diff --git a/internal/ceres/block_structure.cc b/internal/ceres/block_structure.cc
index 39ba082..70f68b2 100644
--- a/internal/ceres/block_structure.cc
+++ b/internal/ceres/block_structure.cc
@@ -1,5 +1,5 @@
 // Ceres Solver - A fast non-linear least squares minimizer
-// Copyright 2015 Google Inc. All rights reserved.
+// Copyright 2023 Google Inc. All rights reserved.
 // http://ceres-solver.org/
 //
 // Redistribution and use in source and binary forms, with or without
@@ -30,8 +30,11 @@
 
 #include "ceres/block_structure.h"
 
-namespace ceres {
-namespace internal {
+#include <vector>
+
+#include "glog/logging.h"
+
+namespace ceres::internal {
 
 bool CellLessThan(const Cell& lhs, const Cell& rhs) {
   if (lhs.block_id == rhs.block_id) {
@@ -40,5 +43,28 @@
   return (lhs.block_id < rhs.block_id);
 }
 
-}  // namespace internal
-}  // namespace ceres
+std::vector<Block> Tail(const std::vector<Block>& blocks, int n) {
+  CHECK_LE(n, blocks.size());
+  std::vector<Block> tail;
+  const int num_blocks = blocks.size();
+  const int start = num_blocks - n;
+
+  int position = 0;
+  tail.reserve(n);
+  for (int i = start; i < num_blocks; ++i) {
+    tail.emplace_back(blocks[i].size, position);
+    position += blocks[i].size;
+  }
+
+  return tail;
+}
+
+int SumSquaredSizes(const std::vector<Block>& blocks) {
+  int sum = 0;
+  for (const auto& b : blocks) {
+    sum += b.size * b.size;
+  }
+  return sum;
+}
+
+}  // namespace ceres::internal