1 files changed, 54 insertions, 0 deletions
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) }
+    }
+}