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spinguice'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"));
	}
}
public class RailFenceCipher {
    private final int rails;

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

    public String getEncryptedData(String clear) {
        StringBuilder sb = new StringBuilder(clear.length());
        Offset offset = new Offset(this.rails);

        for (int rail = 0; rail < this.rails; rail++, offset.move()) {
            out:
            for (int j = rail; ; ) {
                for (int span : offset.values) {
                    if (span != 0) {
                        sb.append(clear.charAt(j));
                    }
                    j += span;
                    if (j >= clear.length()) {
                        break out;
                    }
                }
            }
        }

        return sb.toString();
    }

    public String getDecryptedData(String encrypted) {
        String[] parts = partition(encrypted);
        RailIndex index = new RailIndex(this.rails);

        StringBuilder sb = new StringBuilder(encrypted.length());
        for (int i = 0; i < encrypted.length(); i++, index.nextRail()) {
            String part = parts[index.railIndex()];
            if (index.stringIndex() < part.length()) {
                sb.append(part.charAt(index.stringIndex()));
                index.incrementStringIndex();
            }
        }
        return sb.toString();
    }

    private String[] partition(String encrypted) {
        String[] parts = new String[this.rails];
        Offset offset = new Offset(this.rails);

        int index = 0, startIndex = 0;
        for (int rail = 0; rail < rails; rail++, offset.move()) {
            out:
            for (int j = rail; ; ) {
                for (int span : offset.values) {
                    if (span != 0) {
                        index++;
                    }
                    j += span;
                    if (j >= encrypted.length()) {
                        break out;
                    }
                }
            }
            parts[rail] = encrypted.substring(startIndex, index);
            startIndex = index;
        }

        return parts;
    }


    static class Offset {
        final int[] values;
        final int max;

        Offset(int n) {
            this.max = (n - 1) * 2;
            this.values = new int[]{this.max, 0};
        }

        void move() {
            values[0] -= 2;
            values[1] += 2;
            if (values[0] < 2) {
                values[0] = max;
                values[1] = 0;
            }
        }
    }

    static class RailIndex {
        int index = 0;
        int direction = 1;
        final int[] stringIndexes;

        RailIndex(int size) {
            this.stringIndexes = new int[size];
        }

        int stringIndex() {
            return stringIndexes[index];
        }

        int railIndex() {
            return index;
        }

        void incrementStringIndex() {
            stringIndexes[index]++;
        }

        void nextRail() {
            index += direction;
            if (index == 0) {
                direction = 1;
            } else if (index == stringIndexes.length - 1) {
                direction = -1;
            }
        }
    }
}

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