robin.js

const DUMMY = -1;
// https://github.com/clux/roundrobin/blob/master/robin.js
// returns an array of round representations (array of player pairs).
// http://en.wikipedia.org/wiki/Round-robin_tournament#Scheduling_algorithm
let generateLeague =  function (n, ps) {  // n = num players
  var rs = [];                  // rs = round array
  if (!ps) {
    ps = [];
    for (var k = 1; k <= n; k += 1) {
      ps.push(k);
    }
  } else {
    ps = ps.slice();
  }

  if (n % 2 === 1) {
    ps.push(DUMMY); // so we can match algorithm for even numbers
    n += 1;
  }
  for (var j = 0; j < n - 1; j += 1) {
    rs[j] = []; // create inner match array for round j
    for (var i = 0; i < n / 2; i += 1) {
      if (ps[i] !== DUMMY && ps[n - 1 - i] !== DUMMY) {
        rs[j].push([ps[i], ps[n - 1 - i]]); // insert pair as a match
      }
    }
    ps.splice(1, 0, ps.pop()); // permutate for next round
  }
  return rs;
};