[orx-kdtree] Make orx-kdtree a multiplatform module

orx-kdtree/src/jsMain/kotlin/KDTree.kt

@@ -0,0 +1,71 @@
+package org.openrndr.extra.kdtree
+
+
+actual fun <T> buildKDTree(items: MutableList<T>, dimensions: Int, mapper: (T, Int) -> Double): KDTreeNode<T> {
+    val root = KDTreeNode<T>(dimensions, mapper)
+
+    fun <T> buildTreeTask(
+        node: KDTreeNode<T>,
+        items: MutableList<T>,
+        dimensions: Int,
+        levels: Int,
+        mapper: (T, Int) -> Double
+    ): KDTreeNode<T> {
+
+        if (items.size > 0) {
+            val dimension = levels % dimensions
+            val values = ArrayList<T>()
+            for (item in items) {
+                values.add(item)
+            }
+
+            node.dimension = dimension
+            val median = selectNth(items, items.size / 2) { mapper(it, dimension) }
+
+            val leftItems = mutableListOf<T>()
+            val rightItems = mutableListOf<T>()
+
+            node.median = mapper(median, dimension)
+            node.item = median
+            for (item in items) {
+                if (item === median) {
+                    continue
+                }
+                if (mapper(item, dimension) < node.median) {
+                    leftItems.add(item)
+                } else {
+                    rightItems.add(item)
+                }
+            }
+
+            // validate split
+            if (leftItems.size + rightItems.size + 1 != items.size) {
+                throw IllegalStateException("left: ${leftItems.size}, right: ${rightItems.size}, items: ${items.size}")
+            }
+
+            if (leftItems.size > 0) {
+                node.children[0] = KDTreeNode(dimensions, mapper)
+                node.children[0]?.let {
+                    it.parent = node
+
+
+                    buildTreeTask(it, leftItems, dimensions, levels + 1, mapper)
+
+                }
+            }
+            if (rightItems.size > 0) {
+                node.children[1] = KDTreeNode(dimensions, mapper)
+                node.children[1]?.let {
+                    it.parent = node
+                    buildTreeTask(it, rightItems, dimensions, levels + 1, mapper)
+
+                }
+            }
+        }
+        return node
+    }
+
+
+    buildTreeTask(root, items, dimensions, 0, mapper)
+    return root
+}