* Downstroke A 2D tile-driven game engine for Chicken Scheme, built on SDL2. Targets old-school platformer and arcade games — NES-style and beyond. The API is inspired by Phaser 2: one call to start, lifecycle hooks to fill in. A minimal game is ~20 lines of Scheme. #+begin_src scheme (define my-game (make-game title: "My Game" width: 320 height: 240 preload: (lambda (game) ...) ; load assets create: (lambda (game) ...) ; set up scene update: (lambda (game dt) ...))) ; per-frame logic (physics runs first) (game-run! my-game) #+end_src ** Features - Tweens with easing helpers - Tile-based physics: gravity, velocity, AABB collision against TMX tilemaps - Built-in update pipeline (before your =update:= hook): tweens → acceleration → gravity → velocity → tile collisions → ground detection → entity collisions → group sync → animation; input is polled each frame and available to hooks and your own logic - Entities as association lists (alists) — purely functional, no classes; prefab data is often written as plists and converted when loaded - Data-driven prefab/mixin system for composing entity types - Configurable input: keyboard, joystick, and controller - Asset registry with =preload:= lifecycle hook - Scene and camera management - Sprite animation with frame/tick tracking - SDL2_mixer audio via a thin FFI binding ** Status The engine lives in this repository as modular Chicken units, with demos, docs, and an egg spec (=downstroke.egg=). It still powers [[https://genepasquet.itch.io/macroknight][macroknight]] (Spring Lisp Game Jam 2025). The tree is at release candidate =1.0.0rc1= (=VERSION= in the =Makefile=, same string in the egg and the table below). Macroknight is not yet switched to depend on the published egg — that remains the final validation step. | Milestone | Description | Status | |-----------+------------------------------------+----------| | 1 | Zero-risk module extraction | DONE | | 2 | Configurable input system | DONE | | 3 | Data-driven entity rendering | DONE | | 4 | Renderer abstraction | DONE | | 5 | Asset preloading | DONE | | 6 | Scene loading via =create:= hook | DONE | | 7 | =make-game= / =game-run!= API | DONE | | 8 | Camera follow | DONE | | 9 | Named scene states | DONE | | 10 | Tag-based entity lookup | DONE | | 11 | Package as Chicken egg (v1.0.0rc1) | rc | | 12 | Macroknight ported to use the egg | pending | Milestones 1–6 were pure refactoring. Milestone 7 was the API pivot (=make-game=, =game-run!=, pipeline). Milestones 11–12 are packaging and proving the egg against macroknight. Finer-grained planning may live outside this repo (for example in =downstroke.org= or macroknight's historical =TODO-engine.org=). ** Dependencies System libraries: =SDL2=, =SDL2_image=, =SDL2_mixer=, =SDL2_ttf= Chicken eggs (mirrors =downstroke.egg=; install what you do not already have): #+begin_example chicken-install sdl2 sdl2-image sdl2-ttf expat matchable defstruct list-utils \ srfi-1 srfi-13 srfi-69 srfi-197 simple-logger #+end_example From a git checkout, run tests with the =test= egg (=make test= runs =csi -s= on each suite). Games that ship their own FSM logic (for example with the =states= egg) declare that egg themselves; Downstroke does not depend on it. ** Building #+begin_src sh make # compile all modules to bin/ make test # run unit tests (=test= egg) make demos # build demo games (verifies they compile) make clean # remove bin/ #+end_src Single test module (from repo root; space after each =-I= is required by =csi=): #+begin_src sh csi -I "$(pwd)" -I "$(pwd)/bin" -s tests/physics-test.scm #+end_src The canonical version string is =VERSION= at the top of the =Makefile=; it must match =((version "…")= in =downstroke.egg= and the =v…= marker in the milestone table. Bump all three with =make bump-version NEW=1.2.3= (POSIX =sed=; =NEW= must not contain =&=, backslash, or =#=). ** Architecture Modules live at the project root. Each compiles as a Chicken unit (=csc -c -J -unit=). Compile order follows the dependency graph: : entity → tween → tilemap → world → input → physics → renderer → assets → animation → engine → mixer → sound → prefabs → scene-loader | Module | Responsibility | |----------------+---------------------------------------------------| | =entity= | Alist accessors: =entity-ref=, =entity-set= | | =tilemap= | TMX/TSX parsing (expat), tileset loading | | =world= | Scene struct, entity list ops, camera | | =physics= | Gravity, velocity, AABB tile + entity collisions | | =input= | SDL2 events → action mapping, configurable binds | | =animation= | Frame/tick tracking, sprite ID mapping | | =prefabs= | Mixin composition, entity instantiation | | =renderer= | =draw-sprite=, =draw-tilemap-layer=, =draw-text= | | =assets= | Asset registry for the =preload:= hook | | =scene-loader= | =game-load-scene!=, =instantiate-prefab= | | =mixer= | SDL_mixer FFI (no Scheme deps) | | =sound= | Sound registry, music playback | | =engine= | =make-game=, =game-run!=, lifecycle orchestration | Entities are alists at runtime — no classes, purely functional: #+begin_src scheme '((#:type . player) (#:x . 100) (#:y . 200) (#:width . 16) (#:height . 16) (#:vx . 0) (#:vy . 0) (#:gravity? . #t) (#:on-ground? . #f) (#:tile-id . 29) (#:tags . (player)) (#:anim-name . walk) (#:animations . (((#:name . walk) (#:frames . (27 28)) (#:duration . 10)) ((#:name . idle) (#:frames . (28)) (#:duration . 10))))) #+end_src Entity types are composed from mixins declared in a data file: #+begin_src scheme (mixins (physics-body #:vx 0 #:vy 0 #:gravity? #t #:on-ground? #f) (has-facing #:facing 1)) (prefabs (player physics-body has-facing #:type player #:tile-id 29)) #+end_src ** LLM (AI) disclosure Some engine code was written or refactored with an LLM (aka AI) prior to the initial release. I don't intend to keep using AI for engine code after that. The =docs/= tree is LLM-generated, and that is expected to continue. ** Credits Engine extracted from [[https://genepasquet.itch.io/macroknight][macroknight]] (Spring Lisp Game Jam 2025). - Code: Gene Pasquet - Demo Levels: Owen Pasquet - Demo Art: [[https://kenney.nl][Kenney]] (1-bit pack)