Initial commit

orx-kdtree/src/main/kotlin/QuickSelect.kt

@@ -0,0 +1,37 @@
+package org.openrndr.extra.kdtree
+
+fun <T> selectNth(items: MutableList<T>, n: Int, mapper: (T)->Double): T {
+    var from = 0
+    var to = items.size - 1
+
+    // if from == to we reached the kth element
+    while (from < to) {
+        var r = from
+        var w = to
+        val mid = mapper(items[(r + w) / 2])
+
+        // stop when the reader and writer meet
+        while (r < w) {
+            if (mapper(items[r]) >= mid) { // put the large values at the end
+                val tmp = items[w]
+                items[w] = items[r]
+                items[r] = tmp
+                w--
+            } else { // the value is smaller than the pivot, skip
+                r++
+            }
+        }
+
+        // if we stepped up (r++) we need to step one down
+        if (mapper(items[r]) > mid)
+            r--
+
+        // the r pointer is on the end of the first k elements
+        if (n <= r) {
+            to = r
+        } else {
+            from = r + 1
+        }
+    }
+    return items[n]
+}