function makePairs(readings){
  sortReadingsById(readings); //todo

  let setOfUsedIds = new Set();
  let listOfPairs = [];

  //todo, you determine if simply moving left to right is acceptable
  //or if you need a better heuristic for pairing
  //rather than just "use my nearest sorted neighbor"
  for (let i = 0; i < readings.length; i++){
    //todo: handle edge cases, odd even length....
    let pair = makePair(readings[i], readings[i+1]);
    setOfUsedIds.add(readings[i].id);
    setOfUsedIds.add(readings[i+1].id);
    ++i; //since we used two ids
    listOfPairs.push(pair);
  }
  return listOfPairs;
}

function makePair(r1, r2){
  //todo, check if pair ids are too far away? (i.e not caught by prev sorting)
  let pair = {};
  pair.r1 = r1.id;
  pair.r2 = r2.id;
  pair.pairing = `${r1.id, r2.id}`;
  pair.average = averageOf(r1.reading, r2.reading);
  return pair;
}

function averageOf(r1, r2){
  return (r1.reading + r2.reading) / 2;
}