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to Rotational Cipher in the Java Track

Published at May 27 2019 · 0 comments
Instructions
Test suite
Solution

Create an implementation of the rotational cipher, also sometimes called the Caesar cipher.

The Caesar cipher is a simple shift cipher that relies on transposing all the letters in the alphabet using an integer key between 0 and 26. Using a key of 0 or 26 will always yield the same output due to modular arithmetic. The letter is shifted for as many values as the value of the key.

The general notation for rotational ciphers is ROT + <key>. The most commonly used rotational cipher is ROT13.

A ROT13 on the Latin alphabet would be as follows:

Plain:  abcdefghijklmnopqrstuvwxyz
Cipher: nopqrstuvwxyzabcdefghijklm

It is stronger than the Atbash cipher because it has 27 possible keys, and 25 usable keys.

Ciphertext is written out in the same formatting as the input including spaces and punctuation.

Examples

  • ROT5 omg gives trl
  • ROT0 c gives c
  • ROT26 Cool gives Cool
  • ROT13 The quick brown fox jumps over the lazy dog. gives Gur dhvpx oebja sbk whzcf bire gur ynml qbt.
  • ROT13 Gur dhvpx oebja sbk whzcf bire gur ynml qbt. gives The quick brown fox jumps over the lazy dog.

Setup

Go through the setup instructions for Java to install the necessary dependencies:

https://exercism.io/tracks/java/installation

Running the tests

You can run all the tests for an exercise by entering the following in your terminal:

$ gradle test

Use gradlew.bat if you're on Windows

In the test suites all tests but the first have been skipped.

Once you get a test passing, you can enable the next one by removing the @Ignore("Remove to run test") annotation.

Source

Wikipedia https://en.wikipedia.org/wiki/Caesar_cipher

Submitting Incomplete Solutions

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

RotationalCipherTest.java

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

public class RotationalCipherTest {

    private RotationalCipher rotationalCipher;

    @Test
    public void rotateSingleCharacterBy0() {
        rotationalCipher = new RotationalCipher(0);
        Assert.assertEquals("a", rotationalCipher.rotate("a"));
    }

    @Ignore("Remove to run test")
    @Test
    public void rotateSingleCharacterBy1() {
        rotationalCipher = new RotationalCipher(1);
        Assert.assertEquals("b", rotationalCipher.rotate("a"));
    }

    @Ignore("Remove to run test")
    @Test
    public void rotateSingleCharacterBy26() {
        rotationalCipher = new RotationalCipher(26);
        Assert.assertEquals("a", rotationalCipher.rotate("a"));
    }

    @Ignore("Remove to run test")
    @Test
    public void rotateSingleCharacterBy13() {
        rotationalCipher = new RotationalCipher(13);
        Assert.assertEquals("z", rotationalCipher.rotate("m"));
    }

    @Ignore("Remove to run test")
    @Test
    public void rotateSingleCharacterWithWrapAround() {
        rotationalCipher = new RotationalCipher(13);
        Assert.assertEquals("a", rotationalCipher.rotate("n"));
    }

    @Ignore("Remove to run test")
    @Test
    public void rotateCapitalLetters() {
        rotationalCipher = new RotationalCipher(5);
        Assert.assertEquals("TRL", rotationalCipher.rotate("OMG"));
    }

    @Ignore("Remove to run test")
    @Test
    public void rotateSpaces() {
        rotationalCipher = new RotationalCipher(5);
        Assert.assertEquals("T R L", rotationalCipher.rotate("O M G"));
    }

    @Ignore("Remove to run test")
    @Test
    public void rotateNumbers() {
        rotationalCipher = new RotationalCipher(4);
        Assert.assertEquals("Xiwxmrk 1 2 3 xiwxmrk", rotationalCipher.rotate("Testing 1 2 3 testing"));
    }

    @Ignore("Remove to run test")
    @Test
    public void rotatePunctuation() {
        rotationalCipher = new RotationalCipher(21);
        Assert.assertEquals("Gzo'n zvo, Bmviyhv!", rotationalCipher.rotate("Let's eat, Grandma!"));
    }

    @Ignore("Remove to run test")
    @Test
    public void rotateAllLetters() {
        rotationalCipher = new RotationalCipher(13);
        Assert.assertEquals("The quick brown fox jumps over the lazy dog.",
                rotationalCipher.rotate("Gur dhvpx oebja sbk whzcf bire gur ynml qbt."));
    }
}
import java.util.stream.Collectors;

class RotationalCipher {
    private int shiftKey;

    RotationalCipher(int shiftKey) {
        if (shiftKey < 0 || shiftKey > 26) {
            throw new IllegalArgumentException();
        }
        this.shiftKey = shiftKey % 26;
    }

    String rotate(String data) {
        if (shiftKey == 0) {
            return data;
        }
        return data.chars()
                .mapToObj(ch -> String.valueOf(rotate((char) ch)))
                .collect(Collectors.joining());
    }

    private char rotate(char letter) {
        return Character.isLetter(letter) ? rotateLetter(letter) : letter;
    }

    private char rotateLetter(char letter) {
        char firstLetter = Character.isUpperCase(letter) ? 'A' : 'a';
        int pos = firstLetter + (letter - firstLetter + shiftKey) % 26;
        return (char) pos;
    }
}

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