added quick select algorithm

quick_select.hh

@@ -0,0 +1,53 @@
+/*
+Quick select algorithm
+
+Copyright 2024 Ahmet Inan <inan@aicodix.de>
+*/
+
+#pragma once
+
+namespace DSP {
+
+namespace QUICK {
+
+template <typename TYPE>
+static inline void swap(TYPE *a, int i, int j)
+{
+	TYPE t = a[i];
+	a[i] = a[j];
+	a[j] = t;
+}
+
+template <typename TYPE>
+static int partition(TYPE *a, int l, int h)
+{
+	for (int i = l; i < h; ++i)
+		if (a[i] < a[h])
+			swap(a, i, l++);
+	swap(a, l, h);
+	return l;
+}
+
+template <typename TYPE>
+static TYPE select(TYPE *a, int l, int h, int k)
+{
+	if (l == h)
+		return a[l];
+	int i = partition(a, l, h);
+	if (i == k)
+		return a[i];
+	if (i > k)
+		return select(a, l, i-1, k);
+	return select(a, i+1, h, k);
+}
+
+}
+
+template <typename TYPE>
+TYPE quick_select(TYPE *a, int k, int n)
+{
+	return QUICK::select(a, 0, n-1, k);
+}
+
+}
+