import java.nio.charset.StandardCharsets;
import java.security.MessageDigest;
import java.util.Base64;
import java.util.HashSet;
 
class Test {
  public static void main (String[] args) throws java.lang.Exception {
 
    HashSet<String> set = new HashSet<>();
    int count = 0;
    for (int i = 0; i < 1000000; i++) {
      if (!set.add(hash(""+i))) {
        count++;
      }
    }
    System.out.println("Number of collisions: " + count);
  }
 
  static String hash(String str) throws Exception {
    MessageDigest sha256 = MessageDigest.getInstance("SHA-256");
    byte[] digest = sha256.digest(str.getBytes(StandardCharsets.UTF_8));
    return Base64.getEncoder().encodeToString(digest).substring(0, 7);
  }
}
 

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