From e0857552bb6769774c3f1ae37b051d37e11a664b Mon Sep 17 00:00:00 2001
From: Ahmet Inan <inan@aicodix.de>
Date: Sat, 25 Mar 2023 09:56:27 +0100
Subject: [PATCH] added reversible Fisher-Yates shuffle

---
 permute.hh | 39 +++++++++++++++++++++++++++++++++++++++
 1 file changed, 39 insertions(+)
 create mode 100644 permute.hh

diff --git a/permute.hh b/permute.hh
new file mode 100644
index 0000000..988e48e
--- /dev/null
+++ b/permute.hh
@@ -0,0 +1,39 @@
+/*
+Reversible permutations
+
+Copyright 2023 Ahmet Inan <inan@aicodix.de>
+*/
+
+#pragma once
+
+#include "xorshift.hh"
+
+namespace CODE {
+
+template <int SIZE>
+class ReversibleFisherYatesShuffle
+{
+	int seq[SIZE-1];
+public:
+	ReversibleFisherYatesShuffle()
+	{
+		CODE::Xorshift32 prng;
+		for (int i = 0; i < SIZE-1; ++i)
+			seq[i] = i + prng() % (SIZE - i);
+	}
+	template <typename TYPE>
+	void forward(TYPE *array)
+	{
+		for (int i = 0; i < SIZE-1; ++i)
+			std::swap(array[i], array[seq[i]]);
+	}
+	template <typename TYPE>
+	void reverse(TYPE *array)
+	{
+		for (int i = SIZE-2; i >= 0; --i)
+			std::swap(array[i], array[seq[i]]);
+	}
+};
+
+}
+