diff --git a/2024/src/day11.hpp b/2024/src/day11.hpp
index 806b09e..89956b4 100644
--- a/2024/src/day11.hpp
+++ b/2024/src/day11.hpp
@@ -8,17 +8,17 @@
 #ifndef DAY11_HPP_UOIOTEZD
 #define DAY11_HPP_UOIOTEZD
 
-#include "lib.hpp"    // for DEBUG
-#include <algorithm>  // for count_if
-#include <array>      // for array
-#include <cstddef>    // for size_t
-#include <cstdint>    // for int64_t
-#include <functional> // for plus
-#include <iostream>   // for istream, ostream, cerr
-#include <numeric>    // for transform_reduce
-#include <string>     // for string, to_string, stoi
-#include <variant>    // for variant, get, holds_alternative, visit
-#include <vector>     // for vector
+#include "lib.hpp"       // for DEBUG
+#include "util/math.hpp" // for num_digits, powi
+#include <algorithm>     // for count_if
+#include <array>         // for array
+#include <cstddef>       // for size_t
+#include <cstdint>       // for int64_t
+#include <functional>    // for plus
+#include <iostream>      // for istream, ostream, cerr
+#include <numeric>       // for transform_reduce
+#include <variant>       // for variant, get, holds_alternative, visit
+#include <vector>        // for vector
 
 namespace aoc::day11 {
 
@@ -167,45 +167,6 @@ void HistoryData::set_stone(Stone &stone, stone_value_t value) const {
     }
 }
 
-template <typename IntegerT,
-          int max_digits = std::numeric_limits<IntegerT>::digits10>
-constexpr std::array<IntegerT, max_digits> gen_powers_of_10() {
-    std::array<IntegerT, max_digits> arr;
-    IntegerT x = 1;
-    for (std::size_t i = 0; i < max_digits; ++i) {
-        x *= 10;
-        arr[i] = x;
-    }
-    return arr;
-}
-
-template <std::integral IntegerT>
-int num_digits(IntegerT value) {
-    constexpr auto POWERS = gen_powers_of_10<IntegerT>();
-    return std::distance(
-               POWERS.begin(),
-               std::upper_bound(POWERS.begin(), POWERS.end(), value)) +
-           1;
-}
-
-template <typename IntegerT>
-IntegerT powi(IntegerT base, unsigned int exponent) {
-    if (exponent == 0) {
-        return 1;
-    } else if (exponent == 1) {
-        return base;
-    } else if (exponent == 2) {
-        return base * base;
-    } else {
-        IntegerT tmp = powi(base, exponent / 2);
-        tmp *= tmp;
-        if (exponent % 2 == 1) {
-            tmp *= base;
-        }
-        return tmp;
-    }
-}
-
 // Main update routine.
 void StoneGroup::blink(const HistoryData &history) {
     std::size_t orig_size = stones.size();
@@ -219,9 +180,10 @@ void StoneGroup::blink(const HistoryData &history) {
         if (value == 0) {
             stone = history.get_stone(1);
         } else {
-            int ndigits = num_digits(value);
+            int ndigits = math::num_digits(value);
             if (ndigits % 2 == 0) {
-                stone_value_t modulus = powi<stone_value_t>(10, ndigits / 2);
+                stone_value_t modulus =
+                    math::powi<stone_value_t>(10, ndigits / 2);
                 stone = history.get_stone(value / modulus);
                 stones.push_back(history.get_stone(value % modulus));
             } else {
diff --git a/2024/src/test11.cpp b/aoc_lib/src/test_math.cpp
similarity index 60%
rename from 2024/src/test11.cpp
rename to aoc_lib/src/test_math.cpp
index 3db2c3c..3c391a0 100644
--- a/2024/src/test11.cpp
+++ b/aoc_lib/src/test_math.cpp
@@ -1,29 +1,32 @@
 /******************************************************************************
- * File:        test11.cpp
+ * File:        test_math.cpp
  *
  * Author:      Eric T. Johnson (yut23)
  * Created:     2024-12-11
  *****************************************************************************/
 
-#include "day11.hpp"                  // IWYU pragma: associated
+#include "util/math.hpp" // IWYU pragma: associated
+
 #include "unit_test/pretty_print.hpp" // for repr
 #include "unit_test/unit_test.hpp"
 #include "util/util.hpp" // for demangle
 
-#include <cstddef> // for size_t
-#include <string>  // for string
+#include <cstddef>  // for size_t
+#include <cstdint>  // for int64_t, uint64_t
+#include <iostream> // for cout
+#include <string>   // for string
 // IWYU pragma: no_include <typeinfo>  // for type_info (util::demangle)
 
-namespace aoc::day11::test {
+namespace aoc::math::test {
 
 template <typename IntegerT>
 std::size_t test_powers_of_10() {
     const std::string type_name = util::demangle(typeid(IntegerT).name());
     unit_test::PureTest test(
-        "day11::test_powers_of_10<" + type_name + ">",
+        "aoc::math::gen_powers_of_10<" + type_name + ">",
         +[](IntegerT x) -> IntegerT { return x; });
 
-    constexpr auto POWERS_OF_10 = gen_powers_of_10<IntegerT>();
+    constexpr auto POWERS_OF_10 = aoc::math::gen_powers_of_10<IntegerT>();
     std::size_t size = POWERS_OF_10.size();
     IntegerT max = std::numeric_limits<IntegerT>::max();
     int i = 0;
@@ -51,29 +54,11 @@ std::size_t test_powers_of_10() {
     return test.num_failed();
 }
 
-template <typename IntegerT>
-IntegerT powi(IntegerT base, unsigned int exponent) {
-    if (exponent == 0) {
-        return 1;
-    } else if (exponent == 1) {
-        return base;
-    } else if (exponent == 2) {
-        return base * base;
-    } else {
-        IntegerT tmp = powi(base, exponent / 2);
-        tmp *= tmp;
-        if (exponent % 2 == 1) {
-            tmp *= base;
-        }
-        return tmp;
-    }
-}
-
 template <class IntegerT>
 std::size_t test_num_digits() {
     const std::string type_name = util::demangle(typeid(IntegerT).name());
-    unit_test::PureTest test("day11::test_num_digits<" + type_name + ">",
-                             &aoc::day11::num_digits<IntegerT>);
+    unit_test::PureTest test("aoc::math::num_digits<" + type_name + ">",
+                             &aoc::math::num_digits<IntegerT>);
 
     test(1, 1);
     for (unsigned int i = 0; i <= std::numeric_limits<IntegerT>::digits10;
@@ -91,17 +76,17 @@ std::size_t test_num_digits() {
     return test.done(), test.num_failed();
 }
 
-} // namespace aoc::day11::test
+} // namespace aoc::math::test
 
 int main() {
     std::size_t failed_count = 0;
-    failed_count += aoc::day11::test::test_powers_of_10<int>();
-    failed_count += aoc::day11::test::test_powers_of_10<unsigned int>();
-    failed_count += aoc::day11::test::test_powers_of_10<std::int64_t>();
-    failed_count += aoc::day11::test::test_powers_of_10<std::uint64_t>();
-    failed_count += aoc::day11::test::test_num_digits<int>();
-    failed_count += aoc::day11::test::test_num_digits<unsigned int>();
-    failed_count += aoc::day11::test::test_num_digits<std::int64_t>();
-    failed_count += aoc::day11::test::test_num_digits<std::uint64_t>();
+    failed_count += aoc::math::test::test_powers_of_10<int>();
+    failed_count += aoc::math::test::test_powers_of_10<unsigned int>();
+    failed_count += aoc::math::test::test_powers_of_10<std::int64_t>();
+    failed_count += aoc::math::test::test_powers_of_10<std::uint64_t>();
+    failed_count += aoc::math::test::test_num_digits<int>();
+    failed_count += aoc::math::test::test_num_digits<unsigned int>();
+    failed_count += aoc::math::test::test_num_digits<std::int64_t>();
+    failed_count += aoc::math::test::test_num_digits<std::uint64_t>();
     return unit_test::fix_exit_code(failed_count);
 }
diff --git a/aoc_lib/src/test_pretty_print.cpp b/aoc_lib/src/test_pretty_print.cpp
index 6e4abea..91aa23f 100644
--- a/aoc_lib/src/test_pretty_print.cpp
+++ b/aoc_lib/src/test_pretty_print.cpp
@@ -6,9 +6,10 @@
  * Description: Tests for the unit test pretty-printing code.
  *****************************************************************************/
 
+#include "unit_test/pretty_print.hpp" // IWYU pragma: associated
+
 #include "unit_test/unit_test.hpp"
 
-#include "unit_test/pretty_print.hpp"
 #include <sstream> // for ostringstream
 
 namespace pretty_print::test {
diff --git a/aoc_lib/src/test_unit_test.cpp b/aoc_lib/src/test_unit_test.cpp
index e80548b..48e7134 100644
--- a/aoc_lib/src/test_unit_test.cpp
+++ b/aoc_lib/src/test_unit_test.cpp
@@ -6,7 +6,7 @@
  * Description: Tests of the unit test framework itself.
  *****************************************************************************/
 
-#include "unit_test/unit_test.hpp"
+#include "unit_test/unit_test.hpp" // IWYU pragma: associated
 
 #include <compare>       // for strong_ordering
 #include <cstddef>       // for size_t
diff --git a/aoc_lib/src/util/math.hpp b/aoc_lib/src/util/math.hpp
new file mode 100644
index 0000000..2193de9
--- /dev/null
+++ b/aoc_lib/src/util/math.hpp
@@ -0,0 +1,62 @@
+/******************************************************************************
+ * File:        math.hpp
+ *
+ * Author:      Eric T. Johnson
+ * Created:     2024-12-13
+ * Description: Shared math routines for Advent of Code.
+ *****************************************************************************/
+
+#ifndef MATH_HPP_VHRQFHXN
+#define MATH_HPP_VHRQFHXN
+
+#include <algorithm> // for upper_bound
+#include <array>     // for array
+#include <concepts>  // for integral
+#include <cstddef>   // for size_t
+#include <iterator>  // for distance  // IWYU pragma: keep
+#include <limits>    // for numeric_limits
+
+namespace aoc::math {
+
+template <typename IntegerT,
+          int max_digits = std::numeric_limits<IntegerT>::digits10>
+constexpr std::array<IntegerT, max_digits> gen_powers_of_10() {
+    std::array<IntegerT, max_digits> arr;
+    IntegerT x = 1;
+    for (std::size_t i = 0; i < max_digits; ++i) {
+        x *= 10;
+        arr[i] = x;
+    }
+    return arr;
+}
+
+template <std::integral IntegerT>
+int num_digits(IntegerT value) {
+    constexpr auto POWERS = gen_powers_of_10<IntegerT>();
+    return std::distance(
+               POWERS.begin(),
+               std::upper_bound(POWERS.begin(), POWERS.end(), value)) +
+           1;
+}
+
+template <typename IntegerT>
+IntegerT powi(IntegerT base, unsigned int exponent) {
+    if (exponent == 0) {
+        return 1;
+    } else if (exponent == 1) {
+        return base;
+    } else if (exponent == 2) {
+        return base * base;
+    } else {
+        IntegerT tmp = powi(base, exponent / 2);
+        tmp *= tmp;
+        if (exponent % 2 == 1) {
+            tmp *= base;
+        }
+        return tmp;
+    }
+}
+
+} // namespace aoc::math
+
+#endif /* end of include guard: MATH_HPP_VHRQFHXN */
diff --git a/tools/cpp/iwyu b/tools/cpp/iwyu
index 7905253..4c51777 100755
--- a/tools/cpp/iwyu
+++ b/tools/cpp/iwyu
@@ -45,4 +45,4 @@ done
 # add remaining arguments to include-what-you-use args
 iwyu_args+=("$@")
 
-iwyu-tool -p "../compile_commands_$AOC_YEAR.json" "${tool_args[@]}" "${sources[@]}" -- --no-warnings -Xiwyu --quoted_includes_first -Xiwyu --max_line_length=300 -Xiwyu --mapping_file="$TOOLS_DIR/cpp/iwyu_mappings.imp" -resource-dir "$(clang -print-resource-dir)" "${iwyu_args[@]}"
+iwyu-tool -p "../compile_commands.json" "${tool_args[@]}" "${sources[@]}" -- --no-warnings -Xiwyu --quoted_includes_first -Xiwyu --max_line_length=300 -Xiwyu --mapping_file="$TOOLS_DIR/cpp/iwyu_mappings.imp" -resource-dir "$(clang -print-resource-dir)" "${iwyu_args[@]}"