On Fri, Oct 01 2021, René Scharfe wrote:
> +/*
> + * Perform an iterative mergesort using an array of sublists.
> + *
> + * n is the number of items.
> + * ranks[i] is undefined if n & 2^i == 0, and assumed empty.
> + * ranks[i] contains a sublist of length 2^i otherwise.
> + *
> + * The number of bits in a void pointer limits the number of objects
> + * that can be created, and thus the number of array elements necessary
> + * to be able to sort any valid list.
> + *
> + * Adding an item to this array is like incrementing a binary number;
> + * positional values for set bits correspond to sublist lengths.
> + */
> void *llist_mergesort(void *list,
> void *(*get_next_fn)(const void *),
> void (*set_next_fn)(void *, void *),
> int (*compare_fn)(const void *, const void *))
> {
> - unsigned long l;
> -
> - if (!list)
> - return NULL;
> - for (l = 1; ; l *= 2) {
> - void *curr;
> - struct mergesort_sublist p, q;
> + void *ranks[bitsizeof(void *)];
> + size_t n = 0;
> + int i;
>
> - p.ptr = list;
> - q.ptr = get_nth_next(p.ptr, l, get_next_fn);
> - if (!q.ptr)
> - break;
> - p.len = q.len = l;
> + while (list) {
> + void *next = get_next_fn(list);
> + if (next)
> + set_next_fn(list, NULL);
> + for (i = 0; n & (1 << i); i++)
> + list = llist_merge(ranks[i], list, get_next_fn,
> + set_next_fn, compare_fn);
> + n++;
> + ranks[i] = list;
> + list = next;
> + }
(Commenting on a commit integrated into v2.34.0)
The aCC compiler on HP/UX notes:
"mergesort.c", line 67: warning #2549-D: variable "ranks" is used before its value is set
list = llist_merge(ranks[i], list, get_next_fn,
It's commenting on the ranks[i] within the for-loop-within-while-loop
above.
>
> - if (compare_fn(p.ptr, q.ptr) > 0)
> - list = curr = pop_item(&q, get_next_fn);
> + for (i = 0; n; i++, n >>= 1) {
> + if (!(n & 1))
> + continue;
> + if (list)
> + list = llist_merge(ranks[i], list, get_next_fn,
> + set_next_fn, compare_fn);
> else
> - list = curr = pop_item(&p, get_next_fn);
> -
> - while (p.ptr) {
> - while (p.len || q.len) {
> - void *prev = curr;
> -
> - if (!p.len)
> - curr = pop_item(&q, get_next_fn);
> - else if (!q.len)
> - curr = pop_item(&p, get_next_fn);
> - else if (compare_fn(p.ptr, q.ptr) > 0)
> - curr = pop_item(&q, get_next_fn);
> - else
> - curr = pop_item(&p, get_next_fn);
> - set_next_fn(prev, curr);
> - }
> - p.ptr = q.ptr;
> - p.len = l;
> - q.ptr = get_nth_next(p.ptr, l, get_next_fn);
> - q.len = q.ptr ? l : 0;
> -
> - }
> - set_next_fn(curr, NULL);
> + list = ranks[i];
> }
> return list;
> }
