BLI: support accessing nth last element in Array/Span/Vector

This often helps to make the intend of code more clear compared
to computing the index manually in the caller.

1 files changed, 10 insertions, 8 deletions

diff --git a/source/blender/blenlib/BLI_array.hh b/source/blender/blenlib/BLI_array.hh
index a580f12851e..91dfc81ae27 100644
--- a/source/blender/blenlib/BLI_array.hh
+++ b/source/blender/blenlib/BLI_array.hh
@@ -278,18 +278,20 @@ class Array {
   }
 
   /**
-   * Return a reference to the last element in the array.
-   * This invokes undefined behavior when the array is empty.
+   * Return a reference to the nth last element.
+   * This invokes undefined behavior when the array is too short.
    */
-  const T &last() const
+  const T &last(const int64_t n = 0) const
   {
-    BLI_assert(size_ > 0);
-    return *(data_ + size_ - 1);
+    BLI_assert(n >= 0);
+    BLI_assert(n < size_);
+    return *(data_ + size_ - 1 - n);
   }
-  T &last()
+  T &last(const int64_t n = 0)
   {
-    BLI_assert(size_ > 0);
-    return *(data_ + size_ - 1);
+    BLI_assert(n >= 0);
+    BLI_assert(n < size_);
+    return *(data_ + size_ - 1 - n);
   }
 
   /**