text: const fs = require('fs');
  
  let start = (new Date).getTime();
  
  let frequency = 0;
  let pastFrequencies = {};
  let changes = fs.readFileSync('01input.txt', 'utf8').split('\n').map(r => parseInt(r));
  
  let isRepeating = false;
  while (!isRepeating) {
    for (let change of changes) {
      frequency += change;
      isRepeating = pastFrequencies[`${frequency}`];
      if (isRepeating) {
        console.log('repeating frequency:', frequency);
        console.log(`done in ${(new Date).getTime() - start}ms.`);
        break;
      }
      pastFrequencies[`${frequency}`] = true;
    }
  }