From d319189d7ac995679e04345468b1ff8fe17b4679 Mon Sep 17 00:00:00 2001
From: Ahmet Inan <inan@aicodix.de>
Date: Fri, 8 Mar 2024 09:53:28 +0100
Subject: [PATCH] added merge sort

---
 sort.hh                       | 25 +++++++++++++++++++++++++
 tests/sort_regression_test.cc | 19 ++++++++++++-------
 2 files changed, 37 insertions(+), 7 deletions(-)

diff --git a/sort.hh b/sort.hh
index 441773e..10b3aac 100644
--- a/sort.hh
+++ b/sort.hh
@@ -34,5 +34,30 @@ static void insertion_sort(INDEX *p, TYPE *a, int n)
 	}
 }
 
+template <typename TYPE, int MAX_N>
+class MergeSort
+{
+	TYPE tmp[MAX_N];
+	void merge(TYPE *a, int n, int left, int right, int end)
+	{
+		if (right > n)
+			right = n;
+		if (end > n)
+			end = n;
+		for (int i = left, j = right, k = left; k < end; ++k)
+			tmp[k] = (i < right && (j >= end || a[i] <= a[j])) ? a[i++] : a[j++];
+	}
+public:
+	void operator()(TYPE *a, int n)
+	{
+		for (int l = 1; l < n; l *= 2) {
+			for (int i = 0; i < n; i += 2*l)
+				merge(a, n, i, i+l, i+2*l);
+			for (int i = 0; i < n; ++i)
+				a[i] = tmp[i];
+		}
+	}
+};
+
 }
 
diff --git a/tests/sort_regression_test.cc b/tests/sort_regression_test.cc
index 38ea168..b2f3c3d 100644
--- a/tests/sort_regression_test.cc
+++ b/tests/sort_regression_test.cc
@@ -13,25 +13,30 @@ Copyright 2024 Ahmet Inan <inan@aicodix.de>
 
 int main()
 {
+	const int MAX_N = 128;
 	std::random_device rd;
 	typedef std::default_random_engine generator;
 	typedef std::uniform_int_distribution<int> distribution;
-	auto rand = std::bind(distribution(1, 32), generator(rd()));
-	int a[32], b[32], c[32], d[32], e[32], f[32];
+	auto rand = std::bind(distribution(1, MAX_N), generator(rd()));
+	int a[MAX_N], b[MAX_N], c[MAX_N], d[MAX_N], e[MAX_N], f[MAX_N], g[MAX_N];
+	CODE::MergeSort<int, MAX_N> merge_sort;
 	for (int loop = 0; loop < 1000000; ++loop) {
 		int size = rand();
 		for (int i = 0; i < size; ++i)
-			a[i] = b[i] = c[i] = d[i] = rand();
+			a[i] = b[i] = c[i] = d[i] = e[i] = rand();
 		std::sort(a, a+size);
 		CODE::insertion_sort(b, size);
 		for (int i = 0; i < size; ++i)
 			assert(a[i] == b[i]);
+		merge_sort(c, size);
 		for (int i = 0; i < size; ++i)
-			e[i] = i;
-		std::stable_sort(e, e+size, [c](int i, int j){ return c[i] < c[j]; });
-		CODE::insertion_sort(f, d, size);
+			assert(a[i] == c[i]);
 		for (int i = 0; i < size; ++i)
-			assert(e[i] == f[i]);
+			f[i] = i;
+		std::stable_sort(f, f+size, [d](int i, int j){ return d[i] < d[j]; });
+		CODE::insertion_sort(g, e, size);
+		for (int i = 0; i < size; ++i)
+			assert(f[i] == g[i]);
 	}
 	std::cerr << "Sorting regression test passed!" << std::endl;
 	return 0;