path: root/docs/superpowers/specs/2026-04-05-demos-design.md

# Downstroke Demo Games Design

**Date:** 2026-04-05
**Status:** Approved
**Scope:** `demo/` folder, 5 demo games, Makefile `make demos` target, CLAUDE.md update

---

## Goal

Provide 5 small self-contained demo games in `demo/` that collectively exercise every engine system. Each demo compiles to its own executable (`bin/demo-*`). They replace the macroknight integration test for Milestone 8 and serve as living documentation of the engine API.

---

## File Layout

```
demo/
  assets/                        ← copied from macroknight/assets (not symlinked)
    monochrome-transparent.png   ← tileset spritesheet
    monochrome_transparent.tsx   ← tileset metadata (TSX)
    level-0.tmx                  ← level used by platformer, topdown, sandbox
    DejaVuSans.ttf               ← font for audio demo text
    jump.wav                     ← sound effect (platformer jump, shmup shoot)
    theme.ogg                    ← music (audio demo)
  platformer.scm
  shmup.scm
  topdown.scm
  audio.scm
  sandbox.scm
```

**Omitted from copy:** `prefabs.scm`, `macroknight.tiled-project`, `macroknight.tiled-session` — macroknight-specific files not needed by any demo.

**Audio modules:** `mixer.scm` and `sound.scm` are copied from `macroknight/` into the downstroke repo root and added to the build. They are engine-level modules and are required by any demo that uses audio.

When copying `sound.scm`, add one function that macroknight's version omits:
```scheme
(define (stop-music!) (mix-halt-music))
```
The audio demo uses `stop-music!` to toggle music off.

---

## Build

### Makefile additions

```makefile
DEMO_NAMES := platformer shmup topdown audio sandbox
DEMO_BINS  := $(patsubst %,bin/demo-%,$(DEMO_NAMES))

demos: engine $(DEMO_BINS)

bin/demo-%: demo/%.scm $(OBJECT_FILES) | bin
	csc demo/$*.scm $(OBJECT_FILES) -o bin/demo-$* -I bin
```

- `make` — builds engine modules only (unchanged)
- `make demos` — builds all 5 demo executables; depends on engine being built first
- Demos are compiled as programs (not units), linked against all engine `.o` files
- `$(OBJECT_FILES)` includes `mixer` and `sound` once those modules are added to `MODULE_NAMES`

### `render-scene!` nil guards

`render-scene!` in `renderer.scm` is updated so that entity drawing is **nested inside** the tilemap guard:

- Tilemap drawing only fires if `(scene-tilemap scene)` is not `#f`
- Entity drawing only fires if **both** `(scene-tilemap scene)` AND `(scene-tileset-texture scene)` are not `#f`

```scheme
(when tilemap
  (draw-tilemap renderer camera tileset-texture tilemap)
  (when tileset-texture
    (let ((tileset (tilemap-tileset tilemap)))
      (draw-entities renderer camera tileset tileset-texture entities))))
```

**Consequence for shmup:** since shmup has no tilemap, the engine will not draw its entities. Shmup draws its player, bullets, and enemies as **colored SDL rectangles** in its `render:` hook — matching the original "colored rects" intent. This is acceptable because shmup entities are simple geometric shapes, not sprites.

Audio (no scene at all) works because `engine.scm` guards `render-scene!` with `(when (game-scene game) ...)`.

### CLAUDE.md update

Add to the Build & Test section:

> `make demos` must always succeed. A demo that fails to compile is a build failure. Run `make && make demos` to verify both engine and demos build cleanly.

---

## Demo Code Pattern

Every demo follows this ~30-line structure:

```scheme
(import (prefix sdl2 "sdl2:")
        (prefix sdl2-ttf "ttf:")
        (prefix sdl2-image "img:")
        downstroke/engine
        downstroke/world
        downstroke/tilemap
        downstroke/renderer
        downstroke/input
        downstroke/physics
        downstroke/assets)

(define *game*
  (make-game
    title: "Demo: <Name>" width: 600 height: 400
    preload: (lambda (game) ...)   ;; load tilemap, tileset texture, sounds
    create:  (lambda (game) ...)   ;; build scene, place entities
    update:  (lambda (game dt) ...)  ;; input dispatch, physics calls
    render:  (lambda (game) ...)))   ;; HUD overlay (optional)

(game-run! *game*)
```

Tile IDs in entity plists are placeholder values — to be adjusted visually after first run.

---

## The 5 Demos

### 1. `demo/platformer.scm` — Platformer

**Systems exercised:** `input`, `physics` (gravity + tile collision), `renderer` (tilemap + entities), `world`/scene, camera follow, audio (sound effect)

**Mechanics:**
- Player entity with gravity, left/right movement, jump
- Tile collision via `apply-physics` from `physics.scm`
- Camera follows player horizontally
- Jump sound via `(play-sound 'jump)` (loaded in preload: as `'(jump . "demo/assets/jump.wav")`)
- Level: `demo/assets/level-0.tmx`
- Tile IDs: placeholder (user adjusts)

**Key entity plist:**
```scheme
(list #:type 'player
      #:x 100 #:y 50
      #:width 16 #:height 16
      #:vx 0 #:vy 0
      #:gravity? #t
      #:on-ground? #f
      #:tile-id 1)
```

**Update logic:** read input → set `#:vx` from left/right → jump sets `#:vy` → call physics step → update camera x to follow player x.

---

### 2. `demo/shmup.scm` — Shoot-em-up

**Systems exercised:** `entity` (spawning/removal), manual AABB entity-entity collision (removal-based, not `physics.scm`), `input`, `renderer` (SDL colored rects), `world`/scene (no tilemap)

**Mechanics:**
- Player ship at bottom, moves left/right
- Space bar fires bullet upward (new entity added to scene)
- Enemies spawn from top at random x positions every N frames, move downward
- Bullet-enemy collision: both entities removed from scene
- No tilemap — plain background (black/SDL clear)
- No gravity on any entity
- `jump.wav` plays on shoot

**Key entity plists:** (no `#:tile-id` — entities are drawn as colored rects)
```scheme
;; player
(list #:type 'player #:x 280 #:y 360 #:width 16 #:height 16 #:vx 0 #:vy 0)
;; bullet
(list #:type 'bullet #:x px #:y 340 #:width 4 #:height 8 #:vx 0 #:vy -5)
;; enemy
(list #:type 'enemy  #:x rx #:y 0   #:width 16 #:height 16 #:vx 0 #:vy 2)
```

**Rendering:** shmup has no tilemap, so `render-scene!` draws nothing for this scene. Shmup implements its own `render:` hook using `sdl2:render-fill-rect!` with distinct colors (player = white, bullet = yellow, enemy = red). The camera is at (0,0) so screen coords == world coords.

**Update logic:** move entities by vx/vy each frame → manual AABB collision check between bullets and enemies (not `resolve-entity-collisions` — shmup uses removal, not push-apart) → filter removed entities from scene → spawn new enemy every 60 frames → read input for player movement and shoot.

---

### 3. `demo/topdown.scm` — Top-down explorer

**Systems exercised:** `input` (8-directional), `renderer` (tilemap + entity), `world`/scene, camera follow (both axes), `physics` (no gravity)

**Mechanics:**
- Player entity moves in 8 directions (WASD or arrows)
- No gravity (`#:gravity? #f`)
- Camera follows player on both x and y axes
- Level: `demo/assets/level-0.tmx` (same tilemap, different movement feel)
- Tileset texture loaded in preload: — required for entity sprite rendering (tilemap + tileset-texture both present so render-scene! draws entities via tileset)
- No audio

**Update logic:** read input → set `#:vx` and `#:vy` from direction keys → apply tile collision (no gravity component) → update camera to center on player.

---

### 4. `demo/audio.scm` — Audio showcase

**Systems exercised:** audio (sound effects + music), `renderer` (text via `draw-ui-text`), `input`, `assets`

**Mechanics:**
- Static screen with text instructions rendered via `draw-ui-text`
- Press **J** → play `jump.wav`
- Press **M** → toggle `theme.ogg` music on/off
- Press **Escape** → quit
- No tilemap, no physics, no moving entities
- Uses `DejaVuSans.ttf` for text
- Audio calls via `sound.scm` functions: `load-sounds!`, `play-sound`, `load-music!`, `play-music!`, `stop-music!` (added when copying sound.scm — see File Layout)

**Display:** colored rectangle background + text labels for each key binding.

---

### 5. `demo/sandbox.scm` — Physics sandbox

**Systems exercised:** `physics` (gravity + tile collision + entity-entity collision), `renderer`, `world`/scene, no player input

**Mechanics:**
- 10 entities spawned at random x positions near the top of the screen
- All have `#:gravity? #t` and `#:solid? #t` (required for `resolve-entity-collisions` to participate)
- Physics step runs each frame (gravity accelerates, tile collision stops them)
- Entities rest on floor tiles or bounce (depending on physics.scm behavior)
- No player — pure observation of physics pipeline
- Level: `demo/assets/level-0.tmx`
- After all entities settle (or after 10 seconds), loop: despawn all, respawn at new random positions

---

## Systems Coverage Matrix

| System / Module | platformer | shmup | topdown | audio | sandbox |
|---|---|---|---|---|---|
| `engine` (make-game, game-run!) | ✓ | ✓ | ✓ | ✓ | ✓ |
| `input` | ✓ | ✓ | ✓ | ✓ | — |
| `physics` (gravity) | ✓ | — | — | — | ✓ |
| `physics` (tile collision) | ✓ | — | ✓ | — | ✓ |
| `physics` (entity collision) | — | — | — | — | ✓ |
| manual AABB (removal) | — | ✓ | — | — | — |
| `renderer` (tilemap) | ✓ | — | ✓ | — | ✓ |
| `renderer` (entities) | ✓ | — | ✓ | — | ✓ |
| `renderer` (SDL colored rects) | — | ✓ | — | — | — |
| `renderer` (text) | — | — | — | ✓ | — |
| `world` / scene | ✓ | ✓ | ✓ | — | ✓ |
| `assets` registry | ✓ | ✓ | ✓ | ✓ | ✓ |
| audio (sound) | ✓ | ✓ | — | ✓ | — |
| audio (music) | — | — | — | ✓ | — |
| camera follow | ✓ (x) | — | ✓ (xy) | — | — |

---

## Out of Scope

- Animation state machine (`animation.scm`) — not yet extracted to downstroke
- AI (`ai.scm`) — not yet extracted
- Prefab system — not yet extracted
- Scene transitions — Milestone 9
- Asset-type-specific load helpers (`game-load-tilemap!` etc.) — Milestone 6