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diff --git a/sys/sys/queue_mergesort.h b/sys/sys/queue_mergesort.h
new file mode 100644
--- /dev/null
+++ b/sys/sys/queue_mergesort.h
@@ -0,0 +1,217 @@
+/*-
+ * SPDX-License-Identifier: BSD-2-Clause
+ *
+ * Copyright (c) 2023 Colin Percival
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ * notice, this list of conditions and the following disclaimer in the
+ * documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
+ * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+ * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+ * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+ * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+ * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ */
+
+#ifndef _SYS_QUEUE_MERGESORT_H_
+#define _SYS_QUEUE_MERGESORT_H_
+
+/*
+ * This file defines macros for performing mergesorts on singly-linked lists,
+ * single-linked tail queues, lists, and tail queues as implemented in
+ * <sys/queue.h>.
+ */
+
+/*
+ * Shims to work around _CONCAT and _INSERT_AFTER taking different numbers of
+ * arguments for different types of linked lists.
+ */
+#define STAILQ_CONCAT_4(head1, head2, type, field) \
+ STAILQ_CONCAT(head1, head2)
+#define TAILQ_CONCAT_4(head1, head2, type, field) \
+ TAILQ_CONCAT(head1, head2, field)
+#define SLIST_INSERT_AFTER_4(head, slistelm, elm, field) \
+ SLIST_INSERT_AFTER(slistelm, elm, field)
+#define LIST_INSERT_AFTER_4(head, slistelm, elm, field) \
+ LIST_INSERT_AFTER(slistelm, elm, field)
+
+/*
+ * Generic macros which apply to all types of lists.
+ */
+#define SYSQUEUE_MERGE(sqms_list1, sqms_list2, thunk, sqms_cmp, TYPE, NAME, \
+ M_FIRST, M_INSERT_AFTER, M_INSERT_HEAD, M_NEXT, M_REMOVE_HEAD) \
+do { \
+ struct TYPE *sqms_elm1; \
+ struct TYPE *sqms_elm1_prev; \
+ struct TYPE *sqms_elm2; \
+ \
+ /* Start at the beginning of list1; _prev is the previous node. */ \
+ sqms_elm1_prev = NULL; \
+ sqms_elm1 = M_FIRST(sqms_list1); \
+ \
+ /* Pull entries from list2 and insert them into list1. */ \
+ while ((sqms_elm2 = M_FIRST(sqms_list2)) != NULL) { \
+ /* Remove from list2. */ \
+ M_REMOVE_HEAD(sqms_list2, NAME); \
+ \
+ /* Advance until we find the right place to insert it. */ \
+ while ((sqms_elm1 != NULL) && \
+ (sqms_cmp)(sqms_elm2, sqms_elm1, thunk) >= 0) { \
+ sqms_elm1_prev = sqms_elm1; \
+ sqms_elm1 = M_NEXT(sqms_elm1, NAME); \
+ } \
+ \
+ /* Insert into list1. */ \
+ if (sqms_elm1_prev == NULL) \
+ M_INSERT_HEAD(sqms_list1, sqms_elm2, NAME); \
+ else \
+ M_INSERT_AFTER(sqms_list1, sqms_elm1_prev, \
+ sqms_elm2, NAME); \
+ sqms_elm1_prev = sqms_elm2; \
+ } \
+} while (0)
+
+#define SYSQUEUE_MERGE_SUBL(sqms_sorted, sqms_len1, sqms_len2, sqms_melm, \
+ sqms_mpos, thunk, sqms_cmp, TYPE, NAME, \
+ M_FIRST, M_NEXT, M_REMOVE_HEAD, M_INSERT_AFTER) \
+do { \
+ struct TYPE *sqms_curelm; \
+ size_t sqms_i; \
+ \
+ /* Find the element before the start of the second sorted region. */ \
+ while ((sqms_mpos) < (sqms_len1)) { \
+ (sqms_melm) = M_NEXT((sqms_melm), NAME); \
+ (sqms_mpos)++; \
+ } \
+ \
+ /* Pull len1 entries off the list and insert in the right place. */ \
+ for (sqms_i = 0; sqms_i < (sqms_len1); sqms_i++) { \
+ /* Grab the first element. */ \
+ sqms_curelm = M_FIRST(&(sqms_sorted)); \
+ \
+ /* Advance until we find the right place to insert it. */ \
+ while (((sqms_mpos) < (sqms_len1) + (sqms_len2)) && \
+ ((sqms_cmp)(sqms_curelm, M_NEXT((sqms_melm), NAME), \
+ thunk) >= 0)) { \
+ (sqms_melm) = M_NEXT((sqms_melm), NAME); \
+ (sqms_mpos)++; \
+ } \
+ \
+ /* Move the element in the right place if not already there. */ \
+ if (sqms_curelm != (sqms_melm)) { \
+ M_REMOVE_HEAD(&(sqms_sorted), NAME); \
+ M_INSERT_AFTER(&(sqms_sorted), (sqms_melm), \
+ sqms_curelm, NAME); \
+ (sqms_melm) = sqms_curelm; \
+ } \
+ } \
+} while (0)
+
+#define SYSQUEUE_MERGESORT(sqms_head, thunk, sqms_cmp, TYPE, NAME, M_HEAD, \
+ M_HEAD_INITIALIZER, M_EMPTY, M_FIRST, M_NEXT, M_INSERT_HEAD, \
+ M_INSERT_AFTER, M_CONCAT, M_REMOVE_HEAD) \
+do { \
+ /* \
+ * Invariant: If sqms_slen = 2^a + 2^b + ... + 2^z with a < b < ... < z \
+ * then sqms_sorted is a sequence of 2^a sorted entries followed by a \
+ * list of 2^b sorted entries ... followed by a list of 2^z sorted \
+ * entries. \
+ */ \
+ M_HEAD(, TYPE) sqms_sorted = M_HEAD_INITIALIZER(sqms_sorted); \
+ struct TYPE *sqms_elm; \
+ size_t sqms_slen = 0; \
+ size_t sqms_sortmask; \
+ size_t sqms_mpos; \
+ \
+ /* Move everything from the input list to sqms_sorted. */ \
+ while (!M_EMPTY(sqms_head)) { \
+ /* Pull the head off the input list. */ \
+ sqms_elm = M_FIRST(sqms_head); \
+ M_REMOVE_HEAD(sqms_head, NAME); \
+ \
+ /* Push it onto sqms_sorted. */ \
+ M_INSERT_HEAD(&sqms_sorted, sqms_elm, NAME); \
+ sqms_slen++; \
+ \
+ /* Restore sorting invariant. */ \
+ sqms_mpos = 1; \
+ for (sqms_sortmask = 1; \
+ sqms_sortmask & ~sqms_slen; \
+ sqms_sortmask <<= 1) \
+ SYSQUEUE_MERGE_SUBL(sqms_sorted, sqms_sortmask, \
+ sqms_sortmask, sqms_elm, sqms_mpos, thunk, sqms_cmp,\
+ TYPE, NAME, M_FIRST, M_NEXT, M_REMOVE_HEAD, \
+ M_INSERT_AFTER); \
+ } \
+ \
+ /* Merge the remaining sublists. */ \
+ sqms_elm = M_FIRST(&sqms_sorted); \
+ sqms_mpos = 1; \
+ for (sqms_sortmask = 2; \
+ sqms_sortmask < sqms_slen; \
+ sqms_sortmask <<= 1) \
+ if (sqms_slen & sqms_sortmask) \
+ SYSQUEUE_MERGE_SUBL(sqms_sorted, \
+ sqms_slen & (sqms_sortmask - 1), sqms_sortmask, \
+ sqms_elm, sqms_mpos, thunk, sqms_cmp, \
+ TYPE, NAME, M_FIRST, M_NEXT, M_REMOVE_HEAD, \
+ M_INSERT_AFTER); \
+ \
+ /* Move the sorted list back to the input list. */ \
+ M_CONCAT(sqms_head, &sqms_sorted, TYPE, NAME); \
+} while (0)
+
+/**
+ * Macros for each of the individual data types. They are all invoked as
+ * FOO_MERGESORT(head, thunk, compar, TYPE, NAME)
+ * and
+ * FOO_MERGE(list1, list2, thunk, compar, TYPE, NAME)
+ * where the compar function operates as in qsort_r, i.e. compar(a, b, thunk)
+ * returns an integer less than, equal to, or greater than zero if a is
+ * considered to be respectively less than, equal to, or greater than b.
+ */
+#define SLIST_MERGESORT(head, thunk, cmp, TYPE, NAME) \
+ SYSQUEUE_MERGESORT((head), (thunk), (cmp), TYPE, NAME, SLIST_HEAD, \
+ SLIST_HEAD_INITIALIZER, SLIST_EMPTY, SLIST_FIRST, SLIST_NEXT, \
+ SLIST_INSERT_HEAD, SLIST_INSERT_AFTER_4, SLIST_CONCAT, SLIST_REMOVE_HEAD)
+#define SLIST_MERGE(list1, list2, thunk, cmp, TYPE, NAME) \
+ SYSQUEUE_MERGE((list1), (list2), (thunk), (cmp), TYPE, NAME, SLIST_FIRST, \
+ SLIST_INSERT_AFTER_4, SLIST_INSERT_HEAD, SLIST_NEXT, SLIST_REMOVE_HEAD)
+
+#define LIST_MERGESORT(head, thunk, cmp, TYPE, NAME) \
+ SYSQUEUE_MERGESORT((head), (thunk), (cmp), TYPE, NAME, LIST_HEAD, \
+ LIST_HEAD_INITIALIZER, LIST_EMPTY, LIST_FIRST, LIST_NEXT, \
+ LIST_INSERT_HEAD, LIST_INSERT_AFTER_4, LIST_CONCAT, LIST_REMOVE_HEAD)
+#define LIST_MERGE(list1, list2, thunk, cmp, TYPE, NAME) \
+ SYSQUEUE_MERGE((list1), (list2), (thunk), (cmp), TYPE, NAME, LIST_FIRST, \
+ LIST_INSERT_AFTER_4, LIST_INSERT_HEAD, LIST_NEXT, LIST_REMOVE_HEAD)
+
+#define STAILQ_MERGESORT(head, thunk, cmp, TYPE, NAME) \
+ SYSQUEUE_MERGESORT((head), (thunk), (cmp), TYPE, NAME, STAILQ_HEAD, \
+ STAILQ_HEAD_INITIALIZER, STAILQ_EMPTY, STAILQ_FIRST, STAILQ_NEXT, \
+ STAILQ_INSERT_HEAD, STAILQ_INSERT_AFTER, STAILQ_CONCAT_4, STAILQ_REMOVE_HEAD)
+#define STAILQ_MERGE(list1, list2, thunk, cmp, TYPE, NAME) \
+ SYSQUEUE_MERGE((list1), (list2), (thunk), (cmp), TYPE, NAME, STAILQ_FIRST, \
+ STAILQ_INSERT_AFTER, STAILQ_INSERT_HEAD, STAILQ_NEXT, STAILQ_REMOVE_HEAD)
+
+#define TAILQ_MERGESORT(head, thunk, cmp, TYPE, NAME) \
+ SYSQUEUE_MERGESORT((head), (thunk), (cmp), TYPE, NAME, TAILQ_HEAD, \
+ TAILQ_HEAD_INITIALIZER, TAILQ_EMPTY, TAILQ_FIRST, TAILQ_NEXT, \
+ TAILQ_INSERT_HEAD, TAILQ_INSERT_AFTER, TAILQ_CONCAT_4, TAILQ_REMOVE_HEAD)
+#define TAILQ_MERGE(list1, list2, thunk, cmp, TYPE, NAME) \
+ SYSQUEUE_MERGE((list1), (list2), (thunk), (cmp), TYPE, NAME, TAILQ_FIRST, \
+ TAILQ_INSERT_AFTER, TAILQ_INSERT_HEAD, TAILQ_NEXT, TAILQ_REMOVE_HEAD)
+
+#endif /* !_SYS_QUEUE_MERGESORT_H_ */

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