145 lines
3.5 KiB
C
145 lines
3.5 KiB
C
/*
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* taken from linux kernel lib/list_sort.c, removed uneeded code and adapted
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* for btrfsprogs
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*/
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#include "kerncompat.h"
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#include "list_sort.h"
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#include "list.h"
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#define MAX_LIST_LENGTH_BITS 20
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/*
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* Returns a list organized in an intermediate format suited
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* to chaining of merge() calls: null-terminated, no reserved or
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* sentinel head node, "prev" links not maintained.
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*/
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static struct list_head *merge(void *priv,
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int (*cmp)(void *priv, struct list_head *a,
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struct list_head *b),
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struct list_head *a, struct list_head *b)
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{
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struct list_head head, *tail = &head;
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while (a && b) {
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/* if equal, take 'a' -- important for sort stability */
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if ((*cmp)(priv, a, b) <= 0) {
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tail->next = a;
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a = a->next;
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} else {
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tail->next = b;
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b = b->next;
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}
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tail = tail->next;
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}
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tail->next = a?:b;
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return head.next;
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}
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/*
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* Combine final list merge with restoration of standard doubly-linked
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* list structure. This approach duplicates code from merge(), but
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* runs faster than the tidier alternatives of either a separate final
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* prev-link restoration pass, or maintaining the prev links
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* throughout.
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*/
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static void merge_and_restore_back_links(void *priv,
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int (*cmp)(void *priv, struct list_head *a,
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struct list_head *b),
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struct list_head *head,
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struct list_head *a, struct list_head *b)
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{
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struct list_head *tail = head;
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while (a && b) {
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/* if equal, take 'a' -- important for sort stability */
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if ((*cmp)(priv, a, b) <= 0) {
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tail->next = a;
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a->prev = tail;
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a = a->next;
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} else {
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tail->next = b;
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b->prev = tail;
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b = b->next;
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}
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tail = tail->next;
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}
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tail->next = a ? : b;
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do {
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/*
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* In worst cases this loop may run many iterations.
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* Continue callbacks to the client even though no
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* element comparison is needed, so the client's cmp()
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* routine can invoke cond_resched() periodically.
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*/
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(*cmp)(priv, tail->next, tail->next);
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tail->next->prev = tail;
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tail = tail->next;
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} while (tail->next);
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tail->next = head;
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head->prev = tail;
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}
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/**
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* list_sort - sort a list
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* @priv: private data, opaque to list_sort(), passed to @cmp
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* @head: the list to sort
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* @cmp: the elements comparison function
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*
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* This function implements "merge sort", which has O(nlog(n))
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* complexity.
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*
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* The comparison function @cmp must return a negative value if @a
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* should sort before @b, and a positive value if @a should sort after
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* @b. If @a and @b are equivalent, and their original relative
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* ordering is to be preserved, @cmp must return 0.
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*/
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void list_sort(void *priv, struct list_head *head,
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int (*cmp)(void *priv, struct list_head *a,
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struct list_head *b))
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{
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struct list_head *part[MAX_LIST_LENGTH_BITS+1]; /* sorted partial lists
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-- last slot is a sentinel */
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int lev; /* index into part[] */
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int max_lev = 0;
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struct list_head *list;
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if (list_empty(head))
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return;
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memset(part, 0, sizeof(part));
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head->prev->next = NULL;
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list = head->next;
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while (list) {
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struct list_head *cur = list;
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list = list->next;
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cur->next = NULL;
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for (lev = 0; part[lev]; lev++) {
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cur = merge(priv, cmp, part[lev], cur);
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part[lev] = NULL;
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}
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if (lev > max_lev) {
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if (lev >= ARRAY_SIZE(part)-1) {
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printf("list_sort: list passed to"
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" list_sort() too long for"
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" efficiency\n");
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lev--;
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}
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max_lev = lev;
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}
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part[lev] = cur;
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}
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for (lev = 0; lev < max_lev; lev++)
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if (part[lev])
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list = merge(priv, cmp, part[lev], list);
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merge_and_restore_back_links(priv, cmp, head, part[max_lev], list);
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}
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