From a8e5e723b7e1891c9b352261a3ee4c3d3563e8cf Mon Sep 17 00:00:00 2001
From: Elizabeth Hunt <me@liz.coffee>
Date: Sun, 26 Oct 2025 21:38:22 -0700
Subject: Checkpoint two

---
 .../coffee/liz/abstractionengine/Platform.kt       |  2 +-
 .../kotlin/coffee/liz/abstractionengine/app/App.kt |  3 +-
 .../coffee/liz/abstractionengine/app/ui/Theme.kt   | 25 +++---
 .../abstractionengine/game/AbstractionEngine.kt    |  4 +-
 .../coffee/liz/abstractionengine/game/Game.kt      |  4 +-
 .../liz/abstractionengine/game/ImageCache.kt       |  9 +-
 .../coffee/liz/abstractionengine/game/Renderer.kt  | 15 ++--
 .../commonMain/kotlin/coffee/liz/ecs/DAGWorld.kt   | 29 +++----
 .../src/commonMain/kotlin/coffee/liz/ecs/Entity.kt | 18 ++--
 .../commonMain/kotlin/coffee/liz/ecs/GameLoop.kt   |  3 +-
 .../src/commonMain/kotlin/coffee/liz/ecs/System.kt |  6 +-
 .../kotlin/coffee/liz/ecs/TickedSystem.kt          | 29 +++++--
 .../src/commonMain/kotlin/coffee/liz/ecs/Vec.kt    | 57 +++++++++++--
 .../src/commonMain/kotlin/coffee/liz/ecs/World.kt  |  5 +-
 .../liz/ecs/animation/AnimationComponents.kt       | 35 +++++---
 .../coffee/liz/ecs/animation/AnimationSystem.kt    | 14 +++-
 .../kotlin/coffee/liz/ecs/grid/Components.kt       |  8 ++
 .../commonMain/kotlin/coffee/liz/ecs/grid/Grid.kt  | 54 ++++++++++++
 .../kotlin/coffee/liz/ecs/grid/GridSystem.kt       | 17 ++++
 .../kotlin/coffee/liz/ecs/input/Controlled.kt      |  7 +-
 .../kotlin/coffee/liz/ecs/input/InputProvider.kt   |  7 --
 .../kotlin/coffee/liz/ecs/input/InputSystem.kt     | 19 ++++-
 .../kotlin/coffee/liz/ecs/physics/CollisionTick.kt | 95 ++++++++++++++++++----
 .../kotlin/coffee/liz/ecs/physics/Components.kt    | 25 +++++-
 .../coffee/liz/ecs/physics/IntegrationTick.kt      | 14 ++--
 .../kotlin/coffee/liz/ecs/physics/PhysicsSystem.kt | 25 ++++--
 .../kotlin/coffee/liz/ecs/physics/PhysicsTick.kt   | 10 +++
 27 files changed, 403 insertions(+), 136 deletions(-)
 create mode 100644 composeApp/src/commonMain/kotlin/coffee/liz/ecs/grid/Components.kt
 create mode 100644 composeApp/src/commonMain/kotlin/coffee/liz/ecs/grid/Grid.kt
 create mode 100644 composeApp/src/commonMain/kotlin/coffee/liz/ecs/grid/GridSystem.kt
 delete mode 100644 composeApp/src/commonMain/kotlin/coffee/liz/ecs/input/InputProvider.kt
 create mode 100644 composeApp/src/commonMain/kotlin/coffee/liz/ecs/physics/PhysicsTick.kt

(limited to 'composeApp/src/commonMain/kotlin/coffee')

diff --git a/composeApp/src/commonMain/kotlin/coffee/liz/abstractionengine/Platform.kt b/composeApp/src/commonMain/kotlin/coffee/liz/abstractionengine/Platform.kt
index 17ca317..7c43f74 100644
--- a/composeApp/src/commonMain/kotlin/coffee/liz/abstractionengine/Platform.kt
+++ b/composeApp/src/commonMain/kotlin/coffee/liz/abstractionengine/Platform.kt
@@ -4,4 +4,4 @@ interface Platform {
     val needsTouchscreenControls: Boolean
 }
 
-expect fun getPlatform(): Platform
\ No newline at end of file
+expect fun getPlatform(): Platform
diff --git a/composeApp/src/commonMain/kotlin/coffee/liz/abstractionengine/app/App.kt b/composeApp/src/commonMain/kotlin/coffee/liz/abstractionengine/app/App.kt
index 50e72f0..5b8c967 100644
--- a/composeApp/src/commonMain/kotlin/coffee/liz/abstractionengine/app/App.kt
+++ b/composeApp/src/commonMain/kotlin/coffee/liz/abstractionengine/app/App.kt
@@ -13,7 +13,6 @@ import org.jetbrains.compose.ui.tooling.preview.Preview
 @Composable
 @Preview
 fun App() {
-
     MaterialTheme(colorScheme = GameBoyTheme) {
         Box(modifier = Modifier.fillMaxSize()) {
             if (getPlatform().needsTouchscreenControls) {
@@ -21,4 +20,4 @@ fun App() {
             }
         }
     }
-}
\ No newline at end of file
+}
diff --git a/composeApp/src/commonMain/kotlin/coffee/liz/abstractionengine/app/ui/Theme.kt b/composeApp/src/commonMain/kotlin/coffee/liz/abstractionengine/app/ui/Theme.kt
index be6c3ad..7f181a4 100644
--- a/composeApp/src/commonMain/kotlin/coffee/liz/abstractionengine/app/ui/Theme.kt
+++ b/composeApp/src/commonMain/kotlin/coffee/liz/abstractionengine/app/ui/Theme.kt
@@ -18,15 +18,16 @@ object GameBoyColors {
     val DPadLight = Color(0xFF6B7F82)
 }
 
-val GameBoyTheme = darkColorScheme(
-    primary = GameBoyColors.MediumGreen,
-    onPrimary = GameBoyColors.DarkestGreen,
-    secondary = GameBoyColors.LightGreen,
-    onSecondary = GameBoyColors.DarkestGreen,
-    background = GameBoyColors.DarkestGreen,
-    onBackground = GameBoyColors.LightGreen,
-    surface = GameBoyColors.DarkGreen,
-    onSurface = GameBoyColors.LightGreen,
-    surfaceVariant = GameBoyColors.DPadGray,
-    outline = GameBoyColors.DarkestGreen,
-)
+val GameBoyTheme =
+    darkColorScheme(
+        primary = GameBoyColors.MediumGreen,
+        onPrimary = GameBoyColors.DarkestGreen,
+        secondary = GameBoyColors.LightGreen,
+        onSecondary = GameBoyColors.DarkestGreen,
+        background = GameBoyColors.DarkestGreen,
+        onBackground = GameBoyColors.LightGreen,
+        surface = GameBoyColors.DarkGreen,
+        onSurface = GameBoyColors.LightGreen,
+        surfaceVariant = GameBoyColors.DPadGray,
+        outline = GameBoyColors.DarkestGreen,
+    )
diff --git a/composeApp/src/commonMain/kotlin/coffee/liz/abstractionengine/game/AbstractionEngine.kt b/composeApp/src/commonMain/kotlin/coffee/liz/abstractionengine/game/AbstractionEngine.kt
index 18349c3..1f79dfc 100644
--- a/composeApp/src/commonMain/kotlin/coffee/liz/abstractionengine/game/AbstractionEngine.kt
+++ b/composeApp/src/commonMain/kotlin/coffee/liz/abstractionengine/game/AbstractionEngine.kt
@@ -1,5 +1,3 @@
 package coffee.liz.abstractionengine.game
 
-class AbstractionEngine {
-
-}
\ No newline at end of file
+class AbstractionEngine
diff --git a/composeApp/src/commonMain/kotlin/coffee/liz/abstractionengine/game/Game.kt b/composeApp/src/commonMain/kotlin/coffee/liz/abstractionengine/game/Game.kt
index 0c3a007..20ea74c 100644
--- a/composeApp/src/commonMain/kotlin/coffee/liz/abstractionengine/game/Game.kt
+++ b/composeApp/src/commonMain/kotlin/coffee/liz/abstractionengine/game/Game.kt
@@ -14,7 +14,7 @@ import kotlin.time.Duration.Companion.nanoseconds
 @Composable
 fun Game(
     world: World,
-    content: @Composable () -> Unit
+    content: @Composable () -> Unit,
 ) {
     var frameCount by remember { mutableStateOf(0) }
 
@@ -36,7 +36,7 @@ fun GameCanvas(
     world: World,
     renderSystem: Renderer,
     backgroundColor: Color = Color.Transparent,
-    modifier: Modifier = Modifier
+    modifier: Modifier = Modifier,
 ) {
     Canvas(modifier = modifier) {
         // Clear background
diff --git a/composeApp/src/commonMain/kotlin/coffee/liz/abstractionengine/game/ImageCache.kt b/composeApp/src/commonMain/kotlin/coffee/liz/abstractionengine/game/ImageCache.kt
index 0722535..1d64730 100644
--- a/composeApp/src/commonMain/kotlin/coffee/liz/abstractionengine/game/ImageCache.kt
+++ b/composeApp/src/commonMain/kotlin/coffee/liz/abstractionengine/game/ImageCache.kt
@@ -9,13 +9,14 @@ import androidx.compose.ui.graphics.ImageBitmap
 class ImageCache {
     private val images = mutableMapOf<String, ImageBitmap>()
 
-    fun loadImage(path: String, image: ImageBitmap) {
+    fun loadImage(
+        path: String,
+        image: ImageBitmap,
+    ) {
         images[path] = image
     }
 
-    fun getImage(path: String): ImageBitmap? {
-        return images[path]
-    }
+    fun getImage(path: String): ImageBitmap? = images[path]
 
     fun clear() {
         images.clear()
diff --git a/composeApp/src/commonMain/kotlin/coffee/liz/abstractionengine/game/Renderer.kt b/composeApp/src/commonMain/kotlin/coffee/liz/abstractionengine/game/Renderer.kt
index 57511f3..8e28944 100644
--- a/composeApp/src/commonMain/kotlin/coffee/liz/abstractionengine/game/Renderer.kt
+++ b/composeApp/src/commonMain/kotlin/coffee/liz/abstractionengine/game/Renderer.kt
@@ -21,12 +21,14 @@ import kotlin.time.Duration
  * Depends on AnimationSystem to ensure animations are updated before rendering.
  */
 class Renderer(
-    private val imageCache: ImageCache
+    private val imageCache: ImageCache,
 ) : System {
-
     override val dependencies: Set<KClass<out System>> = setOf(AnimationSystem::class)
 
-    override fun update(world: World, duration: Duration) {
+    override fun update(
+        world: World,
+        duration: Duration,
+    ) {
         // Rendering happens in render() method, not here
     }
 
@@ -34,7 +36,10 @@ class Renderer(
      * Renders all entities with sprites to the given DrawScope.
      * This should be called from a Compose Canvas during the draw phase.
      */
-    fun render(world: World, drawScope: DrawScope) {
+    fun render(
+        world: World,
+        drawScope: DrawScope,
+    ) {
         // Query all entities that can be rendered
         world.query(Position::class, SpriteSheet::class, Animator::class).forEach { entity ->
             val position = entity.get(Position::class) ?: return@forEach
@@ -57,7 +62,7 @@ class Renderer(
                     srcOffset = IntOffset(frameRect.x, frameRect.y),
                     srcSize = IntSize(frameRect.width, frameRect.height),
                     dstOffset = IntOffset(position.x.toInt(), position.y.toInt()),
-                    dstSize = IntSize(frameRect.width, frameRect.height)
+                    dstSize = IntSize(frameRect.width, frameRect.height),
                 )
             }
         }
diff --git a/composeApp/src/commonMain/kotlin/coffee/liz/ecs/DAGWorld.kt b/composeApp/src/commonMain/kotlin/coffee/liz/ecs/DAGWorld.kt
index 77cb30b..c6303d3 100644
--- a/composeApp/src/commonMain/kotlin/coffee/liz/ecs/DAGWorld.kt
+++ b/composeApp/src/commonMain/kotlin/coffee/liz/ecs/DAGWorld.kt
@@ -7,19 +7,20 @@ import kotlin.reflect.KClass
 import kotlin.time.Duration
 
 /**
- * [World] that updates in [System.dependencies] DAG order.
+ * [World] that updates in [System.dependencies] topological order.
  */
 @OptIn(ExperimentalAtomicApi::class)
-class DAGWorld<Outside>(systems: List<System<Outside>>) : World<Outside> {
+class DAGWorld<Outside>(
+    systems: List<System<Outside>>,
+) : World<Outside> {
     private val entities = mutableSetOf<Entity>()
     private val componentCache = mutableMapOf<KClass<out Component>, MutableSet<Entity>>()
     private val systemExecutionOrder: List<System<Outside>> = buildExecutionOrder(systems)
     private val nextEntityId = AtomicInt(0)
 
-    override fun createEntity(): Entity {
-        return Entity(nextEntityId.incrementAndFetch())
+    override fun createEntity(): Entity =
+        Entity(nextEntityId.incrementAndFetch())
             .also { entities.add(it) }
-    }
 
     override fun removeEntity(entity: Entity) {
         entity.componentTypes().forEach { componentType ->
@@ -31,17 +32,18 @@ class DAGWorld<Outside>(systems: List<System<Outside>>) : World<Outside> {
     override fun query(vararg componentTypes: KClass<out Component>): Set<Entity> {
         if (componentTypes.isEmpty()) return entities.toSet()
 
-        // Start with entities that have the first component type
         val firstType = componentTypes[0]
         val candidates = componentCache[firstType] ?: return emptySet()
 
-        // Filter to entities that have all component types
-        return candidates.filter { entity ->
-            componentTypes.all { entity.has(it) }
-        }.toSet()
+        return candidates
+            .filter { entity -> componentTypes.all { entity.has(it) } }
+            .toSet()
     }
 
-    override fun update(state: Outside, deltaTime: Duration) {
+    override fun update(
+        state: Outside,
+        deltaTime: Duration,
+    ) {
         refreshComponentCache()
         systemExecutionOrder.forEach { system ->
             system.update(this, state, deltaTime)
@@ -57,9 +59,6 @@ class DAGWorld<Outside>(systems: List<System<Outside>>) : World<Outside> {
         }
     }
 
-    /**
-     * Build a topologically sorted execution order from system dependencies.
-     */
     private fun buildExecutionOrder(systems: List<System<Outside>>): List<System<Outside>> {
         if (systems.isEmpty()) return emptyList()
 
@@ -88,7 +87,6 @@ class DAGWorld<Outside>(systems: List<System<Outside>>) : World<Outside> {
         val queue = ArrayDeque<KClass<out System<Outside>>>()
         val result = mutableListOf<System<Outside>>()
 
-        // Add all systems with no dependencies to queue
         queue.addAll(inDegree.filterValues { it == 0 }.keys)
 
         while (queue.isNotEmpty()) {
@@ -104,7 +102,6 @@ class DAGWorld<Outside>(systems: List<System<Outside>>) : World<Outside> {
             }
         }
 
-        // Check for cycles
         if (result.size != systems.size) {
             error("Circular dependency detected in systems")
         }
diff --git a/composeApp/src/commonMain/kotlin/coffee/liz/ecs/Entity.kt b/composeApp/src/commonMain/kotlin/coffee/liz/ecs/Entity.kt
index fcc39fb..0ee604c 100644
--- a/composeApp/src/commonMain/kotlin/coffee/liz/ecs/Entity.kt
+++ b/composeApp/src/commonMain/kotlin/coffee/liz/ecs/Entity.kt
@@ -5,7 +5,9 @@ import kotlin.reflect.KClass
 /**
  * An entity is a unique identifier with a collection of components.
  */
-class Entity(val id: Int) {
+class Entity(
+    val id: Int,
+) {
     private val components = mutableMapOf<KClass<out Component>, Component>()
 
     /**
@@ -28,29 +30,25 @@ class Entity(val id: Int) {
      * Get [Component]
      */
     @Suppress("UNCHECKED_CAST")
-    fun <T : Component> get(type: KClass<T>): T {
-        return components[type] as T
-    }
+    fun <T : Component> get(type: KClass<T>): T = components[type] as T
 
     /**
      * Has [Component]
      */
-    fun <T : Component> has(type: KClass<T>): Boolean {
-        return components.containsKey(type)
-    }
+    fun <T : Component> has(type: KClass<T>): Boolean = components.containsKey(type)
 
     /**
      * [Component]s of this entity.
      */
-    fun componentTypes(): Set<KClass<out Component>> {
-        return components.keys.toSet()
-    }
+    fun componentTypes(): Set<KClass<out Component>> = components.keys.toSet()
 
     override fun equals(other: Any?): Boolean {
         if (this === other) return true
         if (other !is Entity) return false
         return id == other.id
     }
+
     override fun hashCode(): Int = id
+
     override fun toString(): String = "Entity($id)"
 }
diff --git a/composeApp/src/commonMain/kotlin/coffee/liz/ecs/GameLoop.kt b/composeApp/src/commonMain/kotlin/coffee/liz/ecs/GameLoop.kt
index 713586d..638a316 100644
--- a/composeApp/src/commonMain/kotlin/coffee/liz/ecs/GameLoop.kt
+++ b/composeApp/src/commonMain/kotlin/coffee/liz/ecs/GameLoop.kt
@@ -12,6 +12,7 @@ abstract class GameLoop<Outside> {
     }
 
     abstract fun render(world: World<Outside>)
+
     abstract fun outsideState(): Outside
 
     fun loop(now: Long) {
@@ -22,4 +23,4 @@ abstract class GameLoop<Outside> {
         render(world)
         lastUpdate = now
     }
-}
\ No newline at end of file
+}
diff --git a/composeApp/src/commonMain/kotlin/coffee/liz/ecs/System.kt b/composeApp/src/commonMain/kotlin/coffee/liz/ecs/System.kt
index e763c9c..0032925 100644
--- a/composeApp/src/commonMain/kotlin/coffee/liz/ecs/System.kt
+++ b/composeApp/src/commonMain/kotlin/coffee/liz/ecs/System.kt
@@ -19,5 +19,9 @@ interface System<Outside> {
      * @param world [World]
      * @param deltaTime [Duration] since last update
      */
-    fun update(world: World<Outside>, state: Outside, deltaTime: Duration)
+    fun update(
+        world: World<Outside>,
+        state: Outside,
+        deltaTime: Duration,
+    )
 }
diff --git a/composeApp/src/commonMain/kotlin/coffee/liz/ecs/TickedSystem.kt b/composeApp/src/commonMain/kotlin/coffee/liz/ecs/TickedSystem.kt
index f512ca7..fa4566d 100644
--- a/composeApp/src/commonMain/kotlin/coffee/liz/ecs/TickedSystem.kt
+++ b/composeApp/src/commonMain/kotlin/coffee/liz/ecs/TickedSystem.kt
@@ -4,14 +4,27 @@ import kotlin.time.Duration
 
 abstract class TickedSystem<Outside>(
     protected val tickRate: Duration,
-    private var lastTick: Duration = Duration.ZERO
-): System<Outside> {
-    abstract fun update(world: World<Outside>, state: Outside, ticks: Int)
+    private var lastTick: Duration = Duration.ZERO,
+) : System<Outside> {
+    abstract fun update(
+        world: World<Outside>,
+        state: Outside,
+        ticks: Int,
+    )
 
-    override fun update(world: World<Outside>, state: Outside, deltaTime: Duration) {
+    override fun update(
+        world: World<Outside>,
+        state: Outside,
+        deltaTime: Duration,
+    ) {
         val ticks = ((deltaTime - lastTick) / tickRate).toInt()
-        lastTick = if (ticks == 0) (lastTick + deltaTime).also {
-            update(world, state, ticks)
-        } else Duration.ZERO
+        lastTick =
+            if (ticks == 0) {
+                (lastTick + deltaTime).also {
+                    update(world, state, ticks)
+                }
+            } else {
+                Duration.ZERO
+            }
     }
-}
\ No newline at end of file
+}
diff --git a/composeApp/src/commonMain/kotlin/coffee/liz/ecs/Vec.kt b/composeApp/src/commonMain/kotlin/coffee/liz/ecs/Vec.kt
index 0b771e4..8047f3e 100644
--- a/composeApp/src/commonMain/kotlin/coffee/liz/ecs/Vec.kt
+++ b/composeApp/src/commonMain/kotlin/coffee/liz/ecs/Vec.kt
@@ -1,19 +1,42 @@
 package coffee.liz.ecs
 
+import kotlin.math.sqrt
+
 /**
  * Rectangle in cartesian space.
  */
 data class Rect(
     val topLeft: Vec2,
-    val dimensions: Vec2
+    val dimensions: Vec2,
 ) {
+    fun contains(point: Vec2): Boolean =
+        point.x >= topLeft.x &&
+            point.x <= topLeft.x + dimensions.x &&
+            point.y >= topLeft.y &&
+            point.y <= topLeft.y + dimensions.y
+
     fun overlaps(other: Rect): Boolean {
-        val xOverlap = topLeft.x <= other.topLeft.x + other.dimensions.x &&
+        val xOverlap =
+            topLeft.x <= other.topLeft.x + other.dimensions.x &&
                 topLeft.x + dimensions.x >= other.topLeft.x
-        val yOverlap = topLeft.y <= other.topLeft.y + other.dimensions.y &&
+        val yOverlap =
+            topLeft.y <= other.topLeft.y + other.dimensions.y &&
                 topLeft.y + dimensions.y >= other.topLeft.y
         return xOverlap && yOverlap
     }
+
+    fun split(newDimensions: Vec2): List<List<Rect>> = List(
+        (dimensions.y / newDimensions.y).toInt()
+    ) { y ->
+        List(
+            (dimensions.x / newDimensions.x).toInt()
+        ) { x ->
+            Rect(
+                topLeft = topLeft.plus(Vec2(x * newDimensions.x, y * newDimensions.y)),
+                dimensions = newDimensions,
+            )
+        }
+    }
 }
 
 /**
@@ -21,13 +44,29 @@ data class Rect(
  */
 data class Vec2(
     val x: Float,
-    val y: Float
+    val y: Float,
 ) {
     fun plus(other: Vec2): Vec2 = Vec2(x + other.x, y + other.y)
+
     fun minus(other: Vec2): Vec2 = Vec2(x - other.x, y - other.y)
-    fun times(scalar: Float): Vec2 = Vec2(x * scalar, y * scalar)
-    fun div(scalar: Float): Vec2 = Vec2(x / scalar, y / scalar)
+
+    fun scale(x: Float, y: Float): Vec2 = Vec2(this.x * x, this.y * y)
+
     fun distanceTo(other: Vec2): Float = (this.minus(other)).length()
-    fun normalize(): Vec2 = this.div(length())
-    fun length(): Float = kotlin.math.sqrt(x * x + y * y)
-}
\ No newline at end of file
+
+    fun normalize(): Vec2 = this.scale(1 / length(), 1 / length())
+
+    fun length(): Float = sqrt(x * x + y * y)
+}
+
+/**
+ * Cardinal directions.
+ */
+enum class CardinalDirection(
+    val direction: Vec2,
+) {
+    NORTH(Vec2(0f, -1f)),
+    SOUTH(Vec2(0f, 1f)),
+    WEST(Vec2(-1f, 0f)),
+    EAST(Vec2(1f, 0f)),
+}
diff --git a/composeApp/src/commonMain/kotlin/coffee/liz/ecs/World.kt b/composeApp/src/commonMain/kotlin/coffee/liz/ecs/World.kt
index 452517f..f49e79f 100644
--- a/composeApp/src/commonMain/kotlin/coffee/liz/ecs/World.kt
+++ b/composeApp/src/commonMain/kotlin/coffee/liz/ecs/World.kt
@@ -26,5 +26,8 @@ interface World<Outside> {
     /**
      * Integrate the [World].
      */
-    fun update(state: Outside, deltaTime: Duration)
+    fun update(
+        state: Outside,
+        deltaTime: Duration,
+    )
 }
diff --git a/composeApp/src/commonMain/kotlin/coffee/liz/ecs/animation/AnimationComponents.kt b/composeApp/src/commonMain/kotlin/coffee/liz/ecs/animation/AnimationComponents.kt
index cb3708f..5a846fb 100644
--- a/composeApp/src/commonMain/kotlin/coffee/liz/ecs/animation/AnimationComponents.kt
+++ b/composeApp/src/commonMain/kotlin/coffee/liz/ecs/animation/AnimationComponents.kt
@@ -10,24 +10,29 @@ import kotlin.jvm.JvmInline
 enum class LoopMode {
     /** Play once **/
     ONCE,
+
     /** Repeat **/
     LOOP,
+
     /** Play forward, then backward, repeat **/
-    PING_PONG
+    PING_PONG,
 }
 
 /**
  * Name of an animation clip.
  */
 @JvmInline
-value class AnimationName(val value: String)
+value class AnimationName(
+    val value: String,
+)
 
 /**
  * Name of a frame.
  */
 @JvmInline
-value class FrameName(val value: String)
-
+value class FrameName(
+    val value: String,
+)
 
 /**
  * Animation Clip details.
@@ -35,7 +40,7 @@ value class FrameName(val value: String)
 data class AnimationClip(
     val frameNames: List<FrameName>,
     val frameTicks: Int,
-    val loopMode: LoopMode = LoopMode.LOOP
+    val loopMode: LoopMode = LoopMode.LOOP,
 )
 
 /**
@@ -43,15 +48,19 @@ data class AnimationClip(
  */
 data class SpriteSheet(
     val imagePath: String,
-    val frames: Map<FrameName, Rect>
+    val frames: Map<FrameName, Rect>,
 ) : Component
 
-enum class AnimationDirection(val step: Int) {
+enum class AnimationDirection(
+    val step: Int,
+) {
     /** Play in forward direction **/
     FORWARD(step = 1),
+
     /** Play in reverse direction **/
-    BACKWARD(step = -1);
+    BACKWARD(step = -1),
 }
+
 /**
  * Current state of an animation.
  */
@@ -61,13 +70,17 @@ data class Animator(
     var frameIndex: Int = 0,
     var elapsedTicks: Int = 0,
     var playing: Boolean = true,
-    var direction: AnimationDirection = AnimationDirection.FORWARD
+    var direction: AnimationDirection = AnimationDirection.FORWARD,
 ) : Component {
     /**
      * Play a specific animation clip by name.
      */
-    fun play(clipName: AnimationName, restart: Boolean = true) {
-        clipName.takeIf { currentClip != it || restart }
+    fun play(
+        clipName: AnimationName,
+        restart: Boolean = true,
+    ) {
+        clipName
+            .takeIf { currentClip != it || restart }
             .run {
                 currentClip = clipName
                 reset()
diff --git a/composeApp/src/commonMain/kotlin/coffee/liz/ecs/animation/AnimationSystem.kt b/composeApp/src/commonMain/kotlin/coffee/liz/ecs/animation/AnimationSystem.kt
index 7968250..db64ac6 100644
--- a/composeApp/src/commonMain/kotlin/coffee/liz/ecs/animation/AnimationSystem.kt
+++ b/composeApp/src/commonMain/kotlin/coffee/liz/ecs/animation/AnimationSystem.kt
@@ -7,8 +7,13 @@ import kotlin.time.Duration
 /**
  * Updates animation playback state for all [Animator] components in [World].
  */
-class AnimationSystem(animationTickRate: Duration) : TickedSystem(animationTickRate) {
-    override fun update(world: World, ticks: Int) {
+class AnimationSystem(
+    animationTickRate: Duration,
+) : TickedSystem(animationTickRate) {
+    override fun update(
+        world: World,
+        ticks: Int,
+    ) {
         world.query(Animator::class).forEach { entity ->
             val animator = entity.get(Animator::class)
             if (!animator.playing) return@forEach
@@ -30,7 +35,10 @@ class AnimationSystem(animationTickRate: Duration) : TickedSystem(animationTickR
     /**
      * Advances the animation to the next frame based on loop mode.
      */
-    private fun advanceFrame(animator: Animator, clip: AnimationClip) {
+    private fun advanceFrame(
+        animator: Animator,
+        clip: AnimationClip,
+    ) {
         animator.frameIndex += animator.direction.step
 
         when (clip.loopMode) {
diff --git a/composeApp/src/commonMain/kotlin/coffee/liz/ecs/grid/Components.kt b/composeApp/src/commonMain/kotlin/coffee/liz/ecs/grid/Components.kt
new file mode 100644
index 0000000..b559aa7
--- /dev/null
+++ b/composeApp/src/commonMain/kotlin/coffee/liz/ecs/grid/Components.kt
@@ -0,0 +1,8 @@
+package coffee.liz.ecs.grid
+
+import coffee.liz.ecs.Component
+import coffee.liz.ecs.Vec2
+
+data class GridPosition(
+    val position: Vec2,
+) : Component
diff --git a/composeApp/src/commonMain/kotlin/coffee/liz/ecs/grid/Grid.kt b/composeApp/src/commonMain/kotlin/coffee/liz/ecs/grid/Grid.kt
new file mode 100644
index 0000000..f4eef43
--- /dev/null
+++ b/composeApp/src/commonMain/kotlin/coffee/liz/ecs/grid/Grid.kt
@@ -0,0 +1,54 @@
+package coffee.liz.ecs.grid
+
+import coffee.liz.ecs.Entity
+import coffee.liz.ecs.Rect
+import coffee.liz.ecs.Vec2
+
+data class Grid(
+    val dimensions: Vec2,
+    val grid: MutableList<MutableList<MutableSet<Entity>>> =
+        MutableList(dimensions.y.toInt()) {
+            MutableList(dimensions.x.toInt()) { mutableSetOf() }
+        },
+) {
+    fun at(position: Vec2): MutableSet<Entity> {
+        if (!Rect(Vec2(0f, 0f), dimensions).contains(position)) {
+            throw IllegalArgumentException()
+        }
+        return grid[position.y.toInt()][position.x.toInt()]
+    }
+
+    fun add(entity: Entity) {
+        if (!entity.has(GridPosition::class)) {
+            throw IllegalArgumentException("Entity must have a GridPosition component to be added to the grid")
+        }
+        val position = entity.get(GridPosition::class).position
+        at(position).add(entity)
+    }
+
+    fun reassignCellEntities(entities: Collection<Entity>) {
+        val remainingEntities = entities.filter { it.has(GridPosition::class) }.toMutableSet()
+        for (y in grid.indices) {
+            for (x in grid[y].indices) {
+                val vec = Vec2(x.toFloat(), y.toFloat())
+                val entities = at(vec)
+                entities.retainAll(
+                    entities
+                        .filter {
+                            if (!it.has(GridPosition::class)) {
+                                return@filter false
+                            }
+                            val currentPosition = it.get(GridPosition::class).position
+                            (vec == currentPosition).also { stillInCell ->
+                                if (!stillInCell) {
+                                    at(currentPosition).add(it)
+                                }
+                            }
+                        }.toSet(),
+                )
+                remainingEntities.removeAll(entities)
+            }
+        }
+        remainingEntities.forEach { add(it) }
+    }
+}
diff --git a/composeApp/src/commonMain/kotlin/coffee/liz/ecs/grid/GridSystem.kt b/composeApp/src/commonMain/kotlin/coffee/liz/ecs/grid/GridSystem.kt
new file mode 100644
index 0000000..50d784d
--- /dev/null
+++ b/composeApp/src/commonMain/kotlin/coffee/liz/ecs/grid/GridSystem.kt
@@ -0,0 +1,17 @@
+package coffee.liz.ecs.grid
+
+import coffee.liz.ecs.System
+import coffee.liz.ecs.World
+import coffee.liz.ecs.physics.PhysicsSystem
+import kotlin.reflect.KClass
+import kotlin.time.Duration
+
+class GridSystem<Outside>(
+    private val grid: Grid
+): System<Outside> {
+    override val dependencies = setOf(PhysicsSystem::class)
+
+    override fun update(world: World<Outside>, state: Outside, deltaTime: Duration) {
+
+    }
+}
\ No newline at end of file
diff --git a/composeApp/src/commonMain/kotlin/coffee/liz/ecs/input/Controlled.kt b/composeApp/src/commonMain/kotlin/coffee/liz/ecs/input/Controlled.kt
index 8c54bfe..ddda2d6 100644
--- a/composeApp/src/commonMain/kotlin/coffee/liz/ecs/input/Controlled.kt
+++ b/composeApp/src/commonMain/kotlin/coffee/liz/ecs/input/Controlled.kt
@@ -2,6 +2,9 @@ package coffee.liz.ecs.input
 
 import coffee.liz.ecs.Component
 
-interface Controlled: Component {
+/**
+ * Processes the [InputState].
+ */
+fun interface Controlled : Component {
     fun handleInput(inputState: InputState)
-}
\ No newline at end of file
+}
diff --git a/composeApp/src/commonMain/kotlin/coffee/liz/ecs/input/InputProvider.kt b/composeApp/src/commonMain/kotlin/coffee/liz/ecs/input/InputProvider.kt
deleted file mode 100644
index 5735665..0000000
--- a/composeApp/src/commonMain/kotlin/coffee/liz/ecs/input/InputProvider.kt
+++ /dev/null
@@ -1,7 +0,0 @@
-package coffee.liz.ecs.input
-
-data class InputState(val activeInputs: Set<String>)
-
-interface InputProvider {
-    val inputState: InputState
-}
\ No newline at end of file
diff --git a/composeApp/src/commonMain/kotlin/coffee/liz/ecs/input/InputSystem.kt b/composeApp/src/commonMain/kotlin/coffee/liz/ecs/input/InputSystem.kt
index 6dc8256..f80c1ad 100644
--- a/composeApp/src/commonMain/kotlin/coffee/liz/ecs/input/InputSystem.kt
+++ b/composeApp/src/commonMain/kotlin/coffee/liz/ecs/input/InputSystem.kt
@@ -1,16 +1,27 @@
 package coffee.liz.ecs.input
 
-import coffee.liz.ecs.System
 import coffee.liz.ecs.TickedSystem
 import coffee.liz.ecs.World
 import kotlin.time.Duration
 
+data class InputState(
+    val activeInputs: Set<String>,
+)
+
+interface InputProvider {
+    val inputState: InputState
+}
+
 class InputSystem<Outside : InputProvider>(
-    pollRate: Duration
+    pollRate: Duration,
 ) : TickedSystem<Outside>(pollRate) {
     private val activeInputs = mutableMapOf<String, Int>()
 
-    override fun update(world: World<Outside>, state: Outside, ticks: Int) {
+    override fun update(
+        world: World<Outside>,
+        state: Outside,
+        ticks: Int,
+    ) {
         state.inputState.activeInputs.forEach {
             activeInputs[it] = (activeInputs[it] ?: 0) + ticks
         }
@@ -20,4 +31,4 @@ class InputSystem<Outside : InputProvider>(
             it.get(Controlled::class).handleInput(activeInputs.toMap())
         }
     }
-}
\ No newline at end of file
+}
diff --git a/composeApp/src/commonMain/kotlin/coffee/liz/ecs/physics/CollisionTick.kt b/composeApp/src/commonMain/kotlin/coffee/liz/ecs/physics/CollisionTick.kt
index bc34ac1..d7d4dda 100644
--- a/composeApp/src/commonMain/kotlin/coffee/liz/ecs/physics/CollisionTick.kt
+++ b/composeApp/src/commonMain/kotlin/coffee/liz/ecs/physics/CollisionTick.kt
@@ -1,28 +1,89 @@
 package coffee.liz.ecs.physics
 
+import coffee.liz.ecs.Entity
 import coffee.liz.ecs.Rect
+import coffee.liz.ecs.Vec2
 import coffee.liz.ecs.World
+import kotlin.math.min
 
 internal class CollisionTick<Outside>(
-    private val collisionResolver: CollisionResolver<Outside>
-) {
-    fun runTick(world: World<Outside>) {
-        // Eh, fast enough for now. Don't need to do any fancy collision detection. There's always later to improve if
-        // it's that bad.
-        world.query(Collidable::class, Position::class).forEach { a ->
-            world.query(Collidable::class, Position::class).forEach { b ->
-                val aHitBoxes = a.get(Position::class).let { pos -> a.get(Collidable::class).hitboxes.map {
-                    Rect(pos.vec2, it.dimensions)
-                }}
-                val bHitBoxes = b.get(Position::class).let { pos -> b.get(Collidable::class).hitboxes.map {
-                    Rect(pos.vec2, it.dimensions)
-                }}
-
-                val collisionDetected = aHitBoxes.any { a -> bHitBoxes.any { b -> a.overlaps(b) } }
-                if (collisionDetected) {
-                    collisionResolver.resolveCollision(world, a, b)
+    private val collisionResolver: CollisionResolver<Outside>,
+    private val subgridDimension: Vec2 = Vec2(50f, 50f)
+) : PhysicsTick<Outside> {
+    override fun runTick(world: World<Outside>) {
+        val subgridEntities = world.collidingBounds().split(subgridDimension).map { subgrid ->
+            subgrid.map { cell ->
+                world.query(Colliding::class, Position::class).filter { entity ->
+                    entity.positionalHitBoxes().any { cell.overlaps(it) }
                 }
             }
         }
+
+        for (y in subgridEntities.indices) {
+            for (x in subgridEntities[y].indices) {
+                resolveCollisions(world, subgridEntities[y][x])
+            }
+        }
+    }
+
+    private fun resolveCollisions(world: World<Outside>, potentialCollisions: Collection<Entity>) {
+        potentialCollisions.forEach { from ->
+            potentialCollisions.filter { to ->
+                val fromColliding = from.get(Colliding::class)
+                val toColliding = to.get(Colliding::class)
+
+                val canCollide = fromColliding.group.collideInto.contains(toColliding.group.collisionGroup)
+                if (!canCollide) {
+                    return@filter false
+                }
+
+                val fromHitBoxes = from.positionalHitBoxes()
+                val toHitBoxes = to.positionalHitBoxes()
+
+                fromHitBoxes.any { a -> toHitBoxes.any { b -> a.overlaps(b) } }
+            }.forEach { collision -> propagateCollision(world, from, collision)}
+        }
+    }
+
+    private fun propagateCollision(world: World<Outside>, from: Entity, to: Entity) {
+        collisionResolver.resolveCollision(world, from, to)
+
+        val fromVelocity = from.takeIf { it.has(Velocity::class) }?.get(Velocity::class) ?: return
+
+        val toVelocity = to.takeIf { it.has(Velocity::class) }?.get(Velocity::class)
+            ?: Velocity(Vec2(0f, 0f))
+        val toColliding = to.get(Colliding::class)
+
+        if (toColliding.propagateMovement) {
+            to.add(Velocity(toVelocity.velocity.plus(fromVelocity.velocity)))
+        }
+    }
+}
+
+private fun Entity.positionalHitBoxes(): Collection<Rect> {
+    val pos = get(Position::class)
+    val hitboxes = get(Colliding::class).hitboxes
+    return hitboxes.map { Rect(pos.position.plus(it.topLeft), it.dimensions) }
+}
+
+fun World<*>.collidingBounds(): Rect {
+    return query(Position::class, Colliding::class).map { entity ->
+        val topLeft = entity.get(Position::class).position
+        val dimensions = entity.get(Colliding::class).hitboxes.merge().dimensions
+        Rect(topLeft, dimensions)
+    }.merge()
+}
+
+fun Collection<Rect>.merge(): Rect {
+    return reduce { a, b ->
+        val newTopLeft = Vec2(
+            min(a.topLeft.x, b.topLeft.x),
+            min(a.topLeft.y, b.topLeft.y)
+        )
+        val dimensions = Vec2(
+            maxOf(a.topLeft.x + a.dimensions.x, b.topLeft.x + b.dimensions.x) - newTopLeft.x,
+            maxOf(a.topLeft.y + a.dimensions.y, b.topLeft.y + b.dimensions.y) - newTopLeft.y
+        )
+        Rect(newTopLeft, dimensions)
     }
 }
\ No newline at end of file
diff --git a/composeApp/src/commonMain/kotlin/coffee/liz/ecs/physics/Components.kt b/composeApp/src/commonMain/kotlin/coffee/liz/ecs/physics/Components.kt
index ffae10b..0b4818d 100644
--- a/composeApp/src/commonMain/kotlin/coffee/liz/ecs/physics/Components.kt
+++ b/composeApp/src/commonMain/kotlin/coffee/liz/ecs/physics/Components.kt
@@ -4,12 +4,29 @@ import coffee.liz.ecs.Component
 import coffee.liz.ecs.Rect
 import coffee.liz.ecs.Vec2
 
-data class Position(val vec2: Vec2): Component
-data class Velocity(val vec2: Vec2) : Component
-data class Acceleration(val vec2: Vec2) : Component
+data class Position(
+    val position: Vec2,
+) : Component
+
+data class Velocity(
+    val velocity: Vec2,
+) : Component
+
+data class Acceleration(
+    val acceleration: Vec2,
+) : Component
+
+interface CollisionGroup<TGroup> {
+    val collisionGroup: TGroup
+    val collideInto: Set<TGroup>
+}
 
 /**
  * @param hitboxes a collection of hitboxes to check collisions against relative to [Rect.topLeft] as the top left of
  * [Position].
  */
-data class Collidable(val hitboxes: Collection<Rect>): Component
+data class Colliding<T : Enum<*>>(
+    val hitboxes: Collection<Rect>,
+    val group: CollisionGroup<T>,
+    val propagateMovement: Boolean = true,
+) : Component
diff --git a/composeApp/src/commonMain/kotlin/coffee/liz/ecs/physics/IntegrationTick.kt b/composeApp/src/commonMain/kotlin/coffee/liz/ecs/physics/IntegrationTick.kt
index 03af835..c3484ae 100644
--- a/composeApp/src/commonMain/kotlin/coffee/liz/ecs/physics/IntegrationTick.kt
+++ b/composeApp/src/commonMain/kotlin/coffee/liz/ecs/physics/IntegrationTick.kt
@@ -2,15 +2,15 @@ package coffee.liz.ecs.physics
 
 import coffee.liz.ecs.World
 
-internal class IntegrationTick<Outside> {
-    fun runTick(world: World<Outside>) {
+internal class IntegrationTick<Outside> : PhysicsTick<Outside> {
+    override fun runTick(world: World<Outside>) {
         world.query(Velocity::class, Acceleration::class).forEach { entity ->
             val velocity = entity.get(Velocity::class)
             val acceleration = entity.get(Acceleration::class)
             entity.add(
                 Velocity(
-                    vec2 = velocity.vec2.plus(acceleration.vec2)
-                )
+                    vec2 = velocity.vec2.plus(acceleration.vec2),
+                ),
             )
         }
 
@@ -19,9 +19,9 @@ internal class IntegrationTick<Outside> {
             val velocity = entity.get(Acceleration::class)
             entity.add(
                 Position(
-                    vec2 = position.vec2.plus(velocity.vec2)
-                )
+                    vec2 = position.vec2.plus(velocity.vec2),
+                ),
             )
         }
     }
-}
\ No newline at end of file
+}
diff --git a/composeApp/src/commonMain/kotlin/coffee/liz/ecs/physics/PhysicsSystem.kt b/composeApp/src/commonMain/kotlin/coffee/liz/ecs/physics/PhysicsSystem.kt
index 703e614..ec9c71b 100644
--- a/composeApp/src/commonMain/kotlin/coffee/liz/ecs/physics/PhysicsSystem.kt
+++ b/composeApp/src/commonMain/kotlin/coffee/liz/ecs/physics/PhysicsSystem.kt
@@ -6,21 +6,34 @@ import coffee.liz.ecs.World
 import kotlin.time.Duration
 
 internal fun interface CollisionResolver<Outside> {
-    fun resolveCollision(world: World<Outside>, a: Entity, b: Entity)
+    fun resolveCollision(
+        world: World<Outside>,
+        a: Entity,
+        b: Entity,
+    )
 }
 
 abstract class PhysicsSystem<Outside>(
-    physicsTickRate: Duration
-): TickedSystem<Outside>(physicsTickRate), CollisionResolver<Outside> {
+    physicsTickRate: Duration,
+) : TickedSystem<Outside>(physicsTickRate),
+    CollisionResolver<Outside> {
     private val integrationTick = IntegrationTick<Outside>()
     private val collisionTick = CollisionTick(this)
 
-    abstract override fun resolveCollision(world: World<Outside>, a: Entity, b: Entity)
+    abstract override fun resolveCollision(
+        world: World<Outside>,
+        a: Entity,
+        b: Entity,
+    )
 
-    override fun update(world: World<Outside>, state: Outside, ticks: Int) {
+    override fun update(
+        world: World<Outside>,
+        state: Outside,
+        ticks: Int,
+    ) {
         for (tick in 1..ticks) {
             collisionTick.runTick(world)
             integrationTick.runTick(world)
         }
     }
-}
\ No newline at end of file
+}
diff --git a/composeApp/src/commonMain/kotlin/coffee/liz/ecs/physics/PhysicsTick.kt b/composeApp/src/commonMain/kotlin/coffee/liz/ecs/physics/PhysicsTick.kt
new file mode 100644
index 0000000..79a95c0
--- /dev/null
+++ b/composeApp/src/commonMain/kotlin/coffee/liz/ecs/physics/PhysicsTick.kt
@@ -0,0 +1,10 @@
+package coffee.liz.ecs.physics
+
+import coffee.liz.ecs.World
+
+/**
+ * Runs a physics tick in [World].
+ */
+fun interface PhysicsTick<Outside> {
+    fun runTick(world: World<Outside>)
+}
-- 
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