#define pr_fmt(fmt) "list_sort_test: " fmt

#include <linux/kernel.h>
#include <linux/bug.h>
#include <linux/compiler.h>
#include <linux/export.h>
#include <linux/string.h>
#include <linux/list_sort.h>
#include <linux/list.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;
        u8 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 >= ARRAY_SIZE(part)-1)) {
                                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);
}
EXPORT_SYMBOL(list_sort);

#ifdef CONFIG_TEST_LIST_SORT

#include <linux/slab.h>
#include <linux/random.h>

/*
 * The pattern of set bits in the list length determines which cases
 * are hit in list_sort().
 */
#define TEST_LIST_LEN (512+128+2) /* not including head */

#define TEST_POISON1 0xDEADBEEF
#define TEST_POISON2 0xA324354C

struct debug_el {
        unsigned int poison1;
        struct list_head list;
        unsigned int poison2;
        int value;
        unsigned serial;
};

/* Array, containing pointers to all elements in the test list */
static struct debug_el **elts __initdata;

static int __init check(struct debug_el *ela, struct debug_el *elb)
{
        if (ela->serial >= TEST_LIST_LEN) {
                pr_err("error: incorrect serial %d\n", ela->serial);
                return -EINVAL;
        }
        if (elb->serial >= TEST_LIST_LEN) {
                pr_err("error: incorrect serial %d\n", elb->serial);
                return -EINVAL;
        }
        if (elts[ela->serial] != ela || elts[elb->serial] != elb) {
                pr_err("error: phantom element\n");
                return -EINVAL;
        }
        if (ela->poison1 != TEST_POISON1 || ela->poison2 != TEST_POISON2) {
                pr_err("error: bad poison: %#x/%#x\n",
                        ela->poison1, ela->poison2);
                return -EINVAL;
        }
        if (elb->poison1 != TEST_POISON1 || elb->poison2 != TEST_POISON2) {
                pr_err("error: bad poison: %#x/%#x\n",
                        elb->poison1, elb->poison2);
                return -EINVAL;
        }
        return 0;
}

static int __init cmp(void *priv, struct list_head *a, struct list_head *b)
{
        struct debug_el *ela, *elb;

        ela = container_of(a, struct debug_el, list);
        elb = container_of(b, struct debug_el, list);

        check(ela, elb);
        return ela->value - elb->value;
}

static int __init list_sort_test(void)
{
        int i, count = 1, err = -ENOMEM;
        struct debug_el *el;
        struct list_head *cur;
        LIST_HEAD(head);

        pr_debug("start testing list_sort()\n");

        elts = kcalloc(TEST_LIST_LEN, sizeof(*elts), GFP_KERNEL);
        if (!elts) {
                pr_err("error: cannot allocate memory\n");
                return err;
        }

        for (i = 0; i < TEST_LIST_LEN; i++) {
                el = kmalloc(sizeof(*el), GFP_KERNEL);
                if (!el) {
                        pr_err("error: cannot allocate memory\n");
                        goto exit;
                }
                 /* force some equivalencies */
                el->value = prandom_u32() % (TEST_LIST_LEN / 3);
                el->serial = i;
                el->poison1 = TEST_POISON1;
                el->poison2 = TEST_POISON2;
                elts[i] = el;
                list_add_tail(&el->list, &head);
        }

        list_sort(NULL, &head, cmp);

        err = -EINVAL;
        for (cur = head.next; cur->next != &head; cur = cur->next) {
                struct debug_el *el1;
                int cmp_result;

                if (cur->next->prev != cur) {
                        pr_err("error: list is corrupted\n");
                        goto exit;
                }

                cmp_result = cmp(NULL, cur, cur->next);
                if (cmp_result > 0) {
                        pr_err("error: list is not sorted\n");
                        goto exit;
                }

                el = container_of(cur, struct debug_el, list);
                el1 = container_of(cur->next, struct debug_el, list);
                if (cmp_result == 0 && el->serial >= el1->serial) {
                        pr_err("error: order of equivalent elements not "
                                "preserved\n");
                        goto exit;
                }

                if (check(el, el1)) {
                        pr_err("error: element check failed\n");
                        goto exit;
                }
                count++;
        }
        if (head.prev != cur) {
                pr_err("error: list is corrupted\n");
                goto exit;
        }


        if (count != TEST_LIST_LEN) {
                pr_err("error: bad list length %d", count);
                goto exit;
        }

        err = 0;
exit:
        for (i = 0; i < TEST_LIST_LEN; i++)
                kfree(elts[i]);
        kfree(elts);
        return err;
}
late_initcall(list_sort_test);
#endif /* CONFIG_TEST_LIST_SORT */