#+TITLE: Entity Model
#+AUTHOR: Downstroke
#+DESCRIPTION: How to create, update, and manage entities in the Downstroke game engine

* Overview

Entities in Downstroke are plain Scheme **plists** (property lists) — alternating keyword/value pairs with no special structure or classes. An entity is just a list:

#+begin_src scheme
(list #:type 'player
      #:x 100 #:y 200
      #:width 16 #:height 16
      #:vx 0 #:vy 0
      #:gravity? #t
      #:on-ground? #f
      #:tile-id 1)
#+end_src

This minimal approach keeps the engine lean: your game defines whatever keys it needs. The shared keys listed below are *conventions* for physics, rendering, and animation — use them to integrate with the engine's built-in systems. Custom keys are always allowed.

* Pipeline skips (~#:skip-pipelines~)

The optional key ~#:skip-pipelines~ holds a list of **symbols** naming frame pipeline steps that should be skipped for that entity. The physics module defines the built-in step names (see ~docs/physics.org~). The predicate ~entity-skips-pipeline?~ and the syntax ~define-pipeline~ live in ~downstroke-entity~ so any subsystem (physics now; rendering or animation later if you extend the engine) can use the same mechanism without a separate “core pipeline” module.

* Creating Entities

There is no ~make-entity~ constructor. Create an entity as a plain list:

#+begin_src scheme
(define my-entity
  (list #:type 'enemy
        #:x 200 #:y 150
        #:width 16 #:height 16
        #:vx 0 #:vy 0
        #:gravity? #t
        #:tile-id 42))
#+end_src

Add whatever additional keys your game needs — tags, state, custom data, anything. Entities are pure data.

* Accessing and Updating Entities

Entities are **immutable**. Use these three functions to read and update them:

** ~entity-ref entity key [default]~

Returns the value associated with ~key~ in the entity plist, or ~default~ if the key is absent.

#+begin_src scheme
(entity-ref player #:x 0)         ; → 100
(entity-ref player #:vx)          ; → 0 (if #:vx is absent)
(entity-ref player #:custom #f)   ; → #f (no #:custom key)
#+end_src

If ~default~ is a procedure, it is called to compute the default:

#+begin_src scheme
(entity-ref player #:x (lambda () (error "x is required")))
#+end_src

** ~entity-type entity~

Shorthand for ~(entity-ref entity #:type #f)~. Returns the entity's ~#:type~ or ~#f~.

#+begin_src scheme
(entity-type player) ; → 'player
#+end_src

** ~entity-set entity key val~

Returns a **new** plist with the key/value pair set (or replaced). The original entity is unchanged — this is functional, immutable update.

#+begin_src scheme
(define updated-player (entity-set player #:vx 5))
;; original player is still unchanged:
(entity-ref player #:vx)          ; → 0
(entity-ref updated-player #:vx)  ; → 5
#+end_src

** ~entity-update entity key proc [default]~

Returns a new entity with ~key~ set to ~(proc (entity-ref entity key default))~. Useful for incrementing or transforming a value:

#+begin_src scheme
(entity-update player #:x (lambda (x) (+ x 3)))  ; move right 3 pixels
#+end_src

** Chaining Updates: The let* Pattern

Since each update returns a new entity, chain updates with ~let*~:

#+begin_src scheme
(let* ((player (entity-set player #:vx 3))
       (player (apply-velocity-x player))
       (player (resolve-tile-collisions-x player tilemap)))
  ;; now use the updated player
  player)
#+end_src

This is how the platformer demo applies physics in order: each step reads the input, computes the next state, and passes it to the next step.

* Plist Key Reference

The engine recognizes these standard keys. Use them to integrate with the physics pipeline, rendering, and animation systems. Custom keys are always allowed.

| Key | Type | Description |
|---|---|---|
| ~#:type~ | symbol | Entity type, e.g., ~'player~, ~'enemy~, ~'coin~. No built-in enforcement; use for ~entity-type~ checks and scene queries. |
| ~#:x~, ~#:y~ | number | World position in pixels (top-left corner of bounding box). Updated by ~apply-velocity-x~, ~apply-velocity-y~, and collision resolvers. |
| ~#:width~, ~#:height~ | number | Bounding box size in pixels. Used for AABB tile collision checks and entity-entity collision. Required for physics. |
| ~#:vx~, ~#:vy~ | number | Velocity in pixels per frame. ~#:vx~ is updated by ~apply-velocity-x~; ~#:vy~ is updated by ~apply-velocity-y~. Both consumed by collision resolvers. |
| ~#:ay~ | number | Y acceleration (e.g., from jumping or knockback). Consumed by ~apply-acceleration~, which adds it to ~#:vy~. Optional; default is 0. |
| ~#:gravity?~ | boolean | Whether gravity applies to this entity. Set to ~#t~ for platformers (gravity pulls down), ~#f~ for top-down or flying entities. Used by ~apply-gravity~. |
| ~#:on-ground?~ | boolean | Whether the entity is supported from below (set by ~detect-on-solid~): solid tile under the feet and/or standing on another solid entity when you pass the scene entity list. Use this to gate jump input. |
| ~#:solid?~ | boolean | Whether this entity participates in entity-entity collision. If ~#t~, ~resolve-entity-collisions~ will check it against other solid entities. |
| ~#:immovable?~ | boolean | If ~#t~ with ~#:solid? #t~, entity–entity resolution only moves the *other* entity (static platforms). Two overlapping immovable solids are not separated. |
| ~#:skip-pipelines~ | list of symbols | Optional. Each symbol names a physics step to skip for this entity (e.g. ~gravity~, ~velocity-x~). See ~docs/physics.org~. |
| ~#:tile-id~ | integer | Sprite index in the tileset (1-indexed). Used by ~render-scene!~ when the scene has a tileset texture and tile metadata (from the tilemap or ~scene-tileset~). Updated automatically by animation (~animate-entity~). |
| ~#:color~ | list | Optional ~(r g b)~ or ~(r g b a)~ (0–255 each). When ~#:tile-id~ is not drawn as a sprite (missing ~#:tile-id~, or no tileset texture), ~render-scene!~ fills the entity rect with this color. |
| ~#:facing~ | number | Horizontal flip direction: ~1~ = right (default), ~-1~ = left. Used by renderer to flip sprite horizontally. Update when changing direction. |
| ~#:tags~ | list of symbols | List of tag symbols, e.g., ~'(player solid)~. Used by ~scene-find-tagged~ and ~scene-find-all-tagged~ for fast lookups. |
| ~#:animations~ | alist | Animation definitions (see Animation section). Keys are animation names (symbols); values are animation specs. |
| ~#:anim-name~ | symbol | Currently active animation name, e.g., ~'walk~, ~'jump~. Set with ~set-animation~; reset by ~animate-entity~. |
| ~#:anim-frame~ | integer | Current frame index within the animation (0-indexed). Updated automatically by ~animate-entity~. |
| ~#:anim-tick~ | integer | Tick counter for frame timing (0 to ~#:duration - 1~). Incremented by ~animate-entity~; resets when frame advances. |

* Entities in Scenes

A **scene** is a level state: it holds a tilemap (optional), an optional standalone tileset for ~#:tile-id~ drawing without TMX, a camera, and a list of entities. These functions manipulate scene entities:

** ~scene-entities scene~

Returns the list of all entities in the scene.

#+begin_src scheme
(define all-entities (scene-entities scene))
#+end_src

** ~scene-entities-set! scene entities~

Mutates the scene to replace the entity list. Use after ~scene-update-entities~ or batch operations.

#+begin_src scheme
(scene-entities-set! scene (list updated-player updated-enemy))
#+end_src

** ~scene-add-entity scene entity~

Adds an entity to the scene and returns the scene. Appends to the entity list.

#+begin_src scheme
(scene-add-entity scene new-enemy)
#+end_src

** ~scene-update-entities scene proc1 proc2 ...~

Maps each procedure over the scene's entities, applying them in sequence. Each proc must be a function of one entity, returning a new entity.

#+begin_src scheme
;; Apply physics pipeline to all entities:
(scene-update-entities scene
  apply-gravity
  apply-velocity-x
  apply-velocity-y)
#+end_src

The result is equivalent to:

#+begin_src scheme
(let* ((es (scene-entities scene))
       (es (map apply-gravity es))
       (es (map apply-velocity-x es))
       (es (map apply-velocity-y es)))
  (scene-entities-set! scene es)
  scene)
#+end_src

** ~scene-filter-entities scene pred~

Removes all entities that do not satisfy the predicate. Use to despawn dead enemies, collected items, etc.

#+begin_src scheme
;; Remove all entities with #:health <= 0:
(scene-filter-entities scene
  (lambda (e) (> (entity-ref e #:health 1) 0)))
#+end_src

** ~scene-find-tagged scene tag~

Returns the first entity whose ~#:tags~ list contains ~tag~, or ~#f~ if none found.

#+begin_src scheme
(scene-find-tagged scene 'player)  ; → player entity or #f
#+end_src

** ~scene-find-all-tagged scene tag~

Returns a list of all entities whose ~#:tags~ list contains ~tag~.

#+begin_src scheme
(scene-find-all-tagged scene 'enemy)  ; → (enemy1 enemy2 enemy3) or ()
#+end_src

* Animation

Entities with the ~#:animations~ key can cycle through sprite frames automatically. Animation is data-driven: define sprite sequences once, then switch between them in your update logic.

** Animation Data Format

The ~#:animations~ key holds an **alist** (association list) of animation entries. Each entry is ~(name #:frames (frame-indices...) #:duration ticks-per-frame)~:

#+begin_src scheme
(list #:type 'player
      ...
      #:animations
        '((idle  #:frames (28)      #:duration 10)
          (walk  #:frames (27 28)   #:duration 6)
          (jump  #:frames (29)      #:duration 10)
          (fall  #:frames (30)      #:duration 10))
      #:anim-name 'idle
      #:anim-frame 0
      #:anim-tick  0)
#+end_src

- ~idle~, ~walk~, ~jump~, ~fall~ are animation **names** (symbols).
- ~#:frames (28)~ means frame 0 of the animation displays sprite tile 28 (1-indexed in tileset).
- ~#:frames (27 28)~ means frame 0 displays tile 27, frame 1 displays tile 28, then loops back to frame 0.
- ~#:duration 6~ means each frame displays for 6 game ticks before advancing to the next frame.

** Switching Animations

Use ~set-animation~ to switch to a new animation, resetting frame and tick counters. If the animation is already active, it is a no-op (avoids restarting mid-loop):

#+begin_src scheme
(set-animation player 'walk)  ; switch to walk animation
;; → entity with #:anim-name 'walk, #:anim-frame 0, #:anim-tick 0
#+end_src

** Advancing Animation

Call ~animate-entity~ once per game frame to step the animation. Pass the entity and its animation table:

#+begin_src scheme
(define animations
  '((idle  #:frames (28)      #:duration 10)
    (walk  #:frames (27 28)   #:duration 6)))

(let* ((player (set-animation player 'walk))
       (player (animate-entity player animations)))
  player)
#+end_src

~animate-entity~ does four things:
1. Increments ~#:anim-tick~ by 1.
2. If ~#:anim-tick~ reaches ~#:duration~, advances ~#:anim-frame~ and resets tick to 0.
3. Updates ~#:tile-id~ to the sprite ID for the current frame.
4. Returns the updated entity (or unchanged entity if ~#:anim-name~ is not set).

** Typical Animation Update Pattern

#+begin_src scheme
(define (update-player-animation player input)
  (let* ((anim (if (input-held? input 'left)  'walk 'idle))
         (player (set-animation player anim)))
    (animate-entity player player-animations)))
#+end_src

* Tags

The ~#:tags~ key is a list of symbols used to label and query entities. Tags are arbitrary — define whatever makes sense for your game.

** Creating Entities with Tags

#+begin_src scheme
(list #:type 'player
      #:tags '(player solid)
      ...)

(list #:type 'enemy
      #:tags '(enemy dangerous)
      ...)

(list #:type 'coin
      #:tags '(collectible)
      ...)
#+end_src

** Querying by Tag

Use the scene tag lookup functions:

#+begin_src scheme
;; Find the first entity tagged 'player:
(define player (scene-find-tagged scene 'player))

;; Find all enemies:
(define enemies (scene-find-all-tagged scene 'enemy))

;; Find all collectibles and remove them:
(scene-filter-entities scene
  (lambda (e) (not (member 'collectible (entity-ref e #:tags '())))))
#+end_src

** Tag Conventions

While tags are free-form, consider using these conventions in your game:

- ~player~: the player character
- ~enemy~: hostile entities
- ~solid~: entities that participate in collision
- ~collectible~: items to pick up
- ~projectile~: bullets, arrows, etc.
- ~hazard~: spikes, lava, etc.

* Example: Complete Entity Setup

Here is a full example showing entity creation, initialization in the scene, and update logic:

#+begin_src scheme
(define (create-player-entity)
  (list #:type 'player
        #:x 100 #:y 200
        #:width 16 #:height 16
        #:vx 0 #:vy 0
        #:ay 0
        #:gravity? #t
        #:on-ground? #f
        #:tile-id 1
        #:facing 1
        #:tags '(player solid)
        #:animations
          '((idle  #:frames (1)      #:duration 10)
            (walk  #:frames (1 2)    #:duration 6)
            (jump  #:frames (3)      #:duration 10))
        #:anim-name 'idle
        #:anim-frame 0
        #:anim-tick  0))

(define (create-hook game)
  (let* ((scene  (game-load-scene! game "assets/level.tmx"))
         (player (create-player-entity)))
    (scene-add-entity scene player)))

(define (update-hook game dt)
  (let* ((input  (game-input game))
         (scene  (game-scene game))
         (player (scene-find-tagged scene 'player))
         (tm     (scene-tilemap scene)))
    ;; Update input-driven velocity
    (let* ((player (entity-set player #:vx
                     (cond
                       ((input-held? input 'left)  -3)
                       ((input-held? input 'right)  3)
                       (else 0))))
           ;; Handle jump
           (player (if (and (input-pressed? input 'a)
                            (entity-ref player #:on-ground? #f))
                       (entity-set player #:ay -5)
                       player))
           ;; Apply physics
           (player (apply-acceleration player))
           (player (apply-gravity player))
           (player (apply-velocity-x player))
           (player (resolve-tile-collisions-x player tm))
           (player (apply-velocity-y player))
           (player (resolve-tile-collisions-y player tm))
           (player (detect-on-solid player tm))
           ;; Update animation
           (player (set-animation player
                     (cond
                       ((not (entity-ref player #:on-ground? #f)) 'jump)
                       ((not (zero? (entity-ref player #:vx 0))) 'walk)
                       (else 'idle))))
           (player (animate-entity player player-animations)))
      ;; Update facing direction
      (let ((player (if (< (entity-ref player #:vx 0) 0)
                        (entity-set player #:facing -1)
                        (entity-set player #:facing 1))))
        (scene-entities-set! scene (list player))))))
#+end_src

Note the let*-chaining pattern: each update builds on the previous result, keeping the data flow clear and each step testable.