Andrew Kesterson
fcad2822a9
Seven new benchmark rows, budgets set from a measured full-scale run rather than guessed, and the three existing rows the work moved re-recorded from that same run. The all-pairs sweep stays and is relabelled `control:` -- it is the cost a caller paid before the library had a broad phase, measured on the same machine in the same run, which is the only honest way to read a reduction. What the numbers say: - The box fast path is two to seven times cheaper than the general solver, and the disjoint case is cheaper still because the proxies' bounds reject it before any shape arithmetic runs. 9 to 20 ns is what a tile game actually pays. - The grid beats the tree by 2.6x on the same population, which is the argument for the default measured here rather than cited from another engine. The tree also rebuilds on every move and that is not in its row, so a moving scene is worse than 2.6x. - A grid `move` that changes nothing is 11.5 ns. That is the incremental claim in one number. Two rows moved on a backend with **no collision world attached**, so no collision code runs in either, and the commit says so rather than letting the feature take credit: - akgl_Actor grew from 415 to 464 bytes -- a 40-byte shape, an override flag and a proxy pointer. The step sweeps the whole pool, so that is about 3 KB more working set per frame. The empty-pool row is unchanged at 58.7 ns, which is what identifies the cost as per live actor rather than per slot. - Reaching the collision check through CATCH cost more than the check. PASS and CATCH call akerr_valid_error_address, which walks AKERR_ARRAY_ERROR, so an early-returning function is not free when it is reached through one. The no-collision path now routes around the machinery entirely, which took the 64-actor sweep from 2,018.7 ns back to 1,588.5. AGENTS.md records that lesson for writing benchmarks; this is the same thing in production code. The remainder is the struct, and it is paid whether or not a game uses collision. Co-Authored-By: Claude Code <noreply@anthropic.com> Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
The libakgl manual
libakgl is a C library for building 2D games on SDL3. It is not an engine: there is 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 you cannot silently ignore.
This manual teaches the library. It deliberately does not re-document its dependencies
— libakerror owns the error-handling protocol, SDL3 owns renderers and events, Tiled owns
the map format, jansson owns the JSON API. Each chapter says what libakgl adds or
constrains, shows what a caller actually writes, and links out for the rest. For
per-function reference, build the Doxygen output with doxygen Doxyfile.
| 1. Introduction | What libakgl is, what it refuses to be, the frame budget, and who owns which documentation |
| 2. Design philosophy | Bounded pools, pluggable backends, bit flags, name-based registries, one world at a time |
| 3. Getting started | Dependencies, add_subdirectory vs akgl.pc, and the smallest program that opens a window |
| 4. Errors and status codes | The status tables, what each code means here, and the traps that fail silently |
| 5. The heap | The five pools, akgl_heap_next_*, akgl_String, and why exhaustion usually means a missing release |
| 6. The registry | The eight registries, configuration properties, and the id-0 silent no-op |
| 7. The game and the frame | Startup order, akgl_game_update, the state lock, iterators, savegames |
| 8. Rendering | The backend vtable, the frame contract, cameras, and embedding a host's own SDL_Renderer |
| 9. Drawing | Points, lines, rectangles, circles, flood fill, and saving a region |
| 10. Spritesheets and sprites | One texture shared by path, the sprite JSON format, frame animation |
| 11. Characters | State-to-sprite mappings, movement constants, the character JSON format |
| 12. Actors | The state bitmask, the six behaviour hooks, parents and children, layers |
| 13. Tilemaps | Tiled TMJ, libakgl's three extensions, and the limits that bind a map |
| 14. Physics | Thrust, environment and velocity; the null and arcade backends; what is not implemented |
| 15. Input | Control maps, push-not-poll dispatch, the keystroke ring, gamepads |
| 16. Text and fonts | Loading, drawing, measuring, and the teardown order that matters |
| 17. Audio | The three-voice synthesizer, and the separate background-music path |
| 18. Utilities | Collision helpers, path resolution, the JSON accessors, static strings |
| 19. Tutorial: a 2D sidescroller | Gravity, a jump, collision you write yourself, coins and hazards |
| 20. Tutorial: a top-down JRPG | A town map, NPCs spawned from map objects, four-way animation, a text box, a follower |
| 21. Appendix | Every limit, every status, every configuration property |
Both tutorials are complete programs under examples/. They build with the
library and run headless in CI, and the chapters quote them rather than restating them — so
a tutorial cannot drift from a program that compiles.
Every example here is checked
The examples in these chapters are compiled, linked, run, and cross-checked against the
source tree by the test suite. A snippet that stops compiling, an excerpt that no longer
matches the header it quotes, or a JSON document the loader would reject turns
ctest -R docs_examples red.
This exists because the documentation it replaces had drifted badly: the previous FAQ's
examples did not compile — two unbalanced PASS() calls, a sprite->frameids = [0, 1, 2, 3];
that is not C in any dialect, and, in the first snippet a reader ever saw, the exact
strncpy call AGENTS.md forbids. The prose had drifted with it, and writing these
chapters turned up twenty-seven header claims that were false against src/.
Where a chapter documents behaviour that is a known defect rather than a design decision,
it says so and points at TODO.md. See MAINTENANCE.md if you are editing an example.
Assets
The tutorial art, tiles and music are CC0, vendored under
tutorials/assets/ with provenance recorded per file in
PROVENANCE.md and geometry documented in
README.md. CC0 specifically, not merely free: a reader who
copies a tutorial into their own game inherits no obligation.