Add slist.h

Add a macro-based singly-linked list implementation to the codebase,
inspired by the doubly-linked list in list.h.

lib/isc/include/isc/slist.h

@@ -0,0 +1,82 @@
+/*
+ * Copyright (C) Internet Systems Consortium, Inc. ("ISC")
+ *
+ * SPDX-License-Identifier: MPL-2.0
+ *
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, you can obtain one at https://mozilla.org/MPL/2.0/.
+ *
+ * See the COPYRIGHT file distributed with this work for additional
+ * information regarding copyright ownership.
+ */
+
+#pragma once
+
+/*! \file isc/slist.h
+ * \brief
+ * Implements macros for singly-linked lists.
+ *
+ * This module provides a generic implementation of singly-linked lists
+ * similar to isc/list.h but optimized for forward-only traversal.
+ */
+
+#define ISC_SLIST_INITIALIZER  \
+	{                      \
+		.head = NULL,  \
+	}
+
+#define ISC_SLINK_INITIALIZER \
+	{                     \
+		.next = NULL, \
+	}
+
+#define ISC_SLIST(type)     \
+	struct {            \
+		type *head; \
+	}
+
+#define ISC_SLINK(type)     \
+	struct {            \
+		type *next; \
+	}
+
+#define ISC_SLIST_HEAD(list)  ((list).head)
+#define ISC_SLIST_EMPTY(list) ((list).head == NULL)
+
+#define ISC_SLIST_PREPEND(list, elt, link) \
+	({                                  \
+		(elt)->link.next = (list).head; \
+		(list).head = (elt);            \
+	})
+
+#define ISC_SLIST_INSERTAFTER(after, elt, link) \
+	({                                       \
+		(elt)->link.next = (after)->link.next; \
+		(after)->link.next = (elt);         \
+	})
+
+#define ISC_SLIST_NEXT(elt, link) ((elt)->link.next)
+
+/* clang-format off */
+#define ISC_SLIST_FOREACH_FROM(elt, list, link, first)                      \
+	for (typeof(first) elt = first,                                      \
+	     elt##_next = (elt != NULL) ? ISC_SLIST_NEXT(elt, link) : NULL; \
+	     elt != NULL;                                                   \
+	     elt = elt##_next,                                              \
+	      elt##_next = (elt != NULL) ? ISC_SLIST_NEXT(elt, link) : NULL)
+
+#define ISC_SLIST_FOREACH(elt, list, link) \
+	ISC_SLIST_FOREACH_FROM(elt, list, link, ISC_SLIST_HEAD(list))
+
+/* clang-format on */
+
+/* Iteration over pointer-to-pointer for safe operations */
+#define ISC_SLIST_FOREACH_PTR(p, head) \
+	for (typeof(head) p = (head); *p != NULL; )
+
+#define ISC_SLIST_PTR_REMOVE(p, elt, link_field) \
+	(*(p) = ISC_SLIST_NEXT(elt, link_field))
+
+#define ISC_SLIST_PTR_ADVANCE(p, link_field) \
+	(p = &ISC_SLIST_NEXT(*p, link_field))