing);
        thing *tmp = malloc(ts * newcap);               // allocate the new area of memory
        memcpy(tmp, self->data, self->capacity * ts);   // copy the old data into the new area of memory, this is the O(N) part
        // you could just use realloc with glibc, but the C spec doesn't guarantee that realloc will keep the contents 
        // if the new size is larger than the old size, only if the new size is smaller or the same size.
        free(self->data);                               // free the old area
        self->data = tmp;                               // update the struct
        self->capacity = newcap;
    }
    self->data[self->size++] = t; // insert the element right after the last element, and then increment the size by 1
}