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, 12 insertions, 10 deletions

diff --git a/source/blender/blenlib/BLI_span.hh b/source/blender/blenlib/BLI_span.hh
index d82f21a57ff..9ab096094de 100644
--- a/source/blender/blenlib/BLI_span.hh
+++ b/source/blender/blenlib/BLI_span.hh
@@ -307,13 +307,14 @@ template<typename T> class Span {
   }
 
   /**
-   * Returns a reference to the last element in the array. This invokes undefined behavior when the
-   * array is empty.
+   * Returns a reference to the nth last element. This invokes undefined behavior when the span is
+   * too short.
    */
-  constexpr const T &last() const
+  constexpr 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];
   }
 
   /**
@@ -673,13 +674,14 @@ template<typename T> class MutableSpan {
   }
 
   /**
-   * Returns a reference to the last element. This invokes undefined behavior when the array is
-   * empty.
+   * Returns a reference to the nth last element. This invokes undefined behavior when the span is
+   * too short.
    */
-  constexpr T &last() const
+  constexpr 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];
   }
 
   /**