sorting tournament seeds

Question

I\'m making a HTML/JS powered single/double elimination bracket web app. I am struggling to figure out how to assign the first round matches from a list of seeded teams/players.

Answer 1

I wrote a solution in PHP (see https://stackoverflow.com/a/45566890/760777). Here is the javascript version.

It returns all seeds in the correct positions. The matches are the same as in his example, but in a prettier order, seed 1 and seed number 8 are on the outside of the schema (as you see in tennis tournaments).

If there are no upsets (meaning a higher seeded player always wins from a lower seeded player), you will end up with seed 1 vs seed 2 in the final.

It actually does two things more:

1. It shows the correct order (which is a requirement for putting byes in the correct positions)

2. It fills in byes in the correct positions (if required)

A perfect explanation about what a single elimination bracket should look like: http://blog.playdriven.com/2011/articles/the-not-so-simple-single-elimination-advantage-seeding/

Code example for 8 participants:

var NUMBER_OF_PARTICIPANTS = 8; // Set the number of participants

if (!String.prototype.format) {
  String.prototype.format = function() {
    var args = arguments;
    return this.replace(/{(\d+)}/g, function(match, number) { 
      return typeof args[number] != 'undefined' ? args[number] : match;
    });
  };
}

var participants = Array.from({length: NUMBER_OF_PARTICIPANTS}, (v, k) => k + 1) ;
var bracket = getBracket(participants);

console.log(bracket);

function getBracket(participants)
{
  var participantsCount = participants.length;	
  var rounds = Math.ceil(Math.log(participantsCount)/Math.log(2));
  var bracketSize = Math.pow(2, rounds);
  var requiredByes = bracketSize - participantsCount;
	
  console.log("Number of participants: {0}".format(participantsCount));
  console.log("Number of rounds: {0}".format(rounds));
  console.log("Bracket size: {0}".format(bracketSize));
  console.log("Required number of byes: {0}".format(requiredByes));    
    
  if(participantsCount < 2) {
    return [];
  }
    
  var matches = [[1,2]];
  
  for(var round = 1; round < rounds; round++) {
    var roundMatches = [];
    var sum = Math.pow(2, round + 1) + 1;
    
    for(var i = 0; i < matches.length; i++) {
      var home = changeIntoBye(matches[i][0], participantsCount);
      var away = changeIntoBye(sum - matches[i][0], participantsCount);
      roundMatches.push([home, away]);
      home = changeIntoBye(sum - matches[i][1], participantsCount);
      away = changeIntoBye(matches[i][1], participantsCount);
      roundMatches.push([home, away]);
    }
    matches = roundMatches;   
    
  }   
  
  return matches;    
}

function changeIntoBye(seed, participantsCount)
{
    //return seed <= participantsCount ?  seed : '{0} (= bye)'.format(seed);  
    return seed <= participantsCount ?  seed : null;
}

Change NUMBER_OF_PARTICIPANTS from 8 to 6 to get two byes.

Good luck. RWC