apollo/src/net/burtleburtle/bob/rand/IsaacRandom.java

package net.burtleburtle.bob.rand;

/**
 * <p>
 * An implementation of the <a href="http://www.burtleburtle.net/bob/rand/isaacafa.html">ISAAC</a> psuedorandom number
 * generator.
 * </p>
 *
 * <pre>
 * ------------------------------------------------------------------------------
 * Rand.java: By Bob Jenkins.  My random number generator, ISAAC.
 *   rand.init() -- initialize
 *   rand.val()  -- get a random value
 * MODIFIED:
 *   960327: Creation (addition of randinit, really)
 *   970719: use context, not global variables, for internal state
 *   980224: Translate to Java
 * ------------------------------------------------------------------------------
 * </pre>
 * <p>
 * This class has been changed to be more conformant to Java and javadoc conventions.
 * </p>
 *
 * @author Bob Jenkins
 */
public final class IsaacRandom {

	/**
	 * The golden ratio.
	 */
	private static final int GOLDEN_RATIO = 0x9e3779b9;

	/**
	 * The log of the size of the result and memory arrays.
	 */
	private static final int LOG_SIZE = 8;

	/**
	 * The size of the result and memory arrays.
	 */
	private static final int SIZE = 1 << LOG_SIZE;

	/**
	 * A mask for pseudorandom lookup.
	 */
	private static int MASK = SIZE - 1 << 2;

	/**
	 * The accumulator.
	 */
	private int a;

	/**
	 * The last result.
	 */
	private int b;

	/**
	 * The counter.
	 */
	private int c;

	/**
	 * The count through the results in the results array.
	 */
	private int count;

	/**
	 * The internal state.
	 */
	private final int[] mem;

	/**
	 * The results given to the user.
	 */
	private final int[] rsl;

	/**
	 * Creates the random number generator without an initial seed.
	 */
	public IsaacRandom() {
		mem = new int[SIZE];
		rsl = new int[SIZE];
		init(false);
	}

	/**
	 * Creates the random number generator with the specified seed.
	 *
	 * @param seed The seed.
	 */
	public IsaacRandom(int[] seed) {
		mem = new int[SIZE];
		rsl = new int[SIZE];
		for (int i = 0; i < seed.length; ++i) {
			rsl[i] = seed[i];
		}
		init(true);
	}

	/**
	 * Initialises this random number generator.
	 *
	 * @param hasSeed Set to {@code true} if a seed was passed to the constructor.
	 */
	private void init(boolean hasSeed) {
		int i;
		int a, b, c, d, e, f, g, h;
		a = b = c = d = e = f = g = h = GOLDEN_RATIO;

		for (i = 0; i < 4; ++i) {
			a ^= b << 11;
			d += a;
			b += c;
			b ^= c >>> 2;
			e += b;
			c += d;
			c ^= d << 8;
			f += c;
			d += e;
			d ^= e >>> 16;
			g += d;
			e += f;
			e ^= f << 10;
			h += e;
			f += g;
			f ^= g >>> 4;
			a += f;
			g += h;
			g ^= h << 8;
			b += g;
			h += a;
			h ^= a >>> 9;
			c += h;
			a += b;
		}

		for (i = 0; i < SIZE; i += 8) { /* fill in mem[] with messy stuff */
			if (hasSeed) {
				a += rsl[i];
				b += rsl[i + 1];
				c += rsl[i + 2];
				d += rsl[i + 3];
				e += rsl[i + 4];
				f += rsl[i + 5];
				g += rsl[i + 6];
				h += rsl[i + 7];
			}
			a ^= b << 11;
			d += a;
			b += c;
			b ^= c >>> 2;
			e += b;
			c += d;
			c ^= d << 8;
			f += c;
			d += e;
			d ^= e >>> 16;
			g += d;
			e += f;
			e ^= f << 10;
			h += e;
			f += g;
			f ^= g >>> 4;
			a += f;
			g += h;
			g ^= h << 8;
			b += g;
			h += a;
			h ^= a >>> 9;
			c += h;
			a += b;
			mem[i] = a;
			mem[i + 1] = b;
			mem[i + 2] = c;
			mem[i + 3] = d;
			mem[i + 4] = e;
			mem[i + 5] = f;
			mem[i + 6] = g;
			mem[i + 7] = h;
		}

		if (hasSeed) { /* second pass makes all of seed affect all of mem */
			for (i = 0; i < SIZE; i += 8) {
				a += mem[i];
				b += mem[i + 1];
				c += mem[i + 2];
				d += mem[i + 3];
				e += mem[i + 4];
				f += mem[i + 5];
				g += mem[i + 6];
				h += mem[i + 7];
				a ^= b << 11;
				d += a;
				b += c;
				b ^= c >>> 2;
				e += b;
				c += d;
				c ^= d << 8;
				f += c;
				d += e;
				d ^= e >>> 16;
				g += d;
				e += f;
				e ^= f << 10;
				h += e;
				f += g;
				f ^= g >>> 4;
				a += f;
				g += h;
				g ^= h << 8;
				b += g;
				h += a;
				h ^= a >>> 9;
				c += h;
				a += b;
				mem[i] = a;
				mem[i + 1] = b;
				mem[i + 2] = c;
				mem[i + 3] = d;
				mem[i + 4] = e;
				mem[i + 5] = f;
				mem[i + 6] = g;
				mem[i + 7] = h;
			}
		}

		isaac();
		count = SIZE;
	}

	/**
	 * Generates 256 results.
	 */
	private void isaac() {
		int i, j, x, y;

		b += ++c;
		for (i = 0, j = SIZE / 2; i < SIZE / 2;) {
			x = mem[i];
			a ^= a << 13;
			a += mem[j++];
			mem[i] = y = mem[(x & MASK) >> 2] + a + b;
			rsl[i++] = b = mem[(y >> LOG_SIZE & MASK) >> 2] + x;

			x = mem[i];
			a ^= a >>> 6;
			a += mem[j++];
			mem[i] = y = mem[(x & MASK) >> 2] + a + b;
			rsl[i++] = b = mem[(y >> LOG_SIZE & MASK) >> 2] + x;

			x = mem[i];
			a ^= a << 2;
			a += mem[j++];
			mem[i] = y = mem[(x & MASK) >> 2] + a + b;
			rsl[i++] = b = mem[(y >> LOG_SIZE & MASK) >> 2] + x;

			x = mem[i];
			a ^= a >>> 16;
			a += mem[j++];
			mem[i] = y = mem[(x & MASK) >> 2] + a + b;
			rsl[i++] = b = mem[(y >> LOG_SIZE & MASK) >> 2] + x;
		}

		for (j = 0; j < SIZE / 2;) {
			x = mem[i];
			a ^= a << 13;
			a += mem[j++];
			mem[i] = y = mem[(x & MASK) >> 2] + a + b;
			rsl[i++] = b = mem[(y >> LOG_SIZE & MASK) >> 2] + x;

			x = mem[i];
			a ^= a >>> 6;
			a += mem[j++];
			mem[i] = y = mem[(x & MASK) >> 2] + a + b;
			rsl[i++] = b = mem[(y >> LOG_SIZE & MASK) >> 2] + x;

			x = mem[i];
			a ^= a << 2;
			a += mem[j++];
			mem[i] = y = mem[(x & MASK) >> 2] + a + b;
			rsl[i++] = b = mem[(y >> LOG_SIZE & MASK) >> 2] + x;

			x = mem[i];
			a ^= a >>> 16;
			a += mem[j++];
			mem[i] = y = mem[(x & MASK) >> 2] + a + b;
			rsl[i++] = b = mem[(y >> LOG_SIZE & MASK) >> 2] + x;
		}
	}

	/**
	 * Gets the next random value.
	 *
	 * @return The next random value.
	 */
	public int nextInt() {
		if (0 == count--) {
			isaac();
			count = SIZE - 1;
		}
		return rsl[count];
	}

}