java 伽罗瓦线性反馈移位寄存器(LFSR)
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/*
* Galois linear feedback shift register (LFSR) in Java
* Copyright (c) 2012 Nayuki Minase
*/
import java.math.BigInteger;
import java.util.Random;
/**
* Galois LFSR random number generator. This can be used in place of java.util.Random.
* In this class, a polynomial is represented as a BigInteger, where the coefficient of
* the <var>x</var><sup><var>k</var></sup> term is represented by bit <var>k</var>.
*/
public class LfsrRandom extends Random {
private BigInteger characteristic;
private int degree;
private BigInteger state;
/**
* Constructs an LFSR with the specified characteristic polynomial and initial state polynomial.
* The characteristic polynomial must have degree at least 2. The state polynomial must have degree
* less than the degree of the characteristic polynomial, and must not be the zero polynomial.
* @param charistic the characteristic polynomial
* @param state the initial state polynomial
* @throws IllegalArgumentException if the polynomials do not satisfy the requirements
*/
public LfsrRandom(BigInteger charis, BigInteger state) {
if (charis.signum() == -1)
throw new IllegalArgumentException("Invalid characteristic polynomial - negative");
if (charis.bitLength() < 2)
throw new IllegalArgumentException("Invalid characteristic polynomial - degree too low");
if (state.equals(BigInteger.ZERO))
throw new IllegalArgumentException("Invalid state polynomial - all zero");
if (state.bitLength() >= charis.bitLength())
throw new IllegalArgumentException("Invalid state polynomial - degree >= char poly degree");
characteristic = charis;
degree = charis.bitLength() - 1;
this.state = state;
}
public boolean nextBoolean() {
boolean result = state.testBit(0); // Use bit 0 in the LFSR state as the result
state = state.shiftLeft(1); // Multiply by x
if (state.testBit(degree)) // If degree of state polynomial matches degree of characteristic polynomial
state = state.xor(characteristic); // Then subtract the characteristic polynomial from the state polynomial
return result;
}
protected int next(int bits) {
int result = 0;
for (int i = 0; i < 32; i++)
result = (result << 1) | (nextBoolean() ? 1 : 0);
return result;
}
public void printDebug() {
StringBuilder sb = new StringBuilder();
sb.append("characteristic: degree=").append(degree).append(" poly = ");
boolean head = true;
for (int i = 0; i <= degree; i++) {
if (characteristic.testBit(i)) {
if (head) head = false;
else sb.append(" + ");
sb.append("x^" + i);
}
}
sb.append(System.getProperty("line.separator")).append("state: degree=").append(state.bitLength() - 1).append(" poly = ");
head = true;
for (int i = 0; i <= degree; i++) {
if (state.testBit(i)) {
if (head) head = false;
else sb.append(" + ");
sb.append("x^" + i);
}
}
System.out.println(sb);
}
}
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