From 974df9b5ce2925be67f1678e14eb287007afdbb5 Mon Sep 17 00:00:00 2001
From: Peter Hutterer <peter.hutterer@who-t.net>
Date: Thu, 22 Jul 2021 09:14:12 +1000
Subject: [PATCH] util: add a list_nth_entry() macro

Returns the nth element for a given list. Note that unlike the other
macros, this takes a type, not a variable as first argument.

Signed-off-by:Peter Hutterer <peter.hutterer@who-t.net>
---
 src/util-list.c | 35 +++++++++++++++++++++++++++++++++++
 src/util-list.h | 21 +++++++++++++++++++++
 2 files changed, 56 insertions(+)

diff --git a/src/util-list.c b/src/util-list.c
index 1d01373..9a79e3b 100644
--- a/src/util-list.c
+++ b/src/util-list.c
@@ -154,4 +154,39 @@ MUNIT_TEST(test_list_append)
 	return MUNIT_OK;
 }
 
+MUNIT_TEST(test_list_nth)
+{
+	struct list_test {
+		int val;
+		struct list node;
+	} tests[] = {
+		{ .val  = 1 },
+		{ .val  = 2 },
+		{ .val  = 3 },
+		{ .val  = 4 },
+	};
+	struct list_test *t;
+	struct list head;
+
+	list_init(&head);
+	ARRAY_FOR_EACH(tests, t) {
+		list_append(&head, &t->node);
+	}
+
+	int idx = 0;
+	ARRAY_FOR_EACH(tests, t) {
+		struct list_test *nth = list_nth_entry(struct list_test,
+						       &head, node, idx);
+		munit_assert_int(nth->val, ==, tests[idx].val);
+		idx++;
+	}
+
+	munit_assert(list_nth_entry(struct list_test, &head, node, 10) == NULL);
+	list_init(&head);
+	munit_assert(list_nth_entry(struct list_test, &head, node, 0) == NULL);
+	munit_assert(list_nth_entry(struct list_test, &head, node, 1) == NULL);
+
+	return MUNIT_OK;
+}
+
 #endif
diff --git a/src/util-list.h b/src/util-list.h
index c943ce5..cbfa814 100644
--- a/src/util-list.h
+++ b/src/util-list.h
@@ -195,3 +195,24 @@ bool list_empty(const struct list *list);
 	     &pos->member != (head);					\
 	     pos = tmp,							\
 	     tmp = list_first_entry(&pos->member, tmp, member))
+
+/**
+ * Return the nth element of the list, starting at 0. This is the equivalent
+ * to `listname[index]` if the list was an array. If the index is outside the
+ * list size, NULL is returned.
+ *
+ * @code
+ *	struct foo *f = get_a_foo();
+ *      struct bar *element = list_nth_entry(struct bar, &f->list_of_bars, link, 2);
+ * @endcode
+ */
+
+#define list_nth_entry(type, head, member, index) \
+ ({ type *nth_ = NULL, *pos_; \
+    __typeof__(index) idx_ = 0; \
+   list_for_each(pos_, head, member) { \
+       if (idx_ == index) { nth_ = pos_; break; } \
+       idx_++; \
+    } \
+    nth_; \
+  })