CavalliumDBEngine/src/main/java/it/cavallium/dbengine/utils/LFSR.java
2022-05-10 00:31:16 +02:00

137 lines
3.6 KiB
Java

package it.cavallium.dbengine.utils;
import java.math.BigInteger;
import java.util.Iterator;
import java.util.Random;
import org.jetbrains.annotations.NotNull;
/**
* Linear feedback shift register
* <p>
* Taps can be found at: See http://www.xilinx.com/support/documentation/application_notes/xapp052.pdf See
* http://mathoverflow.net/questions/46961/how-are-taps-proven-to-work-for-lfsrs/46983#46983 See
* http://www.newwaveinstruments.com/resources/articles/m_sequence_linear_feedback_shift_register_lfsr.htm See
* http://www.yikes.com/~ptolemy/lfsr_web/index.htm See http://seanerikoconnor.freeservers.com/Mathematics/AbstractAlgebra/PrimitivePolynomials/overview.html
*
* @author OldCurmudgeon
*/
public class LFSR implements Iterable<BigInteger> {
private static final Random random = new Random();
// Bit pattern for taps.
private final BigInteger taps;
// Where to start (and end).
private final BigInteger start;
public static LFSR randomInt() {
return random(32, random.nextInt());
}
public static LFSR randomLong() {
return random(64, random.nextLong());
}
public static LFSR randomPositiveLong() {
return random(50, Math.abs(random.nextInt()));
}
public static BigInteger randomPrimitive(int bitsSize) {
// Build the BigInteger.
BigInteger primitive = BigInteger.ZERO;
for (int bitNumber = 0; bitNumber <= bitsSize; bitNumber++) {
if (random.nextBoolean() || bitNumber == 0 || bitNumber == bitsSize) {
primitive = primitive.or(BigInteger.ONE.shiftLeft(bitNumber));
}
}
return primitive;
}
public static LFSR random(int bitsSize, long startNumber) {
return new LFSR(randomPrimitive(bitsSize), BigInteger.valueOf(startNumber));
}
// The poly must be primitive to span the full sequence.
public LFSR(BigInteger primitivePoly, BigInteger start) {
// Where to start from (and stop).
this.start = start.equals(BigInteger.ZERO) ? BigInteger.ONE : start;
// Knock off the 2^0 coefficient of the polynomial for the TAP.
this.taps = primitivePoly.shiftRight(1);
}
@NotNull
@Override
public LFSRIterator iterator() {
return new LFSRIterator(start);
}
public class LFSRIterator implements Iterator<BigInteger> {
// The last one we returned.
private BigInteger last = null;
// The next one to return.
private BigInteger next = null;
public LFSRIterator(BigInteger start) {
// Do not return the seed.
last = start;
}
@Override
public boolean hasNext() {
if (next == null) {
/*
* Uses the Galois form.
*
* Shift last right one.
*
* If the bit shifted out was a 1 - xor with the tap mask.
*/
boolean shiftedOutA1 = last.testBit(0);
// Shift right.
next = last.shiftRight(1);
if (shiftedOutA1) {
// Tap!
next = next.xor(taps);
}
// Never give them `start` again.
if (next.equals(start)) {
// Could set a finished flag here too.
next = null;
}
}
return next != null;
}
@Override
public BigInteger next() {
// Remember this one.
last = hasNext() ? next : null;
// Don't deliver it again.
next = null;
return last;
}
public BigInteger next(BigInteger last) {
this.last = last;
next = null;
return next();
}
@Override
public void remove() {
throw new UnsupportedOperationException("Not supported.");
}
@Override
public String toString() {
return LFSR.this.toString() + "[" + (last != null ? last.toString(16) : "") + "-" + (next != null ? next
.toString(16) : "") + "]";
}
}
@Override
public String toString() {
return "(" + taps.toString(32) + ")-" + start.toString(32);
}
}