Add operator[] to batch in thread pipeline

src/thread_pipeline.hpp

@@ -2,6 +2,7 @@
 
 #include <atomic>
 #include <cassert>
+#include <cstddef>
 #include <cstdint>
 #include <cstdio>
 #include <cstdlib>
@@ -161,9 +162,6 @@ struct ThreadPipeline {
         return static_cast<difference_type>(index_) -
                static_cast<difference_type>(rhs.index_);
       }
-      reference operator[](difference_type n) const {
-        return (*ring)[(index_ + n) & (ring->size() - 1)];
-      }
       friend Iterator operator+(difference_type n, const Iterator &iter) {
         return iter + n;
       }
@@ -211,6 +209,9 @@ struct ThreadPipeline {
 
     [[nodiscard]] size_t size() const { return end_ - begin_; }
     [[nodiscard]] bool empty() const { return end_ == begin_; }
+    T &operator[](uint32_t n) {
+      return (*ring)[(begin_ + n) & (ring->size() - 1)];
+    }
 
   private:
     friend struct ThreadPipeline;