ptrlist: add ptr_list_nth_entry()

Usually ptr lists are accessed iteratively via the FOR/END macros
but in few case we may need to access a given element in a list,
like for example when accessing a given argument of a function.

Create an helper doing that instead of open coding it.

Signed-off-by: Luc Van Oostenryck <luc.vanoostenryck@gmail.com>

2 files changed, 23 insertions, 0 deletions

diff --git a/ptrlist.c b/ptrlist.c
index 3677a347..0fb28127 100644
--- a/ptrlist.c
+++ b/ptrlist.c
@@ -115,6 +115,28 @@ void *last_ptr_list(struct ptr_list *head)
 }
 
 ///
+// get the nth element of a ptrlist
+// @head: the head of the list
+// @return: the nth element of the list or ``NULL`` if the list is too short.
+void *ptr_list_nth_entry(struct ptr_list *list, unsigned int idx)
+{
+	struct ptr_list *head = list;
+
+	if (!head)
+		return NULL;
+
+	do {
+		unsigned int nr = list->nr;
+
+		if (idx < nr)
+			return list->list[idx];
+		else
+			idx -= nr;
+	} while ((list = list->next) != head);
+	return NULL;
+}
+
+///
 // linearize the entries of a list
 //
 // @head: the list to be linearized
diff --git a/ptrlist.h b/ptrlist.h
index 2f023478..2411e745 100644
--- a/ptrlist.h
+++ b/ptrlist.h
@@ -44,6 +44,7 @@ extern bool ptr_list_multiple(const struct ptr_list *head);
 extern int linearize_ptr_list(struct ptr_list *, void **, int);
 extern void *first_ptr_list(struct ptr_list *);
 extern void *last_ptr_list(struct ptr_list *);
+extern void *ptr_list_nth_entry(struct ptr_list *, unsigned int idx);
 extern void pack_ptr_list(struct ptr_list **);
 
 /*