Subject: Re: [PATCH 9/9] mergesort: use ranks stack

From: Ævar Arnfjörð Bjarmason <[email-masked]>
Date: Mon, 17 Jan 2022 18:43:08 +0100

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;
>  }