rri.php

<?php

$options = [
  'max_delta' => 0.01, // if percent change in scores falls below this, assume the scores are stable
  'max_victory_ratio' => 1.5, // cap the margin of victory of an individual game at this ratio
];

protected static function computeRRIInternal($games, $teamsByTeamProxy, $options)
  {
    $delta = 1; // percent change in scores since last round
    $round = 0;
    $powerRating = array_fill_keys(array_unique($teamsByTeamProxy), 100); // every team starts at 100
    $teamHasGames = array_fill_keys(array_unique($teamsByTeamProxy), false);

    while ($delta > $options['max_delta'] && $round < 100)
    {
      $delta = 0;
      $round++;
      $updatePowerRating = $powerRating;

      foreach($games as $game)
      {
        if ($game['home_score'] === null || $game['away_score'] === null)
        {
          continue;
        }

        $homeTeamId = $teamsByTeamProxy[$game['home_team_proxy_id']];
        $awayTeamId = $teamsByTeamProxy[$game['away_team_proxy_id']];

        $teamHasGames[$homeTeamId] = true;
        $teamHasGames[$awayTeamId] = true;

        $predictedHomeRatio = $powerRating[$homeTeamId] / $powerRating[$awayTeamId];
        $actualHomeRatio = $game['home_score'] / ($game['away_score'] ?: 1);

        if (isset($options['max_victory_ratio']))
        {
          $maxRatio = $options['max_victory_ratio'];
          $minRatio = 1/$maxRatio;
          $actualHomeRatio = min(max($minRatio, $actualHomeRatio), $maxRatio);
        }

        $homeDelta = 10 * (log($actualHomeRatio) - log($predictedHomeRatio)); // coefficient should be ~ 10% of the starting PR. this opinion is based on experimentation.

        $updatePowerRating[$homeTeamId] += $homeDelta;
        $updatePowerRating[$awayTeamId] -= $homeDelta;
      }

      foreach($powerRating as $teamId => $oldRating)
      {
        $delta += abs($oldRating - $updatePowerRating[$teamId]) / ($oldRating ?: 1);
      }

      $powerRating = $updatePowerRating;
    }

    foreach($teamHasGames as $teamId => $hasGames)
    {
      if (!$hasGames)
      {
        $powerRating[$teamId] = 0;
      }
    }

    arsort($powerRating);
    return $powerRating;
  }