diff --git a/2023/src/test00_ds.cpp b/2023/src/test00_ds.cpp
index 5bc0192..eb8bc0d 100644
--- a/2023/src/test00_ds.cpp
+++ b/2023/src/test00_ds.cpp
@@ -6,12 +6,17 @@
  * Description: Unit tests for data structures.
  *****************************************************************************/
 
-#include "unit_test/unit_test.hpp"
+#include "data_structures.hpp"        // IWYU pragma: associated
+#include "unit_test/pretty_print.hpp" // for repr
+#include "unit_test/unit_test.hpp"    // for ManualTest, fix_exit_code
+#include "util/util.hpp"              // for demangle
 
-#include "data_structures.hpp"
+#include <array>      // for array
 #include <cstddef>    // for size_t
 #include <functional> // for greater
-#include <string>     // for string, to_string, string_literals
+#include <iostream>   // for cerr
+#include <string>     // for string
+#include <typeinfo>   // for type_info
 
 namespace aoc::ds::test {
 
@@ -81,11 +86,124 @@ std::size_t test_pairing_heap_min() {
     return test.done(), test.num_failed();
 }
 
+template <class T>
+std::size_t test_grid() {
+    const std::string type_name = util::demangle(typeid(T).name());
+    unit_test::ManualTest test("aoc::ds::Grid<" + type_name + ">");
+    std::array<T, 50> arr;
+    {
+        int i = 0;
+        for (auto &x : arr) {
+            x = i++;
+        }
+    }
+    Grid<T> grid(5, 10, arr);
+    DO_TEST(grid.in_bounds(4, 9));
+    DO_TEST(!grid.in_bounds(5, 3));
+    DO_TEST(!grid.in_bounds(0, 10));
+    DO_TEST(!grid.in_bounds(-2, 4));
+    test(
+        [&grid]() {
+            using pretty_print::repr;
+            auto col_it = grid.begin();
+            for (int y = 0; y < grid.height; ++y, ++col_it) {
+                assert(col_it >= grid.begin());
+                assert(col_it == grid.begin() + y);
+                auto row_it = col_it->begin();
+                for (int x = 0; x < grid.width; ++x, ++row_it) {
+                    if (grid.at(x, y) != *row_it) {
+                        std::cerr
+                            << "mismatch at " << x << ", " << y << ": expected "
+                            << pretty_print::repr(grid.at(x, y), true)
+                            << ", got " << pretty_print::repr(*row_it, true)
+                            << "\n";
+                        return false;
+                    }
+                }
+                if (row_it != col_it->end()) {
+                    std::cerr << "iterator for row " << y << " not at end\n";
+                    return false;
+                }
+            }
+            if (col_it != grid.end()) {
+                std::cerr << "grid iterator not at end\n";
+                return false;
+            }
+            return true;
+        },
+        "input iterator");
+    test(
+        [&grid]() {
+            using pretty_print::repr;
+            auto col_it = grid.cbegin();
+            for (int y = 0; y < grid.height; ++y, ++col_it) {
+                assert(col_it >= grid.begin());
+                assert(col_it == grid.begin() + y);
+                assert(col_it >= grid.cbegin());
+                assert(col_it == grid.cbegin() + y);
+                auto row_it = col_it->begin();
+                for (int x = 0; x < grid.width; ++x, ++row_it) {
+                    if (grid.at(x, y) != *row_it) {
+                        std::cerr
+                            << "mismatch at " << x << ", " << y << ": expected "
+                            << pretty_print::repr(grid.at(x, y), true)
+                            << ", got " << pretty_print::repr(*row_it, true)
+                            << "\n";
+                        return false;
+                    }
+                }
+                if (row_it != col_it->end()) {
+                    std::cerr << "iterator for row " << y << " not at end\n";
+                    return false;
+                }
+            }
+            if (col_it != grid.cend()) {
+                std::cerr << "grid iterator not at end\n";
+                return false;
+            }
+            return true;
+        },
+        "input const_iterator");
+    test(
+        [&grid]() {
+            using pretty_print::repr;
+            auto col_it = grid.begin();
+            for (int y = 0; y < grid.height; ++y, ++col_it) {
+                assert(col_it >= grid.begin());
+                assert(col_it == grid.begin() + y);
+                auto row_it = col_it->begin();
+                for (int x = 0; x < grid.width; ++x, ++row_it) {
+                    auto new_val = 1 - *row_it;
+                    *row_it = new_val;
+                    if (grid.at(x, y) != new_val) {
+                        std::cerr << "writing failed at " << x << ", " << y
+                                  << "\n";
+                        return false;
+                    }
+                }
+                if (row_it != col_it->end()) {
+                    std::cerr << "iterator for row " << y << " not at end\n";
+                    return false;
+                }
+            }
+            if (col_it != grid.end()) {
+                std::cerr << "grid iterator not at end\n";
+                return false;
+            }
+            return true;
+        },
+        "output iterator");
+
+    return test.done(), test.num_failed();
+}
+
 } // namespace aoc::ds::test
 
 int main() {
     std::size_t failed_count = 0;
     failed_count += aoc::ds::test::test_pairing_heap_max();
     failed_count += aoc::ds::test::test_pairing_heap_min();
+    failed_count += aoc::ds::test::test_grid<int>();
+    failed_count += aoc::ds::test::test_grid<bool>();
     return unit_test::fix_exit_code(failed_count);
 }
diff --git a/2023/src/unit_test/unit_test.hpp b/2023/src/unit_test/unit_test.hpp
index 038d769..d34e395 100644
--- a/2023/src/unit_test/unit_test.hpp
+++ b/2023/src/unit_test/unit_test.hpp
@@ -4,6 +4,7 @@
 #include <algorithm>        // for transform, min
 #include <cassert>          // for assert
 #include <compare>          // for strong_ordering
+#include <concepts>         // for predicate
 #include <cstddef>          // for size_t
 #include <cstdlib>          // for abs
 #include <filesystem>       // for path
@@ -641,11 +642,23 @@ PureTest(const std::string &, std::function<R(Args...)>, int)
     -> PureTest<R, Args...>;
 
 struct ManualTest : public BaseTest {
+    using runner_type = TestRunner<bool>;
+
     explicit ManualTest(const std::string &name) : BaseTest(name) {}
 
     template <class T>
     bool operator()(T &&passed, const std::string &note = "") {
-        return (*this)(bool(std::forward<T>(passed)), note);
+        if constexpr (std::predicate<T>) {
+            // passed is a function that takes no arguments and returns a
+            // boolean-testable result
+            ++test_num;
+            TestRunner<bool> runner(passed, 0);
+            TestResult result = runner.run_on_args(true, note);
+            record_result(result);
+            return result.passed;
+        } else {
+            return (*this)(bool(std::forward<T>(passed)), note);
+        }
     }
     bool operator()(bool passed, const std::string &note = "") {
         ++test_num;
@@ -658,14 +671,32 @@ struct ManualTest : public BaseTest {
 
 namespace {
 [[maybe_unused]] void _lint_helper() {
-    unit_test::PureTest test(
-        "foobar",
-        +[](const std::vector<int> &vec1, const std::vector<int> &vec2)
-            -> std::strong_ordering { return vec1 <=> vec2; });
-    test({1, 2}, {1, 2}, std::strong_ordering::equal);
-    test({1, 2, 3}, {1, 2}, std::strong_ordering::greater);
-    test({2, 2, 3}, {1, 2}, std::strong_ordering::less, "note");
-    test.done();
+    {
+        unit_test::PureTest test(
+            "foobar",
+            +[](const std::vector<int> &vec1, const std::vector<int> &vec2)
+                -> std::strong_ordering { return vec1 <=> vec2; });
+        test({1, 2}, {1, 2}, std::strong_ordering::equal);
+        test({1, 1}, {1, 2}, std::strong_ordering::less);
+        test({2, 2}, {1, 2}, std::strong_ordering::greater);
+        test({1, 2, 3}, {1, 2}, std::strong_ordering::greater);
+        test({1, 1, 3}, {1, 2}, std::strong_ordering::less, "note");
+        test.done();
+    }
+    {
+        const auto count_chars =
+            +[](const std::string &s) -> std::unordered_map<char, int> {
+            std::unordered_map<char, int> counts;
+            for (char ch : s) {
+                ++counts[ch];
+            }
+            return counts;
+        };
+        unit_test::PureTest test("count_chars", count_chars);
+        test("abc", {{'a', 1}, {'b', 1}, {'c', 1}});
+        test("banana", {{'a', 3}, {'b', 1}, {'n', 2}});
+        test.done();
+    }
 }
 } // namespace