we ain't come this far to omit sorting

quick.hh

@@ -0,0 +1,103 @@
+/*
+Quick algorithms for sorting and selecting
+
+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 inline void median(TYPE *A, int a, int b, int c, int d, int e)
+{
+	if (A[c] < A[a])
+		swap(A, a, c);
+	if (A[d] < A[b])
+		swap(A, b, d);
+	if (A[d] < A[c]) {
+		swap(A, c, d);
+		swap(A, a, b);
+	}
+	if (A[e] < A[b])
+		swap(A, b, e);
+	if (A[e] < A[c]) {
+		swap(A, c, e);
+		if (A[c] < A[a])
+			swap(A, a, c);
+	} else if (A[c] < A[b]) {
+		swap(A, b, c);
+	}
+}
+
+template <typename TYPE>
+static void partition(TYPE *a, int &l, int &h)
+{
+	int half = (h - l) / 2;
+	int quarter = (h - l) / 4;
+	int middle = l + half;
+	median(a, l, l + quarter, middle, middle + quarter, h);
+	TYPE pivot = a[middle];
+	for (int i = l; i <= h;)
+		if (a[i] < pivot)
+			swap(a, i++, l++);
+		else if (a[i] > pivot)
+			swap(a, i, h--);
+		else
+			++i;
+}
+
+template <typename TYPE>
+static void sort(TYPE *a, int l, int h)
+{
+	if (l < h) {
+		int lt = l, gt = h;
+		partition(a, lt, gt);
+		sort(a, l, lt - 1);
+		sort(a, gt + 1, h);
+	}
+}
+
+template <typename TYPE>
+static void select(TYPE *a, int l, int h, int k)
+{
+	while (l < h) {
+		int lt = l, gt = h;
+		partition(a, lt, gt);
+		if (k < lt)
+			h = lt - 1;
+		else if (k > gt)
+			l = gt + 1;
+		else
+			break;
+	}
+}
+
+}
+
+template <typename TYPE>
+void quick_sort(TYPE *a, int n)
+{
+	QUICK::sort(a, 0, n-1);
+}
+
+template <typename TYPE>
+TYPE quick_select(TYPE *a, int k, int n)
+{
+	assert(n && k < n);
+	QUICK::select(a, 0, n-1, k);
+	return a[k];
+}
+
+}
+