# Mkerrigan's solution

## to Rail Fence Cipher in the Java Track

Published at Jul 13 2018 · 0 comments
Instructions
Test suite
Solution

#### Note:

This solution was written on an old version of Exercism. The tests below might not correspond to the solution code, and the exercise may have changed since this code was written.

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 . .
``````

``````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.

# Running the tests

You can run all the tests for an exercise by entering

``````\$ gradle test
``````

## Submitting Incomplete Solutions

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

### RailFenceCipherTest.java

``````import org.junit.Assert;
import org.junit.Ignore;
import org.junit.Test;

public class RailFenceCipherTest {

private RailFenceCipher railFenceCipher;

@Test
public void encodeWithTwoRails() {
railFenceCipher = new RailFenceCipher(2);
Assert.assertEquals("XXXXXXXXXOOOOOOOOO",
railFenceCipher.getEncryptedData("XOXOXOXOXOXOXOXOXO"));
}

@Ignore("Remove to run test")
@Test
public void encodeWithThreeRails() {
railFenceCipher = new RailFenceCipher(3);
Assert.assertEquals("WECRLTEERDSOEEFEAOCAIVDEN",
railFenceCipher.getEncryptedData("WEAREDISCOVEREDFLEEATONCE"));
}

@Ignore("Remove to run test")
@Test
public void encodeWithEndingInTheMiddle() {
railFenceCipher = new RailFenceCipher(4);
Assert.assertEquals("ESXIEECSR",
railFenceCipher.getEncryptedData("EXERCISES"));
}

@Ignore("Remove to run test")
@Test
public void decodeWithThreeRails() {
railFenceCipher = new RailFenceCipher(3);
Assert.assertEquals("THEDEVILISINTHEDETAILS",
railFenceCipher.getDecryptedData("TEITELHDVLSNHDTISEIIEA"));
}

@Ignore("Remove to run test")
@Test
public void decodeWithFiveRails() {
railFenceCipher = new RailFenceCipher(5);
Assert.assertEquals("EXERCISMISAWESOME",
railFenceCipher.getDecryptedData("EIEXMSMESAORIWSCE"));
}

@Ignore("Remove to run test")
@Test
public void decodeWithSixRails() {
railFenceCipher = new RailFenceCipher(6);
Assert.assertEquals("112358132134558914423337761098715972584418167651094617711286",
railFenceCipher.getDecryptedData("133714114238148966225439541018335470986172518171757571896261"));
}
}``````
``````public class RailFenceCipher {
private int railCount;
private StringBuilder[] rails;
private boolean isGoingDown;

public RailFenceCipher(int railCount) {
this.railCount = railCount;
rails = new StringBuilder[railCount];
}

/***
* Get decrypted data given an encrypted input.
*
* @param encryptedData
* @return decrypted data
*/
public String getDecryptedData(String encryptedData) {
initRails();

// Create rail template with '?'
int railPos = 0;
for (int cnt = 0; cnt < encryptedData.length(); cnt++) {
rails[railPos] = rails[railPos].append(String.valueOf("?"));
}

// Place encrypted string on rails
int lengthOfRail;
int charPosition = 0;
for (StringBuilder rail : rails) {
lengthOfRail = rail.length();
rail.replace(0, lengthOfRail, encryptedData.substring(charPosition, charPosition + lengthOfRail));
charPosition = charPosition + lengthOfRail;
}

// Read the cypher up and down along the rails
StringBuilder sb = new StringBuilder();
railPos = 0;
for(int cnt = 0; cnt < encryptedData.length(); cnt++){
sb.append(rails[railPos].charAt(0));
rails[railPos] = rails[railPos].deleteCharAt(0);
}
return sb.toString();
}

/***
* Get encrypted data given a decrypted input.
*
* @param decryptedData
* @return encrypted data
*/
public String getEncryptedData(String decryptedData) {
int railPos = 0;
isGoingDown = true;
initRails();
for (char c : decryptedData.toCharArray()) {
rails[railPos] = rails[railPos].append(String.valueOf(c));
}
StringBuilder sb = new StringBuilder();
for (int cnt = 0; cnt < railCount; cnt++) {
sb.append(rails[cnt]);
}
return sb.toString();
}

if (railPos == railCount - 1) {
isGoingDown = false;
railPos--;
} else if (railPos == 0) {
isGoingDown = true;
railPos++;
} else if (isGoingDown) {
railPos++;
} else if (!isGoingDown) {
railPos--;
}
return railPos;
}

private void initRails() {
isGoingDown = true;
for (int cnt = 0; cnt < railCount; cnt++) {
rails[cnt] = new StringBuilder();
}
}
}``````