euchre_camp/src/__tests__/unit/elo.test.ts

/**
 * Unit Tests: Elo Rating System
 * 
 * Tests the mathematical correctness of Elo calculations and rating updates
 */

import { describe, test, expect } from 'vitest';
import { calculateEloChange, calculateExpectedScore, calculateTeamElo } from '@/lib/elo-utils';

describe('Elo Rating System', () => {
  const K_FACTOR = 32;

  describe('calculateExpectedScore', () => {
    test('should calculate expected score for equal ratings', () => {
      const ratingA = 1500;
      const ratingB = 1500;
      const expected = calculateExpectedScore(ratingA, ratingB);
      
      // Equal ratings should result in 0.5 expected score
      expect(expected).toBeCloseTo(0.5, 4);
    });

    test('should calculate expected score when A is stronger', () => {
      const ratingA = 1600;
      const ratingB = 1500;
      const expected = calculateExpectedScore(ratingA, ratingB);
      
      // Higher rated player should have higher expected score
      expect(expected).toBeGreaterThan(0.5);
      expect(expected).toBeLessThan(1);
    });

    test('should calculate expected score when B is stronger', () => {
      const ratingA = 1500;
      const ratingB = 1600;
      const expected = calculateExpectedScore(ratingA, ratingB);
      
      // Lower rated player should have lower expected score
      expect(expected).toBeLessThan(0.5);
      expect(expected).toBeGreaterThan(0);
    });

    test('should handle large rating differences', () => {
      const ratingA = 2000;
      const ratingB = 1000;
      const expected = calculateExpectedScore(ratingA, ratingB);
      
      // Much higher rated player should have very high expected score
      expect(expected).toBeGreaterThan(0.95);
    });
  });

  describe('calculateEloChange', () => {
    test('should return positive change for win against equal opponent', () => {
      const rating = 1500;
      const opponentRating = 1500;
      const actualScore = 1; // Win
      
      const change = calculateEloChange(rating, opponentRating, actualScore, K_FACTOR);
      
      expect(change).toBeGreaterThan(0);
      // For equal opponents, expected = 0.5, so change = K_FACTOR * (1 - 0.5) = K_FACTOR / 2
      expect(change).toBeCloseTo(K_FACTOR / 2, 0);
    });

    test('should return negative change for loss against equal opponent', () => {
      const rating = 1500;
      const opponentRating = 1500;
      const actualScore = 0; // Loss
      
      const change = calculateEloChange(rating, opponentRating, actualScore, K_FACTOR);
      
      expect(change).toBeLessThan(0);
      // For equal opponents, expected = 0.5, so change = K_FACTOR * (0 - 0.5) = -K_FACTOR / 2
      expect(change).toBeCloseTo(-K_FACTOR / 2, 0);
    });

    test('should return zero change for draw against equal opponent', () => {
      const rating = 1500;
      const opponentRating = 1500;
      const actualScore = 0.5; // Draw
      
      const change = calculateEloChange(rating, opponentRating, actualScore, K_FACTOR);
      
      expect(change).toBeCloseTo(0, 1);
    });

    test('should return larger positive change for win against stronger opponent', () => {
      const rating = 1500;
      const opponentRating = 1800; // Stronger
      const actualScore = 1; // Win (upset)
      
      const change = calculateEloChange(rating, opponentRating, actualScore, K_FACTOR);
      
      // Should gain more than K_FACTOR / 2 for upset win (since expected score is very low)
      expect(change).toBeGreaterThan(K_FACTOR / 2);
    });

    test('should return larger negative change for loss against weaker opponent', () => {
      const rating = 1800;
      const opponentRating = 1500; // Weaker
      const actualScore = 0; // Loss (upset)
      
      const change = calculateEloChange(rating, opponentRating, actualScore, K_FACTOR);
      
      // Should lose more than K_FACTOR / 2 for upset loss (since expected score is very high)
      expect(change).toBeLessThan(-K_FACTOR / 2);
    });
  });

  describe('calculateTeamElo', () => {
    test('should average team member ratings', () => {
      const player1Rating = 1600;
      const player2Rating = 1400;
      
      const teamRating = calculateTeamElo(player1Rating, player2Rating);
      
      expect(teamRating).toBe(1500);
    });

    test('should handle different team compositions', () => {
      const team1 = calculateTeamElo(1500, 1500);
      const team2 = calculateTeamElo(1400, 1600);
      const team3 = calculateTeamElo(1200, 1800);
      
      // All should average to 1500
      expect(team1).toBe(1500);
      expect(team2).toBe(1500);
      expect(team3).toBe(1500);
    });
  });

  describe('Elo Change Distribution', () => {
    test('should conserve total rating points in system', () => {
      const player1Rating = 1500;
      const player2Rating = 1500;
      const player3Rating = 1500;
      const player4Rating = 1500;
      
      // Simulate a match where team 1 wins
      const team1Rating = calculateTeamElo(player1Rating, player2Rating);
      const team2Rating = calculateTeamElo(player3Rating, player4Rating);
      
      // Team 1 wins (score = 1, team 2 score = 0)
      const team1Change = calculateEloChange(team1Rating, team2Rating, 1, K_FACTOR);
      const team2Change = calculateEloChange(team2Rating, team1Rating, 0, K_FACTOR);
      
      // Split changes between team members
      const player1Change = team1Change / 2;
      const player2Change = team1Change / 2;
      const player3Change = team2Change / 2;
      const player4Change = team2Change / 2;
      
      // Total change should be zero (conservation of points)
      const totalChange = player1Change + player2Change + player3Change + player4Change;
      expect(totalChange).toBeCloseTo(0, 10);
    });

    test('should calculate correct ratings after match', () => {
      const p1Initial = 1500;
      const p2Initial = 1500;
      const p3Initial = 1500;
      const p4Initial = 1500;
      
      // Team 1 wins
      const team1Rating = calculateTeamElo(p1Initial, p2Initial);
      const team2Rating = calculateTeamElo(p3Initial, p4Initial);
      
      const team1Change = calculateEloChange(team1Rating, team2Rating, 1, K_FACTOR);
      const team2Change = calculateEloChange(team2Rating, team1Rating, 0, K_FACTOR);
      
      const p1Change = team1Change / 2;
      const p2Change = team1Change / 2;
      const p3Change = team2Change / 2;
      const p4Change = team2Change / 2;
      
      // After match
      const p1Final = p1Initial + p1Change;
      const p2Final = p2Initial + p2Change;
      const p3Final = p3Initial + p3Change;
      const p4Final = p4Initial + p4Change;
      
      // Winners should have higher ratings
      expect(p1Final).toBeGreaterThan(p1Initial);
      expect(p2Final).toBeGreaterThan(p2Initial);
      
      // Losers should have lower ratings
      expect(p3Final).toBeLessThan(p3Initial);
      expect(p4Final).toBeLessThan(p4Initial);
      
      // All final ratings should still be close to 1500
      expect(p1Final).toBeGreaterThan(1480);
      expect(p1Final).toBeLessThan(1520);
    });
  });

  describe('Rating Improvement Over Time', () => {
    test('should reflect consistent winning with rating increase', () => {
      let rating = 1500;
      const opponentRating = 1500;
      
      // Simulate 10 consecutive wins
      for (let i = 0; i < 10; i++) {
        rating += calculateEloChange(rating, opponentRating, 1, K_FACTOR);
      }
      
      // Rating should have increased significantly
      expect(rating).toBeGreaterThan(1600);
    });

    test('should reflect consistent losing with rating decrease', () => {
      let rating = 1500;
      const opponentRating = 1500;
      
      // Simulate 10 consecutive losses
      for (let i = 0; i < 10; i++) {
        rating += calculateEloChange(rating, opponentRating, 0, K_FACTOR);
      }
      
      // Rating should have decreased significantly
      expect(rating).toBeLessThan(1400);
    });
  });
});