diff --git a/internal/ceres/array_utils.cc b/internal/ceres/array_utils.cc
index 3eea042d5..205ddaf27 100644
--- a/internal/ceres/array_utils.cc
+++ b/internal/ceres/array_utils.cc
@@ -30,10 +30,11 @@
 
 #include "ceres/array_utils.h"
 
+#include <algorithm>
 #include <cmath>
 #include <cstddef>
 #include <string>
-
+#include <vector>
 #include "ceres/fpclassify.h"
 #include "ceres/stringprintf.h"
 
@@ -94,5 +95,19 @@ void AppendArrayToString(const int size, const double* x, string* result) {
   }
 }
 
+void MapValuesToContiguousRange(const int size, int* array) {
+  std::vector<int> unique_values(array, array + size);
+  std::sort(unique_values.begin(), unique_values.end());
+  unique_values.erase(std::unique(unique_values.begin(),
+                                  unique_values.end()),
+                      unique_values.end());
+
+  for (int i = 0; i < size; ++i) {
+    array[i] = std::lower_bound(unique_values.begin(),
+                                unique_values.end(),
+                                array[i]) - unique_values.begin();
+  }
+}
+
 }  // namespace internal
 }  // namespace ceres
diff --git a/internal/ceres/array_utils.h b/internal/ceres/array_utils.h
index 34fda6fd4..55e68a8b5 100644
--- a/internal/ceres/array_utils.h
+++ b/internal/ceres/array_utils.h
@@ -67,6 +67,14 @@ void AppendArrayToString(const int size, const double* x, string* result);
 
 extern const double kImpossibleValue;
 
+// array contains a list of (possibly repeating) non-negative
+// integers. Let us assume that we have constructed another array `p`
+// by sorting and uniquing the entries of array.
+//
+// MapValuesToContiguousRange replaces each entry in "array" with its
+// position in `p`.
+void MapValuesToContiguousRange(int size, int* array);
+
 }  // namespace internal
 }  // namespace ceres
 
diff --git a/internal/ceres/array_utils_test.cc b/internal/ceres/array_utils_test.cc
index 96e625d62..203a301d3 100644
--- a/internal/ceres/array_utils_test.cc
+++ b/internal/ceres/array_utils_test.cc
@@ -32,6 +32,7 @@
 
 #include <limits>
 #include <cmath>
+#include <vector>
 #include "gtest/gtest.h"
 
 namespace ceres {
@@ -71,5 +72,51 @@ TEST(ArrayUtils, FindInvalidIndex) {
   EXPECT_EQ(FindInvalidValue(3, x), 0);
 }
 
+TEST(MapValuesToContiguousRange, ContiguousEntries) {
+  vector<int> array;
+  array.push_back(0);
+  array.push_back(1);
+  vector<int> expected = array;
+  MapValuesToContiguousRange(array.size(), &array[0]);
+  EXPECT_EQ(array, expected);
+  array.clear();
+
+  array.push_back(1);
+  array.push_back(0);
+  expected = array;
+  MapValuesToContiguousRange(array.size(), &array[0]);
+  EXPECT_EQ(array, expected);
+}
+
+TEST(MapValuesToContiguousRange, NonContiguousEntries) {
+  vector<int> array;
+  array.push_back(0);
+  array.push_back(2);
+  vector<int> expected;
+  expected.push_back(0);
+  expected.push_back(1);
+  MapValuesToContiguousRange(array.size(), &array[0]);
+  EXPECT_EQ(array, expected);
+}
+
+TEST(MapValuesToContiguousRange, NonContiguousRepeatingEntries) {
+  vector<int> array;
+  array.push_back(3);
+  array.push_back(1);
+  array.push_back(0);
+  array.push_back(0);
+  array.push_back(0);
+  array.push_back(5);
+  vector<int> expected;
+  expected.push_back(2);
+  expected.push_back(1);
+  expected.push_back(0);
+  expected.push_back(0);
+  expected.push_back(0);
+  expected.push_back(3);
+  MapValuesToContiguousRange(array.size(), &array[0]);
+  EXPECT_EQ(array, expected);
+}
+
 }  // namespace internal
 }  // namespace ceres
diff --git a/internal/ceres/solver_impl.cc b/internal/ceres/solver_impl.cc
index 2bf6cd262..fbcaf6b03 100644
--- a/internal/ceres/solver_impl.cc
+++ b/internal/ceres/solver_impl.cc
@@ -34,6 +34,7 @@
 #include <iostream>  // NOLINT
 #include <numeric>
 #include <string>
+#include "ceres/array_utils.h"
 #include "ceres/coordinate_descent_minimizer.h"
 #include "ceres/cxsparse.h"
 #include "ceres/evaluator.h"
@@ -1707,7 +1708,7 @@ bool SolverImpl::ReorderProgramForSchurTypeLinearSolver(
     // Renumber the entries of constraints to be contiguous integers
     // as camd requires that the group ids be in the range [0,
     // parameter_blocks.size() - 1].
-    SolverImpl::CompactifyArray(&constraints);
+    MapValuesToContiguousRange(constraints.size(), &constraints[0]);
 
     // Set the offsets and index for CreateJacobianSparsityTranspose.
     program->SetParameterOffsetsAndIndex();
@@ -1832,20 +1833,5 @@ bool SolverImpl::ReorderProgramForSparseNormalCholesky(
   return true;
 }
 
-void SolverImpl::CompactifyArray(vector<int>* array_ptr) {
-  vector<int>& array = *array_ptr;
-  const set<int> unique_group_ids(array.begin(), array.end());
-  map<int, int> group_id_map;
-  for (set<int>::const_iterator it = unique_group_ids.begin();
-       it != unique_group_ids.end();
-       ++it) {
-    InsertOrDie(&group_id_map, *it, group_id_map.size());
-  }
-
-  for (int i = 0; i < array.size(); ++i) {
-    array[i] = group_id_map[array[i]];
-  }
-}
-
 }  // namespace internal
 }  // namespace ceres
diff --git a/internal/ceres/solver_impl.h b/internal/ceres/solver_impl.h
index 631add59b..0085898bc 100644
--- a/internal/ceres/solver_impl.h
+++ b/internal/ceres/solver_impl.h
@@ -213,13 +213,6 @@ class SolverImpl {
       ParameterBlockOrdering* parameter_block_ordering,
       Program* program,
       string* message);
-
-  // array contains a list of (possibly repeating) non-negative
-  // integers. Let us assume that we have constructed another array
-  // `p` by sorting and uniqueing the entries of array.
-  // CompactifyArray replaces each entry in "array" with its position
-  // in `p`.
-  static void CompactifyArray(vector<int>* array);
 };
 
 }  // namespace internal
diff --git a/internal/ceres/solver_impl_test.cc b/internal/ceres/solver_impl_test.cc
index c2f8c2223..1f156286f 100644
--- a/internal/ceres/solver_impl_test.cc
+++ b/internal/ceres/solver_impl_test.cc
@@ -1038,53 +1038,6 @@ TEST(SolverImpl, ReallocationInCreateJacobianBlockSparsityTranspose) {
   EXPECT_EQ((expected_dense_jacobian - actual_dense_jacobian).norm(), 0.0);
 }
 
-TEST(CompactifyArray, ContiguousEntries) {
-  vector<int> array;
-  array.push_back(0);
-  array.push_back(1);
-  vector<int> expected = array;
-  SolverImpl::CompactifyArray(&array);
-  EXPECT_EQ(array, expected);
-  array.clear();
-
-  array.push_back(1);
-  array.push_back(0);
-  expected = array;
-  SolverImpl::CompactifyArray(&array);
-  EXPECT_EQ(array, expected);
-}
-
-TEST(CompactifyArray, NonContiguousEntries) {
-  vector<int> array;
-  array.push_back(0);
-  array.push_back(2);
-  vector<int> expected;
-  expected.push_back(0);
-  expected.push_back(1);
-  SolverImpl::CompactifyArray(&array);
-  EXPECT_EQ(array, expected);
-}
-
-TEST(CompactifyArray, NonContiguousRepeatingEntries) {
-  vector<int> array;
-  array.push_back(3);
-  array.push_back(1);
-  array.push_back(0);
-  array.push_back(0);
-  array.push_back(0);
-  array.push_back(5);
-  vector<int> expected;
-  expected.push_back(2);
-  expected.push_back(1);
-  expected.push_back(0);
-  expected.push_back(0);
-  expected.push_back(0);
-  expected.push_back(3);
-
-  SolverImpl::CompactifyArray(&array);
-  EXPECT_EQ(array, expected);
-}
-
 TEST(SolverImpl, ProblemHasNanParameterBlocks) {
   Problem problem;
   double x[2];