diff --git a/examples/CurrentManager/include/CurrentManager.hpp b/examples/CurrentManager/include/CurrentManager.hpp
index 12e757805..e3d274f1d 100644
--- a/examples/CurrentManager/include/CurrentManager.hpp
+++ b/examples/CurrentManager/include/CurrentManager.hpp
@@ -5,7 +5,6 @@
 #include <span>
 #include <vector>
 
-#include <sleipnir/SymbolExports.hpp>
 #include <sleipnir/optimization/OptimizationProblem.hpp>
 #include <units/current.h>
 
@@ -15,7 +14,7 @@
  * which subsystem gets less current if the current budget is exceeded.
  * Subsystems with a smaller tolerance are given higher priority.
  */
-class SLEIPNIR_DLLEXPORT CurrentManager {
+class CurrentManager {
  public:
   /**
    * Constructs a CurrentManager.
diff --git a/include/sleipnir/autodiff/Expression.hpp b/include/sleipnir/autodiff/Expression.hpp
index cd1d475e4..2d8363531 100644
--- a/include/sleipnir/autodiff/Expression.hpp
+++ b/include/sleipnir/autodiff/Expression.hpp
@@ -9,10 +9,10 @@
 #include <utility>
 #include <vector>
 
-#include "sleipnir/IntrusiveSharedPtr.hpp"
-#include "sleipnir/Pool.hpp"
-#include "sleipnir/SymbolExports.hpp"
 #include "sleipnir/autodiff/ExpressionType.hpp"
+#include "sleipnir/util/IntrusiveSharedPtr.hpp"
+#include "sleipnir/util/Pool.hpp"
+#include "sleipnir/util/SymbolExports.hpp"
 
 namespace sleipnir::detail {
 
diff --git a/include/sleipnir/autodiff/ExpressionGraph.hpp b/include/sleipnir/autodiff/ExpressionGraph.hpp
index 6c517edbc..346cefd48 100644
--- a/include/sleipnir/autodiff/ExpressionGraph.hpp
+++ b/include/sleipnir/autodiff/ExpressionGraph.hpp
@@ -4,9 +4,9 @@
 
 #include <vector>
 
-#include "sleipnir/SymbolExports.hpp"
 #include "sleipnir/autodiff/Expression.hpp"
 #include "sleipnir/autodiff/Variable.hpp"
+#include "sleipnir/util/SymbolExports.hpp"
 
 namespace sleipnir::detail {
 
diff --git a/include/sleipnir/autodiff/ExpressionType.hpp b/include/sleipnir/autodiff/ExpressionType.hpp
index 2f8297989..2c3c03593 100644
--- a/include/sleipnir/autodiff/ExpressionType.hpp
+++ b/include/sleipnir/autodiff/ExpressionType.hpp
@@ -4,7 +4,7 @@
 
 #include <iosfwd>
 
-#include "sleipnir/SymbolExports.hpp"
+#include "sleipnir/util/SymbolExports.hpp"
 
 namespace sleipnir {
 
diff --git a/include/sleipnir/autodiff/Gradient.hpp b/include/sleipnir/autodiff/Gradient.hpp
index 744f6fc5d..e4e0b5095 100644
--- a/include/sleipnir/autodiff/Gradient.hpp
+++ b/include/sleipnir/autodiff/Gradient.hpp
@@ -6,10 +6,10 @@
 
 #include <Eigen/SparseCore>
 
-#include "sleipnir/SymbolExports.hpp"
 #include "sleipnir/autodiff/Jacobian.hpp"
 #include "sleipnir/autodiff/Profiler.hpp"
 #include "sleipnir/autodiff/Variable.hpp"
+#include "sleipnir/util/SymbolExports.hpp"
 
 namespace sleipnir {
 
diff --git a/include/sleipnir/autodiff/Hessian.hpp b/include/sleipnir/autodiff/Hessian.hpp
index 89b9dfccc..ee487f6e4 100644
--- a/include/sleipnir/autodiff/Hessian.hpp
+++ b/include/sleipnir/autodiff/Hessian.hpp
@@ -5,10 +5,10 @@
 #include <Eigen/Core>
 #include <Eigen/SparseCore>
 
-#include "sleipnir/SymbolExports.hpp"
 #include "sleipnir/autodiff/Jacobian.hpp"
 #include "sleipnir/autodiff/Profiler.hpp"
 #include "sleipnir/autodiff/Variable.hpp"
+#include "sleipnir/util/SymbolExports.hpp"
 
 namespace sleipnir {
 
diff --git a/include/sleipnir/autodiff/Jacobian.hpp b/include/sleipnir/autodiff/Jacobian.hpp
index 79341dc02..6a4dcc5b5 100644
--- a/include/sleipnir/autodiff/Jacobian.hpp
+++ b/include/sleipnir/autodiff/Jacobian.hpp
@@ -6,11 +6,11 @@
 
 #include <Eigen/SparseCore>
 
-#include "sleipnir/SymbolExports.hpp"
 #include "sleipnir/autodiff/Expression.hpp"
 #include "sleipnir/autodiff/ExpressionGraph.hpp"
 #include "sleipnir/autodiff/Profiler.hpp"
 #include "sleipnir/autodiff/Variable.hpp"
+#include "sleipnir/util/SymbolExports.hpp"
 
 namespace sleipnir {
 
diff --git a/include/sleipnir/autodiff/Profiler.hpp b/include/sleipnir/autodiff/Profiler.hpp
index 7296402de..1f36026a6 100644
--- a/include/sleipnir/autodiff/Profiler.hpp
+++ b/include/sleipnir/autodiff/Profiler.hpp
@@ -4,7 +4,7 @@
 
 #include <chrono>
 
-#include "sleipnir/SymbolExports.hpp"
+#include "sleipnir/util/SymbolExports.hpp"
 
 namespace sleipnir {
 
diff --git a/include/sleipnir/autodiff/Variable.hpp b/include/sleipnir/autodiff/Variable.hpp
index b00ebcfd5..f924f2e34 100644
--- a/include/sleipnir/autodiff/Variable.hpp
+++ b/include/sleipnir/autodiff/Variable.hpp
@@ -7,8 +7,8 @@
 #include <Eigen/Core>
 #include <Eigen/SparseCore>
 
-#include "sleipnir/SymbolExports.hpp"
 #include "sleipnir/autodiff/Expression.hpp"
+#include "sleipnir/util/SymbolExports.hpp"
 
 namespace sleipnir {
 
diff --git a/include/sleipnir/autodiff/VariableBlock.hpp b/include/sleipnir/autodiff/VariableBlock.hpp
index 1171a43c3..a5fe43ffe 100644
--- a/include/sleipnir/autodiff/VariableBlock.hpp
+++ b/include/sleipnir/autodiff/VariableBlock.hpp
@@ -9,8 +9,8 @@
 
 #include <Eigen/Core>
 
-#include "sleipnir/SymbolExports.hpp"
 #include "sleipnir/autodiff/Variable.hpp"
+#include "sleipnir/util/SymbolExports.hpp"
 
 namespace sleipnir {
 
diff --git a/include/sleipnir/autodiff/VariableMatrix.hpp b/include/sleipnir/autodiff/VariableMatrix.hpp
index fd09c612f..05a623759 100644
--- a/include/sleipnir/autodiff/VariableMatrix.hpp
+++ b/include/sleipnir/autodiff/VariableMatrix.hpp
@@ -11,9 +11,9 @@
 
 #include <Eigen/Core>
 
-#include "sleipnir/SymbolExports.hpp"
 #include "sleipnir/autodiff/Variable.hpp"
 #include "sleipnir/autodiff/VariableBlock.hpp"
+#include "sleipnir/util/SymbolExports.hpp"
 
 namespace sleipnir {
 
diff --git a/include/sleipnir/control/OCPSolver.hpp b/include/sleipnir/control/OCPSolver.hpp
index fb7fd42f3..b866c34f6 100644
--- a/include/sleipnir/control/OCPSolver.hpp
+++ b/include/sleipnir/control/OCPSolver.hpp
@@ -7,9 +7,9 @@
 
 #include <Eigen/Core>
 
-#include "sleipnir/SymbolExports.hpp"
 #include "sleipnir/autodiff/VariableMatrix.hpp"
 #include "sleipnir/optimization/OptimizationProblem.hpp"
+#include "sleipnir/util/SymbolExports.hpp"
 
 namespace sleipnir {
 
diff --git a/include/sleipnir/optimization/Constraints.hpp b/include/sleipnir/optimization/Constraints.hpp
index 111cebbcc..d7f65515d 100644
--- a/include/sleipnir/optimization/Constraints.hpp
+++ b/include/sleipnir/optimization/Constraints.hpp
@@ -4,9 +4,9 @@
 
 #include <vector>
 
-#include "sleipnir/SymbolExports.hpp"
 #include "sleipnir/autodiff/Variable.hpp"
 #include "sleipnir/autodiff/VariableMatrix.hpp"
+#include "sleipnir/util/SymbolExports.hpp"
 
 namespace sleipnir {
 
diff --git a/include/sleipnir/optimization/OptimizationProblem.hpp b/include/sleipnir/optimization/OptimizationProblem.hpp
index f54fb2cf5..20897ad64 100644
--- a/include/sleipnir/optimization/OptimizationProblem.hpp
+++ b/include/sleipnir/optimization/OptimizationProblem.hpp
@@ -13,7 +13,6 @@
 #include <Eigen/Core>
 #include <Eigen/SparseCore>
 
-#include "sleipnir/SymbolExports.hpp"
 #include "sleipnir/autodiff/Variable.hpp"
 #include "sleipnir/autodiff/VariableMatrix.hpp"
 #include "sleipnir/optimization/Constraints.hpp"
@@ -21,6 +20,7 @@
 #include "sleipnir/optimization/SolverExitCondition.hpp"
 #include "sleipnir/optimization/SolverIterationInfo.hpp"
 #include "sleipnir/optimization/SolverStatus.hpp"
+#include "sleipnir/util/SymbolExports.hpp"
 
 namespace sleipnir {
 
diff --git a/include/sleipnir/optimization/SolverConfig.hpp b/include/sleipnir/optimization/SolverConfig.hpp
index beea42d3c..fcdb97272 100644
--- a/include/sleipnir/optimization/SolverConfig.hpp
+++ b/include/sleipnir/optimization/SolverConfig.hpp
@@ -5,7 +5,7 @@
 #include <chrono>
 #include <limits>
 
-#include "sleipnir/SymbolExports.hpp"
+#include "sleipnir/util/SymbolExports.hpp"
 
 namespace sleipnir {
 
diff --git a/include/sleipnir/optimization/SolverExitCondition.hpp b/include/sleipnir/optimization/SolverExitCondition.hpp
index 5c2ed6e46..865996164 100644
--- a/include/sleipnir/optimization/SolverExitCondition.hpp
+++ b/include/sleipnir/optimization/SolverExitCondition.hpp
@@ -4,7 +4,7 @@
 
 #include <iosfwd>
 
-#include "sleipnir/SymbolExports.hpp"
+#include "sleipnir/util/SymbolExports.hpp"
 
 namespace sleipnir {
 
diff --git a/include/sleipnir/optimization/SolverStatus.hpp b/include/sleipnir/optimization/SolverStatus.hpp
index 4754551a9..04572a1dc 100644
--- a/include/sleipnir/optimization/SolverStatus.hpp
+++ b/include/sleipnir/optimization/SolverStatus.hpp
@@ -2,9 +2,9 @@
 
 #pragma once
 
-#include "sleipnir/SymbolExports.hpp"
 #include "sleipnir/autodiff/Expression.hpp"
 #include "sleipnir/optimization/SolverExitCondition.hpp"
+#include "sleipnir/util/SymbolExports.hpp"
 
 namespace sleipnir {
 
diff --git a/include/sleipnir/Formatters.hpp b/include/sleipnir/util/Formatters.hpp
similarity index 100%
rename from include/sleipnir/Formatters.hpp
rename to include/sleipnir/util/Formatters.hpp
diff --git a/include/sleipnir/IntrusiveSharedPtr.hpp b/include/sleipnir/util/IntrusiveSharedPtr.hpp
similarity index 100%
rename from include/sleipnir/IntrusiveSharedPtr.hpp
rename to include/sleipnir/util/IntrusiveSharedPtr.hpp
diff --git a/include/sleipnir/Pool.hpp b/include/sleipnir/util/Pool.hpp
similarity index 99%
rename from include/sleipnir/Pool.hpp
rename to include/sleipnir/util/Pool.hpp
index 9552f71fa..b7939acbc 100644
--- a/include/sleipnir/Pool.hpp
+++ b/include/sleipnir/util/Pool.hpp
@@ -8,7 +8,7 @@
 #include <memory>
 #include <vector>
 
-#include "sleipnir/SymbolExports.hpp"
+#include "sleipnir/util/SymbolExports.hpp"
 
 namespace sleipnir {
 
diff --git a/include/sleipnir/util/Spy.hpp b/include/sleipnir/util/Spy.hpp
new file mode 100644
index 000000000..baa4bbdb9
--- /dev/null
+++ b/include/sleipnir/util/Spy.hpp
@@ -0,0 +1,48 @@
+// Copyright (c) Sleipnir contributors
+
+#pragma once
+
+#include <iosfwd>
+#include <string_view>
+
+#include <Eigen/SparseCore>
+
+#include "sleipnir/util/SymbolExports.hpp"
+
+namespace sleipnir {
+
+/**
+ * Write the sparsity pattern of a sparse matrix to a file.
+ *
+ * Each character represents an element with "." representing zero, "+"
+ * representing positive, and "-" representing negative. Here's an example for a
+ * 3x3 identity matrix.
+ *
+ * "+.."
+ * ".+."
+ * "..+"
+ *
+ * @param[in] filename The filename.
+ * @param[in] mat The sparse matrix.
+ */
+SLEIPNIR_DLLEXPORT void Spy(std::string_view filename,
+                            const Eigen::SparseMatrix<double>& mat);
+
+/**
+ * Write the sparsity pattern of a sparse matrix to a file.
+ *
+ * Each character represents an element with '.' representing zero, '+'
+ * representing positive, and '-' representing negative. Here's an example for a
+ * 3x3 identity matrix.
+ *
+ * "+.."
+ * ".+."
+ * "..+"
+ *
+ * @param[out] file A file stream.
+ * @param[in] mat The sparse matrix.
+ */
+SLEIPNIR_DLLEXPORT void Spy(std::ostream& file,
+                            const Eigen::SparseMatrix<double>& mat);
+
+}  // namespace sleipnir
diff --git a/include/sleipnir/SymbolExports.hpp b/include/sleipnir/util/SymbolExports.hpp
similarity index 100%
rename from include/sleipnir/SymbolExports.hpp
rename to include/sleipnir/util/SymbolExports.hpp
diff --git a/src/autodiff/Variable.cpp b/src/autodiff/Variable.cpp
index 300083822..ee8301f10 100644
--- a/src/autodiff/Variable.cpp
+++ b/src/autodiff/Variable.cpp
@@ -4,7 +4,6 @@
 
 #include <fmt/core.h>
 
-#include "sleipnir/SymbolExports.hpp"
 #include "sleipnir/autodiff/Expression.hpp"
 #include "sleipnir/autodiff/ExpressionGraph.hpp"
 
diff --git a/src/optimization/OptimizationProblem.cpp b/src/optimization/OptimizationProblem.cpp
index 7c7eda233..49efb63c0 100644
--- a/src/optimization/OptimizationProblem.cpp
+++ b/src/optimization/OptimizationProblem.cpp
@@ -11,18 +11,19 @@
 
 #include <fmt/core.h>
 
-#include "Filter.hpp"
-#include "RegularizedLDLT.hpp"
-#include "ScopeExit.hpp"
+#include "optimization/Filter.hpp"
+#include "optimization/RegularizedLDLT.hpp"
 #include "sleipnir/autodiff/Expression.hpp"
 #include "sleipnir/autodiff/Gradient.hpp"
 #include "sleipnir/autodiff/Hessian.hpp"
 #include "sleipnir/autodiff/Jacobian.hpp"
 #include "sleipnir/autodiff/Variable.hpp"
 #include "sleipnir/optimization/SolverExitCondition.hpp"
-#include "sleipnir/util/AutodiffUtil.hpp"
-#include "sleipnir/util/SparseMatrixBuilder.hpp"
-#include "sleipnir/util/SparseUtil.hpp"
+#include "sleipnir/util/Spy.hpp"
+#include "util/AutodiffUtil.hpp"
+#include "util/ScopeExit.hpp"
+#include "util/SparseMatrixBuilder.hpp"
+#include "util/SparseUtil.hpp"
 
 using namespace sleipnir;
 
diff --git a/src/optimization/RegularizedLDLT.hpp b/src/optimization/RegularizedLDLT.hpp
index f0020c4cb..13b6dff50 100644
--- a/src/optimization/RegularizedLDLT.hpp
+++ b/src/optimization/RegularizedLDLT.hpp
@@ -8,9 +8,9 @@
 #include <Eigen/SparseCholesky>
 #include <Eigen/SparseCore>
 
-#include "Inertia.hpp"
-#include "sleipnir/util/SparseMatrixBuilder.hpp"
-#include "sleipnir/util/SparseUtil.hpp"
+#include "optimization/Inertia.hpp"
+#include "util/SparseMatrixBuilder.hpp"
+#include "util/SparseUtil.hpp"
 
 namespace sleipnir {
 
diff --git a/src/util/AutodiffUtil.cpp b/src/util/AutodiffUtil.cpp
index e252149e1..29d256454 100644
--- a/src/util/AutodiffUtil.cpp
+++ b/src/util/AutodiffUtil.cpp
@@ -1,6 +1,6 @@
 // Copyright (c) Sleipnir contributors
 
-#include "sleipnir/util/AutodiffUtil.hpp"
+#include "util/AutodiffUtil.hpp"
 
 #include <cassert>
 
diff --git a/include/sleipnir/util/AutodiffUtil.hpp b/src/util/AutodiffUtil.hpp
similarity index 61%
rename from include/sleipnir/util/AutodiffUtil.hpp
rename to src/util/AutodiffUtil.hpp
index 2b945d2f4..4f3b69e46 100644
--- a/include/sleipnir/util/AutodiffUtil.hpp
+++ b/src/util/AutodiffUtil.hpp
@@ -6,7 +6,6 @@
 
 #include <Eigen/Core>
 
-#include "sleipnir/SymbolExports.hpp"
 #include "sleipnir/autodiff/Variable.hpp"
 
 namespace sleipnir {
@@ -17,8 +16,8 @@ namespace sleipnir {
  * @param[out] dest The std::vector<Variable>.
  * @param[in] src The Eigen::VectorXd.
  */
-SLEIPNIR_DLLEXPORT void SetAD(std::vector<Variable>& dest,
-                              const Eigen::Ref<const Eigen::VectorXd>& src);
+void SetAD(std::vector<Variable>& dest,
+           const Eigen::Ref<const Eigen::VectorXd>& src);
 
 /**
  * Assigns an Eigen::VectorXd to an Eigen::Vector<Variable>.
@@ -26,21 +25,21 @@ SLEIPNIR_DLLEXPORT void SetAD(std::vector<Variable>& dest,
  * @param[out] dest The Eigen::Vector<Variable>.
  * @param[in] src The Eigen::VectorXd.
  */
-SLEIPNIR_DLLEXPORT void SetAD(Eigen::Ref<VectorXvar> dest,
-                              const Eigen::Ref<const Eigen::VectorXd>& src);
+void SetAD(Eigen::Ref<VectorXvar> dest,
+           const Eigen::Ref<const Eigen::VectorXd>& src);
 
 /**
  * Returns a std::vector<Variable> as an Eigen::VectorXd.
  *
  * @param src The std::vector<Variable>.
  */
-SLEIPNIR_DLLEXPORT Eigen::VectorXd GetAD(std::vector<Variable> src);
+Eigen::VectorXd GetAD(std::vector<Variable> src);
 
 /**
  * Returns an Eigen::Vector<Variable> as an Eigen::VectorXd.
  *
  * @param src The Eigen::Vector<Variable>.
  */
-SLEIPNIR_DLLEXPORT Eigen::VectorXd GetAD(Eigen::Ref<VectorXvar> src);
+Eigen::VectorXd GetAD(Eigen::Ref<VectorXvar> src);
 
 }  // namespace sleipnir
diff --git a/src/ScopeExit.hpp b/src/util/ScopeExit.hpp
similarity index 100%
rename from src/ScopeExit.hpp
rename to src/util/ScopeExit.hpp
diff --git a/include/sleipnir/util/SparseMatrixBuilder.hpp b/src/util/SparseMatrixBuilder.hpp
similarity index 100%
rename from include/sleipnir/util/SparseMatrixBuilder.hpp
rename to src/util/SparseMatrixBuilder.hpp
diff --git a/src/util/SparseUtil.cpp b/src/util/SparseUtil.cpp
index d10a68dd2..2ba1e7ace 100644
--- a/src/util/SparseUtil.cpp
+++ b/src/util/SparseUtil.cpp
@@ -1,6 +1,6 @@
 // Copyright (c) Sleipnir contributors
 
-#include "sleipnir/util/SparseUtil.hpp"
+#include "util/SparseUtil.hpp"
 
 namespace sleipnir {
 
@@ -30,47 +30,4 @@ Eigen::SparseMatrix<double> SparseIdentity(int rows, int cols) {
   return dest;
 }
 
-void Spy(std::string_view filename, const Eigen::SparseMatrix<double>& mat) {
-  std::ofstream file{std::string{filename}};
-  if (!file.is_open()) {
-    return;
-  }
-
-  Spy(file, mat);
-}
-
-void Spy(std::ostream& file, const Eigen::SparseMatrix<double>& mat) {
-  const int cells_width = mat.cols() + 1;
-  const int cells_height = mat.rows();
-
-  std::vector<uint8_t> cells;
-
-  // Allocate space for matrix of characters plus trailing newlines
-  cells.reserve(cells_width * cells_height);
-
-  // Initialize cell array
-  for (int row = 0; row < mat.rows(); ++row) {
-    for (int col = 0; col < mat.cols(); ++col) {
-      cells.emplace_back('.');
-    }
-    cells.emplace_back('\n');
-  }
-
-  // Fill in non-sparse entries
-  for (int k = 0; k < mat.outerSize(); ++k) {
-    for (Eigen::SparseMatrix<double>::InnerIterator it{mat, k}; it; ++it) {
-      if (it.value() < 0.0) {
-        cells[it.row() * cells_width + it.col()] = '-';
-      } else if (it.value() > 0.0) {
-        cells[it.row() * cells_width + it.col()] = '+';
-      }
-    }
-  }
-
-  // Write cell array to file
-  for (const auto& c : cells) {
-    file << c;
-  }
-}
-
 }  // namespace sleipnir
diff --git a/include/sleipnir/util/SparseUtil.hpp b/src/util/SparseUtil.hpp
similarity index 52%
rename from include/sleipnir/util/SparseUtil.hpp
rename to src/util/SparseUtil.hpp
index c35a84fce..c495cf92b 100644
--- a/include/sleipnir/util/SparseUtil.hpp
+++ b/src/util/SparseUtil.hpp
@@ -3,14 +3,12 @@
 #pragma once
 
 #include <algorithm>
+#include <cmath>
 #include <concepts>
-#include <fstream>
 
 #include <Eigen/Core>
 #include <Eigen/SparseCore>
 
-#include "sleipnir/SymbolExports.hpp"
-
 namespace sleipnir {
 
 /**
@@ -18,8 +16,7 @@ namespace sleipnir {
  *
  * @param src Column vector.
  */
-SLEIPNIR_DLLEXPORT Eigen::SparseMatrix<double> SparseDiagonal(
-    const Eigen::VectorXd& src);
+Eigen::SparseMatrix<double> SparseDiagonal(const Eigen::VectorXd& src);
 
 /**
  * Returns sparse identity matrix.
@@ -27,8 +24,7 @@ SLEIPNIR_DLLEXPORT Eigen::SparseMatrix<double> SparseDiagonal(
  * @param rows Number of rows.
  * @param cols Number of columns.
  */
-SLEIPNIR_DLLEXPORT Eigen::SparseMatrix<double> SparseIdentity(int rows,
-                                                              int cols);
+Eigen::SparseMatrix<double> SparseIdentity(int rows, int cols);
 
 /**
  * Returns lₚ-norm of a sparse matrix.
@@ -64,38 +60,4 @@ double SparseLpNorm(const Derived& mat) {
   }
 }
 
-/**
- * Write the sparsity pattern of a sparse matrix to a file.
- *
- * Each character represents an element with "." representing zero, "+"
- * representing positive, and "-" representing negative. Here's an example for a
- * 3x3 identity matrix.
- *
- * "+.."
- * ".+."
- * "..+"
- *
- * @param[in] filename The filename.
- * @param[in] mat The sparse matrix.
- */
-SLEIPNIR_DLLEXPORT void Spy(std::string_view filename,
-                            const Eigen::SparseMatrix<double>& mat);
-
-/**
- * Write the sparsity pattern of a sparse matrix to a file.
- *
- * Each character represents an element with '.' representing zero, '+'
- * representing positive, and '-' representing negative. Here's an example for a
- * 3x3 identity matrix.
- *
- * "+.."
- * ".+."
- * "..+"
- *
- * @param[out] file A file stream.
- * @param[in] mat The sparse matrix.
- */
-SLEIPNIR_DLLEXPORT void Spy(std::ostream& file,
-                            const Eigen::SparseMatrix<double>& mat);
-
 }  // namespace sleipnir
diff --git a/src/util/Spy.cpp b/src/util/Spy.cpp
new file mode 100644
index 000000000..b59df83de
--- /dev/null
+++ b/src/util/Spy.cpp
@@ -0,0 +1,52 @@
+// Copyright (c) Sleipnir contributors
+
+#include "sleipnir/util/Spy.hpp"
+
+#include <fstream>
+
+namespace sleipnir {
+
+void Spy(std::string_view filename, const Eigen::SparseMatrix<double>& mat) {
+  std::ofstream file{std::string{filename}};
+  if (!file.is_open()) {
+    return;
+  }
+
+  Spy(file, mat);
+}
+
+void Spy(std::ostream& file, const Eigen::SparseMatrix<double>& mat) {
+  const int cells_width = mat.cols() + 1;
+  const int cells_height = mat.rows();
+
+  std::vector<uint8_t> cells;
+
+  // Allocate space for matrix of characters plus trailing newlines
+  cells.reserve(cells_width * cells_height);
+
+  // Initialize cell array
+  for (int row = 0; row < mat.rows(); ++row) {
+    for (int col = 0; col < mat.cols(); ++col) {
+      cells.emplace_back('.');
+    }
+    cells.emplace_back('\n');
+  }
+
+  // Fill in non-sparse entries
+  for (int k = 0; k < mat.outerSize(); ++k) {
+    for (Eigen::SparseMatrix<double>::InnerIterator it{mat, k}; it; ++it) {
+      if (it.value() < 0.0) {
+        cells[it.row() * cells_width + it.col()] = '-';
+      } else if (it.value() > 0.0) {
+        cells[it.row() * cells_width + it.col()] = '+';
+      }
+    }
+  }
+
+  // Write cell array to file
+  for (const auto& c : cells) {
+    file << c;
+  }
+}
+
+}  // namespace sleipnir
diff --git a/test/src/FormattersTest.cpp b/test/src/FormattersTest.cpp
index a539e2433..64ffa08b7 100644
--- a/test/src/FormattersTest.cpp
+++ b/test/src/FormattersTest.cpp
@@ -4,7 +4,7 @@
 
 #include <fmt/format.h>
 #include <gtest/gtest.h>
-#include <sleipnir/Formatters.hpp>
+#include <sleipnir/util/Formatters.hpp>
 
 TEST(FormattersTest, Eigen) {
   Eigen::Matrix<double, 3, 2> A{{0.0, 1.0}, {2.0, 3.0}, {4.0, 5.0}};
diff --git a/test/src/IntrusiveSharedPtrTest.cpp b/test/src/IntrusiveSharedPtrTest.cpp
index d10029974..8bdd44c9c 100644
--- a/test/src/IntrusiveSharedPtrTest.cpp
+++ b/test/src/IntrusiveSharedPtrTest.cpp
@@ -6,7 +6,7 @@
 #include <utility>
 
 #include <gtest/gtest.h>
-#include <sleipnir/IntrusiveSharedPtr.hpp>
+#include <sleipnir/util/IntrusiveSharedPtr.hpp>
 
 // NOLINTBEGIN (clang-analyzer-cplusplus.NewDeleteLeaks)
 
diff --git a/test/src/ScopeExitTest.cpp b/test/src/ScopeExitTest.cpp
index 790190463..b83fed09f 100644
--- a/test/src/ScopeExitTest.cpp
+++ b/test/src/ScopeExitTest.cpp
@@ -2,7 +2,7 @@
 
 #include <gtest/gtest.h>
 
-#include "ScopeExit.hpp"
+#include "util/ScopeExit.hpp"
 
 TEST(ScopeExitTest, ScopeExit) {
   int exitCount = 0;
diff --git a/test/src/util/SparseUtilTest.cpp b/test/src/SparseUtilTest.cpp
similarity index 98%
rename from test/src/util/SparseUtilTest.cpp
rename to test/src/SparseUtilTest.cpp
index 5e73057e9..e19140437 100644
--- a/test/src/util/SparseUtilTest.cpp
+++ b/test/src/SparseUtilTest.cpp
@@ -4,7 +4,7 @@
 #include <vector>
 
 #include <gtest/gtest.h>
-#include <sleipnir/util/SparseUtil.hpp>
+#include <util/SparseUtil.hpp>
 
 TEST(SparseUtilTest, SparseDiagonal) {
   Eigen::Vector<double, 5> vec{-2.0, -1.0, 0.0, 1.0, 2.0};