lists: add list sorting routine

We need a list sorting function, just to mention one example user that could
benefit is the lib/tt code to sort columns.

This patch adds list_sort() which uses the Merge Sort algorithm, behaving
nicely in O(nlog(n)), heavily based on the kernel's implementation[1].  The
private data (void *priv) passed to the comparison function as been removed to
adopt a qsort(3)-like syntax, and IMHO we don't really need it anyways.

[1]: http://git.kernel.org/?p=linux/kernel/git/torvalds/linux.git;a=blob;f=lib/list_sort.c;h=d7325c6b103f0be078ff3672c35c468ed35738f1;hb=HEAD

[kzak@redhat.com: - use size_t in list_sort()]

Signed-off-by: Davidlohr Bueso <dave@gnu.org>
Signed-off-by: Karel Zak <kzak@redhat.com>

1 files changed, 132 insertions, 0 deletions

diff --git a/include/list.h b/include/list.h
index f7b17fe37..3ce46caa2 100644
--- a/include/list.h
+++ b/include/list.h
@@ -3,6 +3,7 @@
  * Copyright (C) 1999-2008 by Theodore Ts'o
  *
  * (based on list.h from e2fsprogs)
+ * Merge sort based on kernel's implementation.
  */
 
 #ifndef UTIL_LINUX_LIST_H
@@ -192,6 +193,137 @@ _INLINE_ void list_splice(struct list_head *list, struct list_head *head)
 	for (pos = (head)->next, pnext = pos->next; pos != (head); \
 	     pos = pnext, pnext = pos->next)
 
+#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.
+ */
+_INLINE_ struct list_head *merge(int (*cmp)(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)(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.
+ */
+_INLINE_ void merge_and_restore_back_links(int (*cmp)(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;
+
+	while (a && b) {
+		/* if equal, take 'a' -- important for sort stability */
+		if ((*cmp)(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.
+		 */
+		(*cmp)(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
+ * @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.
+ */
+_INLINE_ void list_sort(struct list_head *head,
+			int (*cmp)(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 */
+	size_t lev;  /* index into part[] */
+	size_t 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(cmp, part[lev], cur);
+			part[lev] = NULL;
+		}
+		if (lev > max_lev) {
+			/* list passed to list_sort() too long for efficiency */
+			if (lev >= ARRAY_SIZE(part) - 1)
+				lev--;
+			max_lev = lev;
+		}
+		part[lev] = cur;
+	}
+
+	for (lev = 0; lev < max_lev; lev++)
+		if (part[lev])
+			list = merge(cmp, part[lev], list);
+
+	merge_and_restore_back_links(cmp, head, part[max_lev], list);
+}
+
 #undef _INLINE_
 
 #endif /* UTIL_LINUX_LIST_H */