From d0ee4f22839e49aaa7b394143c53c593bbd3cf11 Mon Sep 17 00:00:00 2001
From: Falk David <falk@blender.org>
Date: Mon, 9 Oct 2023 13:39:26 +0200
Subject: [PATCH] BLI: Support negative steps in `findlinkfrom`

The function `BLI_findlinkfrom` returns the link that is n-steps
after the given start link. This did not work for negative steps.

This change makes it so that both positive and negative step
values work.
---
 source/blender/blenlib/BLI_listbase.h     |  5 +++--
 source/blender/blenlib/intern/listbase.cc | 15 +++++++++++----
 2 files changed, 14 insertions(+), 6 deletions(-)

diff --git a/source/blender/blenlib/BLI_listbase.h b/source/blender/blenlib/BLI_listbase.h
index 394f5b23f5d..5cb1e5f3cab 100644
--- a/source/blender/blenlib/BLI_listbase.h
+++ b/source/blender/blenlib/BLI_listbase.h
@@ -45,9 +45,10 @@ void *BLI_findlink(const struct ListBase *listbase, int number) ATTR_WARN_UNUSED
     ATTR_NONNULL(1);
 
 /**
- * Returns the nth element after \a link, numbering from 0.
+ * Returns the element before/after \a link that is \a step links away, numbering from 0. \a step
+ * is allowed to be negative. Returns NULL when the link is out-of-bounds.
  */
-void *BLI_findlinkfrom(struct Link *start, int number) ATTR_WARN_UNUSED_RESULT;
+void *BLI_findlinkfrom(struct Link *start, int step) ATTR_WARN_UNUSED_RESULT;
 
 /**
  * Finds the first element of \a listbase which contains the null-terminated
diff --git a/source/blender/blenlib/intern/listbase.cc b/source/blender/blenlib/intern/listbase.cc
index 9af2634c5d3..8e0ddc6ebe5 100644
--- a/source/blender/blenlib/intern/listbase.cc
+++ b/source/blender/blenlib/intern/listbase.cc
@@ -560,17 +560,24 @@ void *BLI_rfindlink(const ListBase *listbase, int number)
   return link;
 }
 
-void *BLI_findlinkfrom(Link *start, int number)
+void *BLI_findlinkfrom(Link *start, int steps)
 {
   Link *link = nullptr;
 
-  if (number >= 0) {
+  if (steps >= 0) {
     link = start;
-    while (link != nullptr && number != 0) {
-      number--;
+    while (link != nullptr && steps != 0) {
+      steps--;
       link = link->next;
     }
   }
+  else {
+    link = start;
+    while (link != nullptr && steps != 0) {
+      steps++;
+      link = link->prev;
+    }
+  }
 
   return link;
 }