Birthday Problem simulation

birthday.js

const num_days = 16_000_000;
const num_participants = 100_000;

const samples = 100_000;
let collisions = 0;

const percentageFormatter = new Intl.NumberFormat('en-US', {
    style: 'percent',
    maximumFractionDigits: 3,
});

for (let i = 0; i < samples; i++) {
    const birthdays = new Set();
    for (let j = 0; j < num_participants; j++) {
        const birthday = Math.floor(Math.random() * num_days);
        if (birthdays.has(birthday)) {
            collisions++;
            break;
        }
        birthdays.add(birthday);
    }

    if ((i + 1) % 1000 === 0) {
        console.log(`Sample ${i + 1} out of ${samples} - ${collisions} collisions so far (${percentageFormatter.format(collisions / (i + 1))})`);
    }
}

console.log(`Collision probability: ${percentageFormatter.format(collisions / samples)} (sample size: ${samples})`);