list_sort.c

#include "list_sort.h"

static int list_cmp(const struct list_head *a, const struct list_head *b)
{
    element_t *a_ele = list_entry(a, element_t, list);
    element_t *b_ele = list_entry(b, element_t, list);
    return strcmp(a_ele->value, b_ele->value);
}

static struct list_head *list_merge(list_cmp_func_t cmp,
                                    struct list_head *a,
                                    struct list_head *b)
{
    struct list_head *head = NULL, **tail = &head;

    for (;;) {
        /* if equal, take 'a' -- important for sort stability */
        if (cmp(a, b) <= 0) {
            *tail = a;
            tail = &a->next;
            a = a->next;
            if (!a) {
                *tail = b;
                break;
            }
        } else {
            *tail = b;
            tail = &b->next;
            b = b->next;
            if (!b) {
                *tail = a;
                break;
            }
        }
    }
    return head;
}
static void merge_final(list_cmp_func_t cmp,
                        struct list_head *head,
                        struct list_head *a,
                        struct list_head *b)
{
    struct list_head *tail = head;
    long long count = 0;

    for (;;) {
        /* if equal, take 'a' -- important for sort stability */
        if (cmp(a, b) <= 0) {
            tail->next = a;
            a->prev = tail;
            tail = a;
            a = a->next;
            if (!a)
                break;
        } else {
            tail->next = b;
            b->prev = tail;
            tail = b;
            b = b->next;
            if (!b) {
                b = a;
                break;
            }
        }
    }

    /* Finish linking remainder of list b on to tail */
    tail->next = b;
    do {
        /*
         * If the merge is highly unbalanced (e.g. the input is
         * already sorted), 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(b, b);
        b->prev = tail;
        tail = b;
        b = b->next;
    } while (b);

    /* And the final links to make a circular doubly-linked list */
    tail->next = head;
    head->prev = tail;
}
void list_sort(struct list_head *head, list_cmp_func_t cmp)
{
    struct list_head *list = head->next, *pending = NULL;
    size_t count = 0; /* Count of pending */

    if (list == head->prev) /* Zero or one elements */
        return;

    /* Convert to a null-terminated singly-linked list. */
    head->prev->next = NULL;

    /*
     * Data structure invariants:
     * - All lists are singly linked and null-terminated; prev
     *   pointers are not maintained.
     * - pending is a prev-linked "list of lists" of sorted
     *   sublists awaiting further merging.
     * - Each of the sorted sublists is power-of-two in size.
     * - Sublists are sorted by size and age, smallest & newest at front.
     * - There are zero to two sublists of each size.
     * - A pair of pending sublists are merged as soon as the number
     *   of following pending elements equals their size (i.e.
     *   each time count reaches an odd multiple of that size).
     *   That ensures each later final merge will be at worst 2:1.
     * - Each round consists of:
     *   - Merging the two sublists selected by the highest bit
     *     which flips when count is incremented, and
     *   - Adding an element from the input as a size-1 sublist.
     */
    do {
        size_t bits;
        struct list_head **tail = &pending;

        /* Find the least-significant clear bit in count */
        for (bits = count; bits & 1; bits >>= 1)
            tail = &(*tail)->prev;
        /* Do the indicated merge */
        if (likely(bits)) {
            struct list_head *a = *tail, *b = a->prev;

            a = list_merge(cmp, b, a);
            /* Install the merged result in place of the inputs */
            a->prev = b->prev;
            *tail = a;
        }

        /* Move one element from input list to pending */
        list->prev = pending;
        pending = list;
        list = list->next;
        pending->next = NULL;
        count++;
    } while (list);

    /* End of input; merge together all the pending lists. */
    list = pending;
    pending = pending->prev;
    for (;;) {
        struct list_head *next = pending->prev;

        if (!next)
            break;
        list = list_merge(cmp, pending, list);
        pending = next;
    }
    /* The final merge, rebuilding prev links */
    merge_final(cmp, head, pending, list);
}