// CS 1538 Fall 2009
// This is a mixed linear congruetial generator taken from an old
// CS 0445 textbook (Data Structures in C++ by Ford and Topp -- I
// changed some things to make it work in Java but it is essentially the
// same as the generator presented in the Ford and Topp textbook).

// Note that value returned by this generator is only 16 bits, giving
// it a maximum range of 2^16.  This was common with older C++ compilers in
// which a long int was only 32 bits (and in fact many implementations of
// the rand() function in C/C++ still have a range of 2^16).  It is well-known
// that the lower-order bits in a linear congruential generator with a modulus
// of 2^b do not have a very good "random" distribution.  However, a modulus
// of 2^b is easier and faster to calculate than other moduli, so it is
// frequently used.  Thus, most linear congruential generators of this type
// will shift out the lower order bits so that the "usable" range of results
// is actually smaller than the entire range of the generator.

// If you examine m, a and c, you will see that they meet the criteria for a
// full-period generator. The shortcomings of the smaller range can be seen
// when the desired subrange of values is a significant (but not divisible)
// part of the overall range.  In this case, assuming that the original value
// generated is uniform, when it is "mapped" onto the desired subrange the
// probabilities will no longer be uniform.  See notes on board for more
// details.

// Also note the last method, bogusInt.  This uses all of the 32 bits
// generated and will likely have very poor results, due to the lack of
// "randomness" of those lower order bits.  However, note that this poor
// behavior may not necessarily be detected by a uniformity test such
// as Chi-Square.  We need to also test for independence.

public class Rand1
{

	private final long mask  = (1L << 32) - 1; 		// this is "m" = 2^32
	private final long multiplier = 1194211693L; 	// this is "a"
	private final long adder = 12345L;				// this is "c"
    private long randSeed;

    public Rand1()
    {
		randSeed = System.currentTimeMillis();
	}

    public Rand1(long s)
    {
		randSeed = s;
	}

    public int nextShort(int n)
	{
		randSeed = (multiplier * randSeed + adder) & mask;
		int shortAns = Math.abs((int) randSeed >> 16);
		return (shortAns % n);
	}

    public int bogusInt(int n)
	{
		randSeed = (multiplier * randSeed + adder) & mask;
		int intAns = Math.abs((int)randSeed);
		return (intAns % n);
	}

}