libakgl/README.md

# libakgl

A C library for building 2D games on SDL3. Not an engine: no editor, no scripting layer, no
inheritance, and no runtime `malloc`. Behaviour attaches to a struct as function pointers,
objects come from fixed pools, and every call reports failure through an error context the
caller cannot silently ignore.

## The manual

**[`docs/`](docs/README.md) is the manual** — twenty-one chapters covering every subsystem,
plus two complete tutorial games. Start at
[Chapter 3, Getting started](docs/03-getting-started.md) if you want a window on screen, or
[Chapter 2, Design philosophy](docs/02-design-philosophy.md) if you want to know why the
library is shaped the way it is.

The two tutorials build real, running programs that live under [`examples/`](examples):

| | |
|---|---|
| [A 2D sidescroller](docs/20-tutorial-sidescroller.md) | Gravity, a jump, collision you write yourself, coins and hazards |
| [A top-down JRPG](docs/21-tutorial-jrpg.md) | A town map, NPCs spawned from map objects, four-way animation, a text box, a follower |

For per-function reference, build the Doxygen output with `doxygen Doxyfile`; it is a CI
gate, so an undocumented symbol fails the build.

This README covers the development process only — hooks, mutation testing, benchmarks and
memory checking. Everything about *using* the library is in `docs/`.

> **The task-oriented FAQ that used to live here has moved into `docs/`, corrected.** It was
> not merely incomplete: its examples did not compile. Two unbalanced `PASS()` calls, a
> `sprite->frameids = [0, 1, 2, 3];` that is not C in any dialect, a stray `9` inside a
> bitmask expression, an `int screenwidth = NULL`, and — in the first snippet a reader ever
> saw — the exact `strncpy` call `AGENTS.md` forbids. The prose had drifted with it: its
> claim that the engine "ONLY supports TilED TMJ tilemaps with tileset external references"
> is backwards, and the loader accepts only *embedded* tilesets. Writing the manual turned
> up twenty-seven such claims across the headers. Every example in `docs/` is now compiled,
> linked, run or matched against the source tree by `ctest`, so that class of drift fails a
> build instead of reaching a reader.

## Documentation examples

Every fenced example in `docs/` carries an info string saying what it is, and the
`docs_examples` test acts on it: `c` blocks are compiled, `c run=` blocks are linked against
the library and executed headless with their output compared byte for byte, `c excerpt=`
blocks must still appear verbatim in the file they quote, `json` blocks are loaded through
the real `akgl_*_load_json`, and `c screenshot=` blocks generate the figures in
`docs/images/`. A block with no info string is a hard error, because the failure mode the
harness exists to prevent is passing while checking nothing.

```sh
ctest --test-dir build -R docs_examples --output-on-failure
ctest --test-dir build -R docs_screenshots --output-on-failure
cmake --build build --target docs_screenshots   # regenerate the figures
```

While editing one chapter, run it directly rather than the whole suite:

```sh
./tests/docs_examples.sh --root . \
    --cflags-file build/docs_cflags.txt \
    --ldflags-file build/docs_ldflags.txt \
    --checkjson build/akgl_docs_checkjson \
    docs/14-physics.md
```

`docs/MAINTENANCE.md` is the reference for the info strings, the preludes and the fixtures.
There is deliberately no docs-path filter in CI: documentation goes stale because the *code*
moved, not because somebody edited a chapter.

## Git hooks

The repository ships a `pre-commit` hook that keeps committed C sources in the
project's canonical format (Emacs `cc-mode` "stroustrup"; see `AGENTS.md` for the
full style guide). The hook lives in `scripts/hooks/` and is version controlled,
but **Git configuration is not cloned**, so every clone has to be pointed at it
once:

```sh
git config core.hooksPath scripts/hooks
```

Confirm it took effect:

```sh
git rev-parse --git-path hooks     # should print: scripts/hooks
```

That is the whole installation. The hook is already committed with its
executable bit set, so nothing needs `chmod`.

### What the hook does

On each commit it looks at the **staged** content of any added, copied, modified,
or renamed `.c`/`.h` file under `src/`, `include/`, `tests/`, or `util/`, and
reindents it if it does not already match the canonical style. Checking the
staged content rather than the working tree means what lands in the commit is
what was actually verified.

When a file needs reindenting, the hook fixes it in the working tree and
re-stages it — but only when the index and working tree agree for that file. If
they differ, you have staged part of a file with `git add -p`, and re-staging
would sweep your unstaged work into the commit. Rather than do that silently the
hook stops and prints the commands to run yourself:

```sh
scripts/reindent.sh path/to/file.c
git add path/to/file.c
```

`include/akgl/SDL_GameControllerDB.h` is generated and always skipped.

### Requirements

The hook drives Emacs in batch mode, because `cc-mode`'s indentation engine is
the definition of the style and `clang-format` cannot reproduce it exactly. If
`emacs` is not on `PATH` the hook prints a warning and allows the commit — a
hook that refuses to run without an optional tool only teaches people to reach
for `--no-verify`. Install Emacs to get the check; without it, formatting is on
you.

### Bypassing and uninstalling

Skip the hook for a single commit:

```sh
git commit --no-verify
```

Remove it entirely:

```sh
git config --unset core.hooksPath
```

### If you already have local hooks

`core.hooksPath` **replaces** the hooks directory outright — once it is set, Git
stops reading `.git/hooks/` altogether, so any hooks you keep there will silently
stop firing. If that matters, leave `core.hooksPath` unset and symlink just this
one hook instead:

```sh
ln -s ../../scripts/hooks/pre-commit .git/hooks/pre-commit
```

### Formatting without the hook

The hook is a convenience, not the source of truth. The same check is available
directly, and is what you would run in CI:

```sh
scripts/reindent.sh --check     # list non-conforming files; exit 1 if any
scripts/reindent.sh             # reindent every tracked C source in place
scripts/reindent.sh src/game.c  # reindent specific files
```

`--check` exits `0` when everything conforms, `1` when a file needs reindenting,
and `2` if Emacs is missing or the script cannot run — so a CI job that treats
any non-zero status as failure will not mistake a broken toolchain for a clean
tree.

## Mutation testing

The mutation harness makes one deliberate source-code change at a time in a scratch copy, then rebuilds and runs the passing CTest suite to measure whether tests detect the change. The known-failing `character` test is excluded by default.

```sh
cmake --build build --target mutation
scripts/mutation_test.py --target src/tilemap.c --list
scripts/mutation_test.py --target src/tilemap.c --max-mutants 10
scripts/mutation_test.py --threshold 40 --junit mutation-junit.xml
```

The default run covers all libakgl-owned files under `src/`. Use repeated `--target` options to narrow the scope. A surviving mutant identifies behavior that the current tests do not verify; the script prints its file, line, operator, and exact edit. The real working tree is never mutated.

## Performance testing

`tests/perf.c` and `tests/perf_render.c` are benchmarks rather than unit tests: they drive the hot paths — pool acquire and release, the registry, the physics sweep, the per-actor update and render, the tilemap draw, text, and asset loading — and print what each one costs per operation.

```sh
ctest --test-dir build -L perf --output-on-failure   # benchmarks only, about 30 seconds
ctest --test-dir build -LE perf                      # everything except the benchmarks
AKGL_BENCH_SCALE=0.1 ctest --test-dir build -L perf  # a tenth of the iterations
```

Each measurement is the best of five runs and is held to a budget set at roughly ten times the recorded baseline, so a suite that turns red means an algorithmic regression rather than a busy machine. Budgets are enforced only in an optimized build at full scale; below `AKGL_BENCH_SCALE=1.0`, and in a coverage build, they are reported without failing.

`PERFORMANCE.md` carries the recorded baseline, the frame budget it adds up to, and what the numbers say — including the raw-SDL control rows that separate what libakgl costs from what the rasterizer costs. The six defects the stress tests turned up, and the targets the numbers are measured against, are in `TODO.md` under **Performance**.

## Memory checking

`memcheck` runs the suites that already exist under valgrind rather than adding suites of its own. The perf binaries carry most of it: they are the only programs in the tree that load assets, draw a scene, and run a frame loop in one process, and `tests/benchutil.h` cuts their iteration counts by three orders of magnitude when it detects valgrind — a benchmark walks one path a hundred thousand times, a leak check wants every path walked once.

```sh
cmake --build build --target memcheck   # everything, about 30 seconds
scripts/memcheck.sh -R tilemap          # one suite; ctest selection flags pass through
scripts/memcheck.sh -LE perf            # skip the benchmarks
```

The run forces `SDL_VIDEO_DRIVER=dummy`, `SDL_RENDER_DRIVER=software` and `SDL_AUDIO_DRIVER=dummy` so the vendor GPU stack is never loaded — that removes thousands of unfixable findings inside the driver without suppressing anything. Definite leaks, invalid accesses and uninitialised reads are counted and set the exit status; "still reachable" is not, because that is what SDL and FreeType keep for the process lifetime. Suppressions for genuine third-party findings are in `scripts/valgrind.supp`.

What it found the first time it ran is in `TODO.md` under **Memory checking**.

## What is compiled in, and the notice it carries

**[libccd](https://github.com/danfis/libccd) is compiled into `libakgl.so`.** It is the
collision narrowphase -- GJK, EPA and MPR, the same code vendored in ODE, FCL and Bullet --
and it is **BSD-3-Clause**. Its licence is `deps/libccd/BSD-LICENSE`, and `cmake --install`
puts a copy at `share/doc/akgl/BSD-LICENSE.libccd`. If you redistribute a binary built from
this tree, that notice has to travel with it.

It is the only dependency in that position. SDL3, SDL3_image, SDL3_mixer, SDL3_ttf, jansson,
libakerror and libakstdlib are all linked as separate shared objects and carry their own
notices; `deps/semver` is a single header, MIT, installed alongside ours; and `deps/tg` is
vendored but **compiled into nothing** -- see `TODO.md`, "tg is vendored and has no
consumer".

libccd is given a static arena rather than the allocator it ships with, so "libakgl does not
call `malloc` at runtime" stays literally true with it linked in. That is
`src/collision_arena.c`, and [Chapter 15](docs/15-collision.md) explains why.