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to Rotational Cipher in the Kotlin Track

Published at Sep 05 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 Kotlin to install the necessary dependencies:

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

Making the test suite pass

Execute the tests with:

$ gradlew 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 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.kt

import org.junit.Ignore
import org.junit.Test
import kotlin.test.assertEquals

class RotationalCipherTest {

    @Test
    fun testRotateLowercaseABy0() {
        val cipher = RotationalCipher(0)
        assertEquals("a", cipher.encode("a"))
    }

    @Ignore
    @Test
    fun testRotateLowercaseABy1NoWrapAround() {
        val cipher = RotationalCipher(1)
        assertEquals("b", cipher.encode("a"))
    }

    @Ignore
    @Test
    fun testRotateLowercaseABy26SingleWrapAround() {
        val cipher = RotationalCipher(26)
        assertEquals("a", cipher.encode("a"))
    }

    @Ignore
    @Test
    fun testRotateLowercaseMBy13NoWrapAround() {
        val cipher = RotationalCipher(13)
        assertEquals("z", cipher.encode("m"))
    }

    @Ignore
    @Test
    fun testRotateLowercaseNBy1SingleWrapAround() {
        val cipher = RotationalCipher(13)
        assertEquals("a", cipher.encode("n"))
    }

    @Ignore
    @Test
    fun testRotateCapitalLettersNoWrapAround() {
        val cipher = RotationalCipher(5)
        assertEquals("TRL", cipher.encode("OMG"))
    }

    @Ignore
    @Test
    fun testSpacesAreUnalteredByRotation() {
        val cipher = RotationalCipher(5)
        assertEquals("T R L", cipher.encode("O M G"))
    }

    @Ignore
    @Test
    fun testNumbersAreUnalteredByRotation() {
        val cipher = RotationalCipher(4)
        assertEquals("Xiwxmrk 1 2 3 xiwxmrk", cipher.encode("Testing 1 2 3 testing"))
    }

    @Ignore
    @Test
    fun testPunctuationIsUnalteredByRotation() {
        val cipher = RotationalCipher(21)
        assertEquals("Gzo'n zvo, Bmviyhv!", cipher.encode("Let's eat, Grandma!"))
    }

    @Ignore
    @Test
    fun testAllLettersRotateCorrectly() {
        val cipher = RotationalCipher(13)
        assertEquals(
                "Gur dhvpx oebja sbk whzcf bire gur ynml qbt.",
                cipher.encode("The quick brown fox jumps over the lazy dog."))
    }

}
class RotationalCipher(val pos: Int) {
    val chars = ('a'..'z').toList()

    fun encode(s: String): String {
        return s.toCharArray().map {
            val case = it.isLowerCase()
            val index = chars.indexOf(it.toLowerCase())
            if (index == -1) {
                return@map it
            }
            val num = (index + pos) % chars.size
            return@map if (case) chars[num] else chars[num].toUpperCase()
        }.fold("", { a, b -> a + b })
    }

}

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