🎉 Exercism Research is now launched. Help Exercism, help science and have some fun at research.exercism.io 🎉
Avatar of dougal

dougal's solution

to Rail Fence Cipher in the Go Track

Published at Jan 18 2021 · 0 comments
Instructions
Test suite
Solution

Implement encoding and decoding for the rail fence cipher.

The Rail Fence cipher is a form of transposition cipher that gets its name from the way in which it's encoded. It was already used by the ancient Greeks.

In the Rail Fence cipher, the message is written downwards on successive "rails" of an imaginary fence, then moving up when we get to the bottom (like a zig-zag). Finally the message is then read off in rows.

For example, using three "rails" and the message "WE ARE DISCOVERED FLEE AT ONCE", the cipherer writes out:

W . . . E . . . C . . . R . . . L . . . T . . . E
. E . R . D . S . O . E . E . F . E . A . O . C .
. . A . . . I . . . V . . . D . . . E . . . N . .

Then reads off:

WECRLTEERDSOEEFEAOCAIVDEN

To decrypt a message you take the zig-zag shape and fill the ciphertext along the rows.

? . . . ? . . . ? . . . ? . . . ? . . . ? . . . ?
. ? . ? . ? . ? . ? . ? . ? . ? . ? . ? . ? . ? .
. . ? . . . ? . . . ? . . . ? . . . ? . . . ? . .

The first row has seven spots that can be filled with "WECRLTE".

W . . . E . . . C . . . R . . . L . . . T . . . E
. ? . ? . ? . ? . ? . ? . ? . ? . ? . ? . ? . ? .
. . ? . . . ? . . . ? . . . ? . . . ? . . . ? . .

Now the 2nd row takes "ERDSOEEFEAOC".

W . . . E . . . C . . . R . . . L . . . T . . . E
. E . R . D . S . O . E . E . F . E . A . O . C .
. . ? . . . ? . . . ? . . . ? . . . ? . . . ? . .

Leaving "AIVDEN" for the last row.

W . . . E . . . C . . . R . . . L . . . T . . . E
. E . R . D . S . O . E . E . F . E . A . O . C .
. . A . . . I . . . V . . . D . . . E . . . N . .

If you now read along the zig-zag shape you can read the original message.

Coding the solution

Look for a stub file having the name rail_fence_cipher.go and place your solution code in that file.

Running the tests

To run the tests run the command go test from within the exercise directory.

If the test suite contains benchmarks, you can run these with the --bench and --benchmem flags:

go test -v --bench . --benchmem

Keep in mind that each reviewer will run benchmarks on a different machine, with different specs, so the results from these benchmark tests may vary.

Further information

For more detailed information about the Go track, including how to get help if you're having trouble, please visit the exercism.io Go language page.

Source

Wikipedia https://en.wikipedia.org/wiki/Transposition_cipher#Rail_Fence_cipher

Submitting Incomplete Solutions

It's possible to submit an incomplete solution so you can see how others have completed the exercise.

cases_test.go

package railfence

// Source: exercism/problem-specifications
// Commit: 88db37b rail-fence-cipher: apply "input" policy
// Problem Specifications Version: 1.1.0

type testCase struct {
	description string
	message     string
	rails       int
	expected    string
}

// encode
var encodeTests = []testCase{

	{"encode with two rails",
		"XOXOXOXOXOXOXOXOXO",
		2,
		"XXXXXXXXXOOOOOOOOO"},

	{"encode with three rails",
		"WEAREDISCOVEREDFLEEATONCE",
		3,
		"WECRLTEERDSOEEFEAOCAIVDEN"},

	{"encode with ending in the middle",
		"EXERCISES",
		4,
		"ESXIEECSR"},
}

// decode
var decodeTests = []testCase{

	{"decode with three rails",
		"TEITELHDVLSNHDTISEIIEA",
		3,
		"THEDEVILISINTHEDETAILS"},

	{"decode with five rails",
		"EIEXMSMESAORIWSCE",
		5,
		"EXERCISMISAWESOME"},

	{"decode with six rails",
		"133714114238148966225439541018335470986172518171757571896261",
		6,
		"112358132134558914423337761098715972584418167651094617711286"},
}

rail_fence_cipher_test.go

package railfence

import "testing"

func testCases(op func(string, int) string, cases []testCase, t *testing.T) {
	for _, tc := range cases {
		if actual := op(tc.message, tc.rails); actual != tc.expected {
			t.Fatalf("FAIL: %s\nExpected: %q\nActual: %q", tc.description, tc.expected, actual)
		}
		t.Logf("PASS: %s", tc.description)
	}
}

func TestEncode(t *testing.T) { testCases(Encode, encodeTests, t) }
func TestDecode(t *testing.T) { testCases(Decode, decodeTests, t) }
package railfence

// Encode takes a string and encodes it using the rail fence cipher, with
// appropriate number of rails.
func Encode(message string, rails int) string {
	rmessage := []rune(message)
	lines := [][]rune{}
	for r := 0; r < rails; r++ {
		lines = append(lines, []rune{})
	}

	pos, line, dir := 0, 0, -1
	for pos < len(rmessage) {
		lines[line] = append(lines[line], rmessage[pos])

		// Flip direction when on top or bottom rail.
		if line == 0 || line == rails-1 {
			dir *= -1
		}

		line += dir
		pos++
	}

	ciphertext := []rune{}
	for _, l := range lines {
		ciphertext = append(ciphertext, l...)
	}
	return string(ciphertext)
}

// Decode takes a string and decodes it using the rail fence cipher, with
// appropriate number of rails.
func Decode(ciphertext string, rails int) string {
	lines := make([][]rune, rails)
	for r := 0; r < rails; r++ {
		lines[r] = make([]rune, len(ciphertext))
	}

	chars := []rune(ciphertext)

	// Break the ciphertext into fence rails.
	var pos int
	for i := 0; i < rails; i++ {
		line, col, dir := 0, 0, 1
		for range ciphertext {
			if line == i {
				lines[line][col] = chars[pos]
				pos++
			}
			col++
			line += dir

			// Flip direction on top or bottom rail.
			if line == rails-1 || line == 0 {
				dir *= -1
			}
		}
	}

	// Travel up and down the rails collecting letters.
	res := make([]rune, 0, len(ciphertext))
	line, col, dir := 0, 0, 1
	for range ciphertext {
		res = append(res, lines[line][col])
		col++
		line += dir

		// Flip direction on the top or bottom rail.
		if line == rails-1 || line == 0 {
			dir *= -1
		}
	}

	return string(res)
}

Community comments

Find this solution interesting? Ask the author a question to learn more.

dougal's Reflection

This is the first evercise I did not enjoy. Being able to see how I would solve this in a language with rich enumerable, such as Ruby, it was frustrating to do so much manual iteration.