Calculate ELO

eloCalculator.ts

const K_FACTOR = 20;
const RATING_SCALING = 400;

interface Player {
  isWin: boolean;
  elo: number;
}

interface Result {
  firstPlayer: Player;
  secondPlayer: Player;
}

function calculateElo({ firstPlayer, secondPlayer }: Result): [number, number] {
  const expectedOutcomeFirst = 1 / (1 + 10 ** ((secondPlayer.elo - firstPlayer.elo) / RATING_SCALING));
  const expectedOutcomeSecond = 1 - expectedOutcomeFirst;

  const actualOutcomeFirst = firstPlayer.isWin ? 1 : 0;
  const actualOutcomeSecond = secondPlayer.isWin ? 1 : 0;

  const newEloFirst = firstPlayer.elo + K_FACTOR * (actualOutcomeFirst - expectedOutcomeFirst);
  const newEloSecond = secondPlayer.elo + K_FACTOR * (actualOutcomeSecond - expectedOutcomeSecond);

  return [newEloFirst, newEloSecond];
}

function predictOutcome(firstPlayerElo: number, secondPlayerElo: number): [number, number] {
  const expectedOutcomeFirst = 1 / (1 + 10 ** ((secondPlayerElo - firstPlayerElo) / RATING_SCALING));
  const expectedOutcomeSecond = 1 - expectedOutcomeFirst;

  return [expectedOutcomeFirst, expectedOutcomeSecond];
}

async function main() {
  const firstPlayerScore = 1000
  const secondPlayerScore = 1026;

  const [newEloFirstPlayer, newEloSecondPlayer] = calculateElo({
    firstPlayer: {
      isWin: true,
      elo: firstPlayerScore,
    },
    secondPlayer: {
      isWin: false,
      elo: secondPlayerScore,
    },
  });

  console.log("Updated Elo Ratings:");
  console.log(`First Player: ${newEloFirstPlayer}`);
  console.log(`Second Player: ${newEloSecondPlayer}`);

  const [expectedOutcomeFirst, expectedOutcomeSecond] = predictOutcome(newEloFirstPlayer, newEloSecondPlayer);
  console.log("Expected Outcome:");
  console.log(`First Player: ${expectedOutcomeFirst}`);
  console.log(`Second Player: ${expectedOutcomeSecond}`);
}

main();