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

---
 .../kotlin/coffee/liz/ecs/physics/CollisionTick.kt | 95 ++++++++++++++++++----
 1 file changed, 78 insertions(+), 17 deletions(-)

(limited to 'composeApp/src/commonMain/kotlin/coffee/liz/ecs/physics/CollisionTick.kt')

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)
     }
 }
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