8215441: Increase uniformity of the distribution of BigIntegers constructed by BigInteger(int, Random)

[Douglas Surber]

I wrote the following simple test case to look at the uniformity of the distribution. I don't see any problem running it up to 4096 buckets. Admittedly  I did not do any statistical tests on the buckets but by eye they look uniformly distributed.


  public static void main(String[] args) throws Throwable {
    SecureRandom sr = SecureRandom.getInstance("SHA1PRNG");
    final int nBits = 4096;
    final int nBuckets = 128;
    final int count = 100000;
    Pair[] buckets = new Pair[nBuckets];
    BigInteger max = BigInteger.TWO.pow(nBits);
    BigInteger limit = max;
    BigInteger step = limit.divide(BigInteger.valueOf(buckets.length));
    for (int i = 0; i < buckets.length; i++) buckets[i] = new Pair(limit = limit.subtract(step));
    for (int i = 0; i < count; ++i) {
      // biased towards high numbers. never chooses below a high limit 
      BigInteger number = new BigInteger(nBits, sr);
      int j;
      for (j = 0; buckets[j].limit.compareTo(number) > 0; j++) {}
      buckets[j].count++;
    }
    for (int i = buckets.length; i > 0; i--) System.out.print(buckets[i-1].count + (i%8==0 ? "\n" : "\t"));
    System.out.println();
  }


Douglas