ranking 2.0.0

A library to compute rankings of players. Uses ELO for dynamic rankings, and Bradley-Terry for static rankings.

example/ranking_example.dart

// Copyright 2019 Florian Loitsch
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// https://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

import 'package:ranking/ranking.dart';

var players = [
  'Novak',
  'Rafael',
  'Roger',
  'Dominic',
  'Alexander',
  'Stefanos',
];

var games = [
  ['Stefanos', 'Dominic', 0.0, '2018-01-01'], // quarter
  ['Stefanos', 'Dominic', 0.0, '2018-03-05'], // ro64
  ['Alexander', 'Rafael', 0.0, '2018-04-02'], // round robin.
  ['Dominic', 'Novak', 1.0, '2018-04-16'],
  ['Stefanos', 'Dominic', 1.0, '2018-04-16'], // quarter
  ['Dominic', 'Rafael', 0.0, '2018-04-16'],
  ['Stefanos', 'Rafael', 0.0, '2018-04-23'], // final
  ['Dominic', 'Rafael', 1.0, '2018-05-07'],
  ['Alexander', 'Dominic', 1.0, '2018-05-07'],
  ['Rafael', 'Novak', 1.0, '2018-05-14'],
  ['Alexander', 'Rafael', 0.0, '2018-05-14'],
  ['Stefanos', 'Dominic', 0.0, '2018-05-28'], // ro64
  ['Alexander', 'Dominic', 0.0, '2018-05-28'], // quarter
  ['Dominic', 'Rafael', 0.0, '2018-05-28'],
  ['Rafael', 'Novak', 0.0, '2018-07-02'],
  ['Stefanos', 'Alexander', 0.0, '2018-07-30'], // semi
  ['Stefanos', 'Dominic', 1.0, '2018-08-06'], // ro32
  ['Stefanos', 'Novak', 1.0, '2018-08-06'], // round of 16
  ['Stefanos', 'Alexander', 1.0, '2018-08-06'], // quarter
  ['Stefanos', 'Rafael', 0.0, '2018-08-06'], // final
  ['Roger', 'Novak', 0.0, '2018-08-13'],
  ['Dominic', 'Rafael', 0.0, '2018-08-27'],
  ['Alexander', 'Novak', 0.0, '2018-10-08'],
  ['Roger', 'Novak', 0.0, '2018-10-29'],
  ['Alexander', 'Novak', 0.0, '2018-11-12'], // Earlier round robin.
  ['Dominic', 'Roger', 0.0, '2018-11-12'], // round robin
  ['Alexander', 'Roger', 1.0, '2018-11-12'],
  ['Alexander', 'Novak', 1.0, '2018-11-12'],
  ['Roger', 'Rafael', 1.0, '2019-03-04'],
  ['Dominic', 'Roger', 1.0, '2019-03-04'],
  ['Stefanos', 'Roger', 1.0, '2019-01-14'], // round of 16
  ['Stefanos', 'Rafael', 0.0, '2019-01-14'], // semi
  ['Rafael', 'Novak', 0.0, '2019-01-14'], // final
  ['Stefanos', 'Roger', 0.0, '2019-02-25'], // final
  ['Dominic', 'Rafael', 1.0, '2019-04-22'],
  ['Stefanos', 'Alexander', 1.0, '2019-05-06'], // quarter
  ['Dominic', 'Roger', 1.0, '2019-05-06'],
  ['Stefanos', 'Rafael', 1.0, '2019-05-06'], // semi
  ['Dominic', 'Novak', 0.0, '2019-05-06'], // semi
  ['Stefanos', 'Novak', 0.0, '2019-05-06'], // final
  ['Stefanos', 'Roger', 1.0, '2019-05-13'], // quarter
  ['Stefanos', 'Rafael', 0.0, '2019-05-13'], // semi
  ['Rafael', 'Novak', 1.0, '2019-05-13'], // final
];

void bradleyTerryExample() {
  // Bradley-Terry is better suited for players that don't change their skill
  // level. As such, using the data of Tennis games isn't great, since
  // Tennis players change their skill level over time.
  // However, it's still interesting to compare the result to the ELO algorithm.
  var input = games.map((list) {
    if (list[2] == 1.0) return [list[0], list[1]];
    return [list[1], list[0]];
  }).toList();

  var scores = computeBradleyTerryScores(input);

  var ranked = players.toList()
    ..sort((a, b) => -scores[a]!.compareTo(scores[b]!));
  print('Bradley-Terry Scores:');
  for (var player in ranked) {
    print('  $player: ${scores[player]}');
  }
}

void eloExample() {
  // The data we are using is from Tennis.
  //
  // The ranking changes over time, whenever a new game has been played.
  // Players are naturally changing their skills (relative to others) over time
  // and as such order matters. This means that `ELO` is a good model to
  // rank the players. (At least, if we used all played games, and not just
  // a tiny subset of them).

  var elo = Elo(defaultInitialRating: 100, n: 70, kFactor: 15);

  for (var game in games) {
    elo.recordResult(game[0], game[1], game[2] as double);
  }
  var ratings = elo.ratings;
  var ranked = players.toList()
    ..sort((a, b) => -ratings[a]!.compareTo(ratings[b]!));
  print('ELO Scores:');
  for (var player in ranked) {
    print('  $player: ${ratings[player]}');
  }
}

void main() {
  eloExample();
  print('');
  bradleyTerryExample();
}