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mergesort.c

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+/*
+ * Copyright 2008-2013 Various Authors
+ * Copyright 2004 Timo Hirvonen
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License as
+ * published by the Free Software Foundation; either version 2 of the
+ * License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include "mergesort.h"
+#include "list.h"
+
+void list_mergesort(struct list_head *head,
+	int (*compare)(const struct list_head *, const struct list_head *))
+{
+	LIST_HEAD(empty);
+	struct list_head *unsorted_head, *sorted_head, *p, *q, *tmp;
+	int psize, qsize, K, count;
+
+	if (list_empty(head))
+		return;
+
+	unsorted_head = head;
+	sorted_head = &empty;
+	K = 1;
+	while (1) {
+		p = unsorted_head->next;
+		count = 0;
+		do {
+			q = p;
+			psize = 0;
+			while (psize < K) {
+				if (q == unsorted_head)
+					break;
+				psize++;
+				q = q->next;
+			}
+			qsize = K;
+			while (1) {
+				struct list_head *e;
+
+				if (q == unsorted_head)
+					qsize = 0;
+				if (psize == 0 && qsize == 0)
+					break;
+				if (!psize || (qsize && compare(p, q) > 0)) {
+					e = q;
+					q = q->next;
+					qsize--;
+				} else {
+					e = p;
+					p = p->next;
+					psize--;
+				}
+				list_del(e);
+				list_add_tail(e, sorted_head);
+			}
+			count++;
+			p = q;
+		} while (p != unsorted_head);
+
+		if (count == 1) {
+			head->next = sorted_head->next;
+			head->prev = sorted_head->prev;
+			sorted_head->prev->next = head;
+			sorted_head->next->prev = head;
+			return;
+		}
+		tmp = unsorted_head;
+		unsorted_head = sorted_head;
+		sorted_head = tmp;
+		K *= 2;
+	}
+}