artichoke.js

#!/usr/bin/env node
"use strict";
var program = require('commander');
var x = 'abcdefghijklmnopqrstuvwxyz&';
var matrix = [];
for (let i=0; i<x.length; i++) {
  let cols = [];
  for (let j=0; j<x.length; j++) {
    cols.push(x[(j+i)%x.length]);
  }
  matrix.push(cols);
}

function encode(phrase, keyword='artichoke') {
  let result = [];
  let j = 0;
  for (let i=0; i<phrase.length; i++) {
    let c = phrase[i];
    if (x.indexOf(c) < 0) {
      result.push(c);
    } else {
      let r = keyword[j++%keyword.length];
      let row = x.indexOf(r);
      let col = (x.indexOf(c)+1)%x.length;
      result.push(matrix[row][col]);      
    }
  }
  return result.join('');
}

function decode(phrase, keyword='artichoke') {
  let result = [];
  let j = 0;
  for (let i=0; i<phrase.length; i++) {
    let c = phrase[i];

    if (x.indexOf(c) < 0) {
      result.push(c);
    } else {
      let r = keyword[j++%keyword.length];
      let row = x.indexOf(r);

      for (var p=0; p<matrix[row].length; p++) {
        if (matrix[row][p] == c) {
          break;
        }
      }
      p = p-1; 
      if (p<0) {
        p = x.length-1; // handle negative -1
      }
      result.push(x[p]);
    }
  }
  return result.join('');
}

program
  .option('-k, --keyword <keyword>', 'keyword to use. Default "artichoke"')
  .option('-e, --encode', 'encode the given phrase. Default')
  .option('-d, --decode', 'decode the given phrase')
  .arguments('<phrase>')
  .action((phrase) => {
    phrase = phrase.toLowerCase();
    if (program.decode) {
      console.log(decode(phrase, program.keyword));
    } else {
      console.log(encode(phrase, program.keyword));
    }
  })
.parse(process.argv);