// SPDX-License-Identifier: GPL-2.0

/* Copied from the Linux kernel lib/list_sort.c file */

#include <stdlib.h>

#include <string.h>			/* memset */

#include <libipset/list_sort.h>

#define MAX_LIST_LENGTH_BITS 20

/*

 * Returns a list organized in an intermediate format suited

 * to chaining of merge() calls: null-terminated, no reserved or

 * sentinel head node, "prev" links not maintained.

 */

static struct list_head *merge(void *priv,

				int (*cmp)(void *priv, struct list_head *a,

					struct list_head *b),

				struct list_head *a, struct list_head *b)

{

	struct list_head head, *tail = &head;

	while (a && b) {

		/* if equal, take 'a' -- important for sort stability */

		if ((*cmp)(priv, a, b) <= 0) {

			tail->next = a;

			a = a->next;

		} else {

			tail->next = b;

			b = b->next;

		}

		tail = tail->next;

	}

	tail->next = a?:b;

	return head.next;

}

/*

 * Combine final list merge with restoration of standard doubly-linked

 * list structure.  This approach duplicates code from merge(), but

 * runs faster than the tidier alternatives of either a separate final

 * prev-link restoration pass, or maintaining the prev links

 * throughout.

 */

static void merge_and_restore_back_links(void *priv,

				int (*cmp)(void *priv, struct list_head *a,

					struct list_head *b),

				struct list_head *head,

				struct list_head *a, struct list_head *b)

{

	struct list_head *tail = head;

	int count = 0;

	while (a && b) {

		/* if equal, take 'a' -- important for sort stability */

		if ((*cmp)(priv, a, b) <= 0) {

			tail->next = a;

			a->prev = tail;

			a = a->next;

		} else {

			tail->next = b;

			b->prev = tail;

			b = b->next;

		}

		tail = tail->next;

	}

	tail->next = a ? : b;

	do {

		/*

		 * In worst cases this loop may run many iterations.

		 * Continue callbacks to the client even though no

		 * element comparison is needed, so the client's cmp()

		 * routine can invoke cond_resched() periodically.

		 */

		if (unlikely(!(++count)))

			(*cmp)(priv, tail->next, tail->next);

		tail->next->prev = tail;

		tail = tail->next;

	} while (tail->next);

	tail->next = head;

	head->prev = tail;

}

/**

 * list_sort - sort a list

 * @priv: private data, opaque to list_sort(), passed to @cmp

 * @head: the list to sort

 * @cmp: the elements comparison function

 *

 * This function implements "merge sort", which has O(nlog(n))

 * complexity.

 *

 * The comparison function @cmp must return a negative value if @a

 * should sort before @b, and a positive value if @a should sort after

 * @b. If @a and @b are equivalent, and their original relative

 * ordering is to be preserved, @cmp must return 0.

 */

void list_sort(void *priv, struct list_head *head,

		int (*cmp)(void *priv, struct list_head *a,

			struct list_head *b))

{

	struct list_head *part[MAX_LIST_LENGTH_BITS+1]; /* sorted partial lists

						-- last slot is a sentinel */

	int lev;  /* index into part[] */

	int max_lev = 0;

	struct list_head *list;

	if (list_empty(head))

		return;

	memset(part, 0, sizeof(part));

	head->prev->next = NULL;

	list = head->next;

	while (list) {

		struct list_head *cur = list;

		list = list->next;

		cur->next = NULL;

		for (lev = 0; part[lev]; lev++) {

			cur = merge(priv, cmp, part[lev], cur);

			part[lev] = NULL;

		}

		if (lev > max_lev) {

			if (unlikely(lev >= MAX_LIST_LENGTH_BITS)) {

				// printk_once(KERN_DEBUG "list too long for efficiency\n");

				lev--;

			}

			max_lev = lev;

		}

		part[lev] = cur;

	}

	for (lev = 0; lev < max_lev; lev++)

		if (part[lev])

			list = merge(priv, cmp, part[lev], list);

	merge_and_restore_back_links(priv, cmp, head, part[max_lev], list);

}