diff --git a/.github/workflows/gen_matrix.py b/.github/workflows/gen_matrix.py
index 61ddca1..e4bdc8b 100755
--- a/.github/workflows/gen_matrix.py
+++ b/.github/workflows/gen_matrix.py
@@ -49,6 +49,12 @@ def find_include_paths(line: str) -> Generator[Path, None, None]:
                         yield curr_path
                         break
 
+    elif path.name == "Makefile":
+
+        def find_include_paths(line: str) -> Generator[Path, None, None]:
+            if line.strip().startswith("include "):
+                yield path.parent / line.strip().removeprefix("include ")
+
     else:
         return frozenset()
 
@@ -72,31 +78,40 @@ def get_transitive_dependencies(path: Path, *include_dirs: Path) -> frozenset[Pa
 @dataclass(order=True, frozen=True, kw_only=True)
 class Target:
     base_dir: Path
-    day: int
+    day: int | None
     prefix: str
     extra: str
 
+    @property
+    def _day_str(self) -> str:
+        if self.day is None:
+            return ""
+        return f"{self.day:02d}"
+
     @property
     def suffix(self) -> str:
-        return f"{self.day:02d}{self.extra}"
+        return f"{self._day_str}{self.extra}"
 
     def __str__(self) -> str:
-        return f"{self.prefix}{self.day:02d}{self.extra}"
+        return f"{self.prefix}{self._day_str}{self.extra}"
 
     @classmethod
     def from_file(cls, base_dir: Path, source_file: Path) -> Target:
         m = re.match(
-            r"(?P<prefix>day|test)(?P<day>\d+)(?P<extra>.*)$", source_file.stem
+            r"(?P<prefix>day|test)(?P<day>\d+)?(?P<extra>.*)$", source_file.stem
         )
         assert m is not None, source_file
-        return cls(
-            base_dir=base_dir, day=int(m["day"]), prefix=m["prefix"], extra=m["extra"]
-        )
+        if m["day"] is None:
+            assert m["prefix"] == "test", "only unit tests may omit a day number"
+            day = None
+        else:
+            day = int(m["day"])
+        return cls(base_dir=base_dir, day=day, prefix=m["prefix"], extra=m["extra"])
 
     def get_deps(self, mode: str) -> frozenset[Path]:
         deps = set()
         src = self.base_dir / "src"
-        aoc_lib = self.base_dir.parent / "aoc_lib"
+        aoc_lib = self.base_dir.parent / "aoc_lib/src"
         deps.add(src / f"{self}.cpp")
         for included_file in get_transitive_dependencies(
             src / f"{self}.cpp", src, aoc_lib
diff --git a/.github/workflows/test_gen_matrix.py b/.github/workflows/test_gen_matrix.py
index edc3e92..5c6cddc 100644
--- a/.github/workflows/test_gen_matrix.py
+++ b/.github/workflows/test_gen_matrix.py
@@ -1,4 +1,12 @@
-from gen_matrix import ROOT, WORKFLOWS_DIR, Matrix, get_dependencies
+from pathlib import Path
+
+from gen_matrix import (
+    ROOT,
+    WORKFLOWS_DIR,
+    Matrix,
+    get_dependencies,
+    get_transitive_dependencies,
+)
 
 ACTIONS_DIR = ROOT / ".github/actions"
 
@@ -6,7 +14,7 @@
 def test_get_dependencies_cpp():
     base_dir = ROOT / "2023"
     src = base_dir / "src"
-    aoc_lib = base_dir.parent / "aoc_lib"
+    aoc_lib = base_dir.parent / "aoc_lib/src"
     include_dirs = (src, aoc_lib)
     assert get_dependencies(src / "day01.cpp", *include_dirs) == {aoc_lib / "lib.hpp"}
     assert get_dependencies(src / "day05.cpp", *include_dirs) == {
@@ -19,6 +27,39 @@ def test_get_dependencies_cpp():
         aoc_lib / "unit_test/unit_test.hpp",
     }
 
+    assert get_dependencies(ROOT / "2022/src/day16.cpp", *include_dirs) == {
+        ROOT / "2022/src/lib.h"
+    }
+
+
+def test_get_transitive_dependencies():
+    base_dir = ROOT / "2023"
+    src = base_dir / "src"
+    aoc_lib = base_dir.parent / "aoc_lib/src"
+    include_dirs = (src, aoc_lib)
+    assert get_transitive_dependencies(src / "day22.cpp", *include_dirs) == frozenset(
+        {
+            src / "day22.hpp",
+            aoc_lib / "lib.hpp",
+            aoc_lib / "util/hash.hpp",
+            aoc_lib / "util/concepts.hpp",
+            aoc_lib / "graph_traversal.hpp",
+        }
+    )
+
+    assert get_transitive_dependencies(aoc_lib / "test_ds.cpp", aoc_lib) == frozenset(
+        {
+            aoc_lib / "ds/grid.hpp",
+            aoc_lib / "ds/pairing_heap.hpp",
+            aoc_lib / "unit_test/pretty_print.hpp",
+            aoc_lib / "unit_test/unit_test.hpp",
+            aoc_lib / "util/util.hpp",
+            aoc_lib / "lib.hpp",
+            aoc_lib / "util/hash.hpp",
+            aoc_lib / "util/concepts.hpp",
+        }
+    )
+
 
 def test_get_dependencies_workflows():
     assert get_dependencies(WORKFLOWS_DIR / "unit-tests.yml") == {
@@ -41,60 +82,95 @@ def test_get_dependencies_workflows():
     }
 
 
-def matrix_helper(*changed_files: str) -> dict[str, set[str]]:
-    result = {}
-    for mode in ("build", "answer", "unit"):
-        matrix = Matrix(mode)
-        for f in changed_files:
-            matrix.process_changed_file(ROOT / f)
-        result[mode] = {
-            "{directory}/{name}".format(**target.to_dict()) for target in matrix.targets
-        }
-    return result
+class MatrixHelper:
+    def __init__(self) -> None:
+        self.matrices = {mode: Matrix(mode) for mode in ("build", "answer", "unit")}
+
+    def get_targets(self, *changed_files: str | Path) -> dict[str, set[str]]:
+        result = {}
+        for mode in ("build", "answer", "unit"):
+            matrix = self.matrices[mode]
+            for f in changed_files:
+                matrix.process_changed_file(ROOT / f)
+            result[mode] = {
+                "{directory}/{name}".format(**target.to_dict())
+                for target in matrix.targets
+            }
+            matrix.targets.clear()
+        return result
 
 
 def test_matrix():
     # pylint: disable=use-dict-literal
-    everything = matrix_helper("2022/Makefile", "tools/cpp/Makefile")
+    helper = MatrixHelper()
+    everything = helper.get_targets(*ROOT.glob("*/Makefile"), "tools/cpp/Makefile")
 
     # C++ source files
-    assert matrix_helper("2023/src/day05.cpp") == dict(
+    assert helper.get_targets("2023/src/day05.cpp") == dict(
         build={"2023/day05"}, answer={"2023/day05"}, unit=set()
     )
-    assert matrix_helper("2023/src/day05.hpp") == dict(
-        build={"2023/day05", "2023/test05"}, answer={"2023/day05"}, unit={"2023/test05"}
+    assert helper.get_targets("2023/src/day05.hpp") == dict(
+        build={"2023/day05", "2023/test05"},
+        answer={"2023/day05"},
+        unit={"2023/test05"},
     )
-    assert matrix_helper("2023/src/test05.cpp") == dict(
+    assert helper.get_targets("2023/src/test05.cpp") == dict(
         build={"2023/test05"}, answer=set(), unit={"2023/test05"}
     )
 
+    # 2022 doesn't use aoc_lib/src/lib.hpp
+    matrix = Matrix("build")
+    matrix.process_changed_file(ROOT / "aoc_lib/src/lib.hpp")
+    assert all(target.base_dir != ROOT / "2022" for target in matrix.targets)
+
+    # unit tests for aoc_lib
+    assert helper.get_targets("aoc_lib/src/test_graph.cpp") == dict(
+        build={"aoc_lib/test_graph"},
+        answer=set(),
+        unit={"aoc_lib/test_graph"},
+    )
+    assert "aoc_lib/test_lib" in helper.get_targets("aoc_lib/src/lib.hpp")["unit"]
+    assert (
+        "aoc_lib/test_graph"
+        in helper.get_targets("aoc_lib/src/graph_traversal.hpp")["unit"]
+    )
+    assert "aoc_lib/test_ds" in helper.get_targets("aoc_lib/src/ds/grid.hpp")["unit"]
+    assert (
+        "aoc_lib/test_ds"
+        in helper.get_targets("aoc_lib/src/ds/pairing_heap.hpp")["unit"]
+    )
+    assert (
+        "aoc_lib/test_unit_test"
+        in helper.get_targets("aoc_lib/src/unit_test/unit_test.hpp")["unit"]
+    )
+
     # answer tests
-    assert matrix_helper("2023/answer_tests/day01/example2.txt") == dict(
+    assert helper.get_targets("2023/answer_tests/day01/example2.txt") == dict(
         build=set(), answer={"2023/day01"}, unit=set()
     )
 
     # workflow files
-    assert matrix_helper(".github/workflows/gen_matrix.py") == everything
-    assert matrix_helper(".github/workflows/generate-matrix.yml") == everything
-    assert matrix_helper(".github/workflows/docker-build.yml") == everything
+    assert helper.get_targets(".github/workflows/gen_matrix.py") == everything
+    assert helper.get_targets(".github/workflows/generate-matrix.yml") == everything
+    assert helper.get_targets(".github/workflows/docker-build.yml") == everything
 
-    assert matrix_helper(".github/workflows/test-build.yml") == dict(
+    assert helper.get_targets(".github/workflows/test-build.yml") == dict(
         build=everything["build"], answer=set(), unit=set()
     )
-    assert matrix_helper(".github/workflows/answer-tests.yml") == dict(
+    assert helper.get_targets(".github/workflows/answer-tests.yml") == dict(
         build=set(), answer=everything["answer"], unit=set()
     )
-    assert matrix_helper(".github/workflows/unit-tests.yml") == dict(
+    assert helper.get_targets(".github/workflows/unit-tests.yml") == dict(
         build=set(), answer=set(), unit=everything["unit"]
     )
 
     # workflow action helpers
-    assert matrix_helper(".github/actions/build-action/action.yml") == matrix_helper(
-        ".github/workflows/test-build.yml"
-    )
-    assert matrix_helper(
+    assert helper.get_targets(
+        ".github/actions/build-action/action.yml"
+    ) == helper.get_targets(".github/workflows/test-build.yml")
+    assert helper.get_targets(
         ".github/actions/answer-test-action/action.yml"
-    ) == matrix_helper(".github/workflows/answer-tests.yml")
-    assert matrix_helper(
+    ) == helper.get_targets(".github/workflows/answer-tests.yml")
+    assert helper.get_targets(
         ".github/actions/unit-test-action/action.yml"
-    ) == matrix_helper(".github/workflows/unit-tests.yml")
+    ) == helper.get_targets(".github/workflows/unit-tests.yml")
diff --git a/.lvimrc b/.lvimrc
index a8d84a6..a641977 100644
--- a/.lvimrc
+++ b/.lvimrc
@@ -1,5 +1,5 @@
 " add the aoc_lib dir to 'path', so gf works as expected
-let &path = lh#path#munge(&path, lh#path#fix(g:localvimrc_script_dir . '/aoc_lib'))
+let &path = lh#path#munge(&path, lh#path#fix(g:localvimrc_script_dir . '/aoc_lib/src'))
 
 if &filetype ==# 'cpp'
   let b:surround_{char2nr('D')} = "if constexpr (aoc::DEBUG) {\n\r\n}"
diff --git a/aoc_lib/.autoenv.zsh b/aoc_lib/.autoenv.zsh
new file mode 100644
index 0000000..7ee2433
--- /dev/null
+++ b/aoc_lib/.autoenv.zsh
@@ -0,0 +1,13 @@
+local wrapper_cmd=${0:h}/make_wrapper.sh
+if [[ $autoenv_event == 'enter' ]]; then
+  alias iwyu=${0:h}/iwyu
+  alias make="$wrapper_cmd -f ${(q)0:h}/Makefile"
+  compdef "$wrapper_cmd"=make
+  export AOC_YEAR=aoc_lib
+else
+  unalias iwyu make
+  # remove completion definition
+  unset "_comps[$wrapper_cmd]"
+  unset "_services[$wrapper_cmd]"
+  unset AOC_YEAR
+fi
diff --git a/aoc_lib/Makefile b/aoc_lib/Makefile
new file mode 100644
index 0000000..de3b016
--- /dev/null
+++ b/aoc_lib/Makefile
@@ -0,0 +1,3 @@
+# set default target
+test:
+include ../tools/cpp/Makefile
diff --git a/aoc_lib/iwyu b/aoc_lib/iwyu
new file mode 120000
index 0000000..bcca1ed
--- /dev/null
+++ b/aoc_lib/iwyu
@@ -0,0 +1 @@
+../tools/cpp/iwyu
\ No newline at end of file
diff --git a/aoc_lib/make_wrapper.sh b/aoc_lib/make_wrapper.sh
new file mode 120000
index 0000000..fb7fc1f
--- /dev/null
+++ b/aoc_lib/make_wrapper.sh
@@ -0,0 +1 @@
+../tools/cpp/make_wrapper.sh
\ No newline at end of file
diff --git a/aoc_lib/ds/grid.hpp b/aoc_lib/src/ds/grid.hpp
similarity index 100%
rename from aoc_lib/ds/grid.hpp
rename to aoc_lib/src/ds/grid.hpp
diff --git a/aoc_lib/ds/pairing_heap.hpp b/aoc_lib/src/ds/pairing_heap.hpp
similarity index 100%
rename from aoc_lib/ds/pairing_heap.hpp
rename to aoc_lib/src/ds/pairing_heap.hpp
diff --git a/aoc_lib/gauss_elim.hpp b/aoc_lib/src/gauss_elim.hpp
similarity index 100%
rename from aoc_lib/gauss_elim.hpp
rename to aoc_lib/src/gauss_elim.hpp
diff --git a/aoc_lib/graph_traversal.hpp b/aoc_lib/src/graph_traversal.hpp
similarity index 100%
rename from aoc_lib/graph_traversal.hpp
rename to aoc_lib/src/graph_traversal.hpp
diff --git a/aoc_lib/lib.hpp b/aoc_lib/src/lib.hpp
similarity index 100%
rename from aoc_lib/lib.hpp
rename to aoc_lib/src/lib.hpp
diff --git a/2023/src/test00_ds.cpp b/aoc_lib/src/test_ds.cpp
similarity index 99%
rename from 2023/src/test00_ds.cpp
rename to aoc_lib/src/test_ds.cpp
index e545ec1..87aa054 100644
--- a/2023/src/test00_ds.cpp
+++ b/aoc_lib/src/test_ds.cpp
@@ -1,5 +1,5 @@
 /******************************************************************************
- * File:        test00_ds.cpp
+ * File:        test_ds.cpp
  *
  * Author:      Eric T. Johnson (yut23)
  * Created:     2024-01-01
diff --git a/2023/src/test00_graph.cpp b/aoc_lib/src/test_graph.cpp
similarity index 98%
rename from 2023/src/test00_graph.cpp
rename to aoc_lib/src/test_graph.cpp
index 1c7944b..35363e7 100644
--- a/2023/src/test00_graph.cpp
+++ b/aoc_lib/src/test_graph.cpp
@@ -1,5 +1,5 @@
 /******************************************************************************
- * File:        test00_graph.cpp
+ * File:        test_graph.cpp
  *
  * Author:      Eric T. Johnson (yut23)
  * Created:     2024-01-15
diff --git a/2023/src/test00.cpp b/aoc_lib/src/test_lib.cpp
similarity index 99%
rename from 2023/src/test00.cpp
rename to aoc_lib/src/test_lib.cpp
index aa172b1..1d14816 100644
--- a/2023/src/test00.cpp
+++ b/aoc_lib/src/test_lib.cpp
@@ -1,5 +1,5 @@
 /******************************************************************************
- * File:        test00.cpp
+ * File:        test_lib.cpp
  *
  * Author:      Eric T. Johnson (yut23)
  * Created:     2023-12-19
diff --git a/2023/src/test00_unit_test.cpp b/aoc_lib/src/test_unit_test.cpp
similarity index 98%
rename from 2023/src/test00_unit_test.cpp
rename to aoc_lib/src/test_unit_test.cpp
index 93caea6..e80548b 100644
--- a/2023/src/test00_unit_test.cpp
+++ b/aoc_lib/src/test_unit_test.cpp
@@ -1,5 +1,5 @@
 /******************************************************************************
- * File:        test00_unit_test.cpp
+ * File:        test_unit_test.cpp
  *
  * Author:      Eric T. Johnson (yut23)
  * Created:     2024-01-23
diff --git a/aoc_lib/unit_test/pretty_print.hpp b/aoc_lib/src/unit_test/pretty_print.hpp
similarity index 100%
rename from aoc_lib/unit_test/pretty_print.hpp
rename to aoc_lib/src/unit_test/pretty_print.hpp
diff --git a/aoc_lib/unit_test/unit_test.hpp b/aoc_lib/src/unit_test/unit_test.hpp
similarity index 100%
rename from aoc_lib/unit_test/unit_test.hpp
rename to aoc_lib/src/unit_test/unit_test.hpp
diff --git a/aoc_lib/util/concepts.hpp b/aoc_lib/src/util/concepts.hpp
similarity index 100%
rename from aoc_lib/util/concepts.hpp
rename to aoc_lib/src/util/concepts.hpp
diff --git a/aoc_lib/util/hash.hpp b/aoc_lib/src/util/hash.hpp
similarity index 100%
rename from aoc_lib/util/hash.hpp
rename to aoc_lib/src/util/hash.hpp
diff --git a/aoc_lib/util/util.hpp b/aoc_lib/src/util/util.hpp
similarity index 100%
rename from aoc_lib/util/util.hpp
rename to aoc_lib/src/util/util.hpp
diff --git a/tools/cpp/Makefile b/tools/cpp/Makefile
index 28d627d..041784b 100644
--- a/tools/cpp/Makefile
+++ b/tools/cpp/Makefile
@@ -13,7 +13,7 @@ ifeq ($(USE_LIBCXX),TRUE)
 endif
 
 # These are searched left-to-right
-INCLUDES = -Isrc -I../aoc_lib
+INCLUDES = -Isrc -I../aoc_lib/src
 
 LOCAL_CXXFLAGS = -Wall -Wextra -O3 -g -std=c++20 $(INCLUDES) $(STDLIB_CXXFLAGS)
 DEBUG_CXXFLAGS = $(LOCAL_CXXFLAGS) -O0 -DDEBUG_MODE