Generate a random binary number with a variable proportion of '1' bits

Question

I need a function to generate random integers. (assume Java long type for now, but this will be extended to BigInteger or BitSet later

Answer 1

Distribute proportional number of bits throughuot the number. Pseudocode:

long generateNumber( double probability ){
  int bitCount = 64 * probability;
  byte[] data = new byte[64]; // 0-filled

  long indexes = getRandomLong();

  for 0 to bitCount-1 {
    do { 
      // distribute this bit to some postition with 0.
      int index = indexes & 64;
      indexes >> 6;
      if( indexes == 0 ) indexes = getRandomLong();
    } while ( data[index] == 0 );
    data[index] = 1;
  }

  return bytesToLong( data );
}    

I hope you get what I mean. Perhaps the byte[] could be replaced with a long and bit operations to make it faster.