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brunnock's solution

to Atbash Cipher in the JavaScript 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.

Create an implementation of the atbash cipher, an ancient encryption system created in the Middle East.

The Atbash cipher is a simple substitution cipher that relies on transposing all the letters in the alphabet such that the resulting alphabet is backwards. The first letter is replaced with the last letter, the second with the second-last, and so on.

An Atbash cipher for the Latin alphabet would be as follows:

Plain:  abcdefghijklmnopqrstuvwxyz
Cipher: zyxwvutsrqponmlkjihgfedcba

It is a very weak cipher because it only has one possible key, and it is a simple monoalphabetic substitution cipher. However, this may not have been an issue in the cipher's time.

Ciphertext is written out in groups of fixed length, the traditional group size being 5 letters, and punctuation is excluded. This is to make it harder to guess things based on word boundaries.

Examples

  • Encoding test gives gvhg
  • Decoding gvhg gives test
  • Decoding gsvjf rxpyi ldmul cqfnk hlevi gsvoz abwlt gives thequickbrownfoxjumpsoverthelazydog

Setup

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

http://exercism.io/languages/javascript/installation

Running the test suite

The provided test suite uses Jasmine. You can install it by opening a terminal window and running the following command:

npm install -g jasmine

Run the test suite from the exercise directory with:

jasmine atbash-cipher.spec.js

In many test suites all but the first test have been marked "pending". Once you get a test passing, activate the next one by changing xit to it.

Source

Wikipedia http://en.wikipedia.org/wiki/Atbash

Submitting Incomplete Solutions

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

atbash-cipher.spec.js

var atbash = require('./atbash-cipher');

describe('encode', function () {
  it('encodes no', function () {
    expect(atbash.encode('no')).toEqual('ml');
  });

  xit('encodes yes', function () {
    expect(atbash.encode('yes')).toEqual('bvh');
  });

  xit('encodes OMG', function () {
    expect(atbash.encode('OMG')).toEqual('lnt');
  });

  xit('encodes O M G', function () {
    expect(atbash.encode('O M G')).toEqual('lnt');
  });

  xit('encodes long words', function () {
    expect(atbash.encode('mindblowingly')).toEqual('nrmwy oldrm tob');
  });

  xit('encodes numbers', function () {
    expect(atbash.encode('Testing, 1 2 3, testing.'))
      .toEqual('gvhgr mt123 gvhgr mt');
  });

  xit('encodes sentences', function () {
    expect(atbash.encode('Truth is fiction.')).toEqual('gifgs rhurx grlm');
  });

  xit('encodes all the things', function () {
    expect(atbash.encode('The quick brown fox jumps over the lazy dog.'))
      .toEqual('gsvjf rxpyi ldmul cqfnk hlevi gsvoz abwlt');
  });
});
module.exports = {
  encode : function (text) {
     text = text.toLowerCase();
     var alpha = 'abcdefghijklmnopqrstuvwxyz'.split('');
     var alpha_rev = alpha.slice(0);
     alpha_rev.reverse();

     var code = 
     text.split('').map(function(x){ 
	  return (/\d/.test(x) ? x : alpha_rev[alpha.indexOf(x)]) }).join('');

     if (code.length > 5) {
       var code_arr = code.match(/.{1,5}/g);
       code = code_arr.join(' ');
     }
     return code;
  }
}

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