RatingSystem

package me.rabrg.kitpvp.rating;

/**
 * A class which implements the Elo rating system.
 * @author Ryan Greene
 *
 */
public final class RatingSystem {

    /**
     * The rating system instance.
     */
    private static final RatingSystem INSTANCE = new RatingSystem();

    /**
     * Gets the rating system instance.
     * @return The rating system instance.
     */
    public static RatingSystem getInstance() {
        return INSTANCE;
    }

    /**
     * A private constructor to prevent initialization.
     */
    private RatingSystem() {

    }

    /**
     * Gets the new rating of a player.
     * @param rating The current rating of the player.
     * @param opponentRating The current rating of the player's opponent.
     * @param result The result of the match.
     * @param kFactor The K-factor of the player.
     * @return The new rating of the player.
     */
    public int getNewRating(final int rating, final int opponentRating, final Result result, final int kFactor) {
        double expectedScore = getExpectedScore(rating, opponentRating);
        int newRating = calculateNewRating(rating, result.getScore(), expectedScore, kFactor);
        return newRating;
    }

    /**
     * Gets the expected score of a match.
     * @param rating The rating of the player.
     * @param opponentRating The rating of the player's opponent.
     * @return The expected score of a match.
     */
    private double getExpectedScore(final int rating, final int opponentRating) {
        return 1.0 / (1.0 + Math.pow(10.0, ((double) (opponentRating - rating) / 400.0)));
    }

    /**
     * Calculates the new rating of a player.
     * @param oldRating The old rating of the player.
     * @param score The score of the match.
     * @param expectedScore The expected score of the match.
     * @param kFactor The K-factor of the player.
     * @return The new rating of the player.
     */
    private int calculateNewRating(final int oldRating, double score, double expectedScore, double kFactor) {
        return oldRating + (int) (kFactor * (score - expectedScore));
    }
}