From e55c2bc0833a685f7be31aa56f019198ad40da03 Mon Sep 17 00:00:00 2001
From: Ahmet Inan <inan@aicodix.de>
Date: Tue, 28 Mar 2023 11:44:15 +0200
Subject: [PATCH] added a simple reversible block wise shuffle

---
 permute.hh | 42 ++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 42 insertions(+)

diff --git a/permute.hh b/permute.hh
index 283ffbd..27b8906 100644
--- a/permute.hh
+++ b/permute.hh
@@ -35,6 +35,48 @@ public:
 	}
 };
 
+template <int SIZE, int BLOCK>
+class ReversibleBlockShuffle
+{
+	static_assert(SIZE >= 2 * BLOCK, "SIZE not large enough");
+	static_assert(SIZE % BLOCK == 0, "BLOCK does not divide SIZE");
+	static constexpr int BLOCKS = SIZE / BLOCK;
+	int seq[SIZE-1];
+public:
+	ReversibleBlockShuffle()
+	{
+		CODE::Xorshift32 prng;
+		for (int j = 0; j < BLOCKS; ++j) {
+			if (j < BLOCKS-1)
+				seq[BLOCK*j] = j + prng() % (BLOCKS - j);
+			for (int i = 0; i < BLOCK-1; ++i)
+				seq[BLOCK*j+1+i] = i + prng() % (BLOCK - i);
+		}
+	}
+	template <typename TYPE>
+	void forward(TYPE *array)
+	{
+		for (int j = 0; j < BLOCKS; ++j) {
+			if (j < BLOCKS-1)
+				for (int i = 0; i < BLOCK; ++i)
+					std::swap(array[BLOCK*j+i], array[BLOCK*seq[BLOCK*j]+i]);
+			for (int i = 0; i < BLOCK-1; ++i)
+				std::swap(array[BLOCK*j+i], array[BLOCK*j+seq[BLOCK*j+1+i]]);
+		}
+	}
+	template <typename TYPE>
+	void reverse(TYPE *array)
+	{
+		for (int j = BLOCKS-1; j >= 0; --j) {
+			for (int i = BLOCK-2; i >= 0; --i)
+				std::swap(array[BLOCK*j+i], array[BLOCK*j+seq[BLOCK*j+1+i]]);
+			if (j < BLOCKS-1)
+				for (int i = 0; i < BLOCK; ++i)
+					std::swap(array[BLOCK*j+i], array[BLOCK*seq[BLOCK*j]+i]);
+		}
+	}
+};
+
 template <int SIZE, typename TYPE>
 static void BitReversalPermute(TYPE *array)
 {