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travisamiller'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 . .

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.

Running the tests

You can run all the tests for an exercise by entering

$ gradle test

in your terminal.

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.

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"));
	}
}
import java.util.*;
import java.util.stream.*;

class RailFenceCipher {

    private int height;

    public RailFenceCipher(int height) {
        this.height = height;
    }

    public String getEncryptedData(String plaintext) {
        return (new Fence(this.height, plaintext)).stream()
            .map(sb -> sb.toString())
            .collect(Collectors.joining());
    }

    public String getDecryptedData(String ciphertext) {
        // build temp fence so I don't have to calc rail lengths
        Fence fence = new Fence (this.height, ciphertext);

        // replace the rails with the correct values
        for (int idx = 0, pointer = 0; idx < fence.getNumberOfRails(); idx++) {
            int length = fence.getNumberOfPostsInRail(idx);
            fence.replaceRailValues(idx, ciphertext.substring(pointer, pointer + length));
            pointer += length;
        }

        // reading fence gives plaintext
        return fence.toString();
    }

    class Fence {
        private StringBuffer[] rails;
        private int length = 0;

        private Fence(int rails, String text) {
            this.rails = new StringBuffer[rails];
            for (int idx = 0; idx < rails; idx++) {
                this.rails[idx] = new StringBuffer();
            }

            Oscillator oscillator = new Oscillator(rails - 1);
            for (Character character : text.toCharArray()) {
                this.rails[oscillator.next()].append(character);
                length++;
            }
        }

        private void replaceRailValues(int rail, String letters) {
            length -= rails[rail].length();
            rails[rail] = new StringBuffer(letters);
            length += letters.length();
        }

        private int getNumberOfPostsInRail(int rail) {
            return rails[rail].length();
        }

        private int getNumberOfRails() {
            return rails.length;
        }

        public String toString() {
            Oscillator oscillator = new Oscillator(getNumberOfRails() - 1);
            StringBuilder string = new StringBuilder(length);

            for (int idx = 0; idx < length; idx++) {
                string.append(pop(oscillator.next()));
            }

            return string.toString();
        }

        private Character pop(int rail) {
            Character character = rails[rail].charAt(0);
            rails[rail].deleteCharAt(0);
            return character;
        }

        private Stream<StringBuffer> stream() {
            return Arrays.stream(rails);
        }
    }

    /**
     * (new Oscillator(1)).next() -> 0 1 0 1 0 1 0 1 0 1 0 1 0
     * (new Oscillator(2)).next() -> 0 1 2 1 0 1 2 1 0 1 2 1 0
     * (new Oscillator(3)).next() -> 0 1 2 3 2 1 0 1 2 3 2 1 0
     */
    class Oscillator {
        private int max;
        private int min = 0;
        private int current = -1;
        private int step = 1;

        private Oscillator(int max) {
            this.max = max;
        }

        private int next() {
            if (current == -1) {
                current = 0;
            } else {
                current += step;
                if (current == min || current == max) {
                    step *= -1;
                }
            }

            return current;
        }
    }
}

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