From d19993026bfee1c2885c60eba93ce34097b34b8a Mon Sep 17 00:00:00 2001
From: Tammo Behrends <behr.tam@gmail.com>
Date: Tue, 1 Dec 2015 20:31:43 +0100
Subject: [PATCH] rail-fence-cipher: Implement new exercise

---
 config.json                                 |  1 +
 rail-fence-cipher/example.py                | 17 +++++++++++
 rail-fence-cipher/rail_fence_cipher_test.py | 34 +++++++++++++++++++++
 3 files changed, 52 insertions(+)
 create mode 100644 rail-fence-cipher/example.py
 create mode 100644 rail-fence-cipher/rail_fence_cipher_test.py

diff --git a/config.json b/config.json
index 41ff709f69e..f254a3282c7 100644
--- a/config.json
+++ b/config.json
@@ -40,6 +40,7 @@
     "pythagorean-triplet",
     "robot-name",
     "matrix",
+    "rail-fence-cipher",
     "nth-prime",
     "saddle-points",
     "beer-song",
diff --git a/rail-fence-cipher/example.py b/rail-fence-cipher/example.py
new file mode 100644
index 00000000000..29942b74fe5
--- /dev/null
+++ b/rail-fence-cipher/example.py
@@ -0,0 +1,17 @@
+from itertools import cycle, chain
+
+
+def fence_pattern(rails, size):
+    zig_zag = cycle(chain(range(rails), range(rails - 2, 0, -1)))
+    return zip(zig_zag, range(size))
+
+
+def encode(msg, rails):
+    fence = fence_pattern(rails, len(msg))
+    return ''.join(msg[i] for _, i in sorted(fence))
+
+
+def decode(msg, rails):
+    fence = fence_pattern(rails, len(msg))
+    fence_msg = zip(msg, sorted(fence))
+    return ''.join(char for char, _ in sorted(fence_msg, key=lambda item: item[1][1]))
diff --git a/rail-fence-cipher/rail_fence_cipher_test.py b/rail-fence-cipher/rail_fence_cipher_test.py
new file mode 100644
index 00000000000..d46a7ec0628
--- /dev/null
+++ b/rail-fence-cipher/rail_fence_cipher_test.py
@@ -0,0 +1,34 @@
+import unittest
+
+from rail_fence_cipher import encode, decode
+
+
+class RailFenceTests(unittest.TestCase):
+
+    def test_encode_with_two_rails(self):
+        self.assertMultiLineEqual('XXXXXXXXXOOOOOOOOO',
+                                  encode('XOXOXOXOXOXOXOXOXO', 2))
+
+    def test_encode_with_three_rails(self):
+        self.assertMultiLineEqual('WECRLTEERDSOEEFEAOCAIVDEN',
+                                  encode('WEAREDISCOVEREDFLEEATONCE', 3))
+
+    def test_encode_with_middle_stop(self):
+        self.assertMultiLineEqual('ESXIEECSR', encode('EXERCISES', 4))
+
+    def test_decode_with_three_rails(self):
+        self.assertMultiLineEqual('THEDEVILISINTHEDETAILS',
+                                  decode('TEITELHDVLSNHDTISEIIEA', 3))
+
+    def test_decode_with_five_rails(self):
+        self.assertMultiLineEqual('EXERCISMISAWESOME',
+                                  decode('EIEXMSMESAORIWSCE', 5))
+
+    def test_decode_with_six_rails(self):
+        self.assertMultiLineEqual(
+            '112358132134558914423337761098715972584418167651094617711286',
+            decode('133714114238148966225439541018335470986172518171757571896261', 6)
+        )
+
+if __name__ == '__main__':
+    unittest.main()