runlength-encoding.js

#!/usr/bin/env node
'use strict';

const runlength = {};

runlength.decode = function(str) {
  return str.match(/\d*\w/g)
    .reduce((prev, curr) =>
      prev + (curr.length > 1
        ? curr.slice(-1) 
          .repeat(curr.slice(0, curr.length - 1))
        : curr)
    , '');
}

runlength.encode = function(str) {
  return str.match(/(.)\1*/g)
    .reduce((prev, curr) =>
      prev + (curr.length === 1
        ? curr
        : curr.length + curr.slice(-1))
    , '');
}

const test = [
  [ 'foo', 'f2o' ],
  [ 'WWWWWWWWWWWWBWWWWWWWWWWWWBBBWWWWWWWWWWWWWWWWWWWWWWWWBWWWWWWWWWWWWWW', '12WB12W3B24WB14W' ],
  [ 'abcdefghijklmnopqrstuvwxyzzzzzzzzzzzz', 'abcdefghijklmnopqrstuvwxy12z' ]
];

console.log('encoding');
test.forEach(t => console.log(runlength.encode(t[0]) == t[1], `${t[0]} == ${t[1]}`));

console.log('decoding');
test.forEach(t => console.log(runlength.decode(t[1]) == t[0], `${t[1]} == ${t[0]}`));