Birthday.java

import java.security.SecureRandom;

class Birthday {
    public static final int NUM_BITS = 8;
    public static final int MAX_ID = (1 << NUM_BITS);
    private static final SecureRandom random = new SecureRandom();

    public static void main(String[] args) {
        // 0.001 = 0.1%, 1 = 100%
        double targetProbability = 0.001d;
        long idCount = 1500;

        double invProbability = 1 / (1 - targetProbability);
        long twoD = MAX_ID * 2;

        double result = Math.sqrt(twoD * Math.log(invProbability));
        System.out.println("Largest ID: " + MAX_ID);
        System.out.println("Largest ID in Hex: " + Long.toHexString(MAX_ID));
        System.out.printf(
                "Collision probability of %f%% after %f IDs\n", targetProbability * 100, result);

        double exponent = (double) (idCount * (idCount - 1)) / 2;
        double base = (double) (MAX_ID - 1) / MAX_ID;

        double collisionProbability = (1d - Math.pow(base, exponent)) * 100;


        System.out.printf(
                "Collision probability after  %d IDs: %f%%\n", idCount, collisionProbability);
    }

    public static int generateId() {
        return random.nextInt(MAX_ID);
    }
}