LFSR.java

package com.thealgorithms.ciphers.a5;

import java.util.BitSet;

public class LFSR implements BaseLFSR {
    private final BitSet register;
    private final int length;
    private final int clockBitIndex;
    private final int[] tappingBitsIndices;

    public LFSR( int length, int clockBitIndex, int[] tappingBitsIndices ) {
        this.length = length;
        this.clockBitIndex = clockBitIndex;
        this.tappingBitsIndices = tappingBitsIndices;
        register = new BitSet( length );
    }

    @Override
    public void initialize( BitSet sessionKey, BitSet frameCounter ) {
        register.clear();
        clock( sessionKey, SESSION_KEY_LENGTH );
        clock( frameCounter, FRAME_COUNTER_LENGTH );
    }

    private void clock( BitSet key, int keyLength ) {
        // We start from reverse because LFSR 0 index is the left most bit
        // while key 0 index is right most bit, so we reverse it
        for ( int i = keyLength - 1; i >= 0; --i ) {
            var newBit = key.get( i ) ^ xorTappingBits();
            pushBit( newBit );
        }
    }

    @Override
    public boolean clock() {
        return pushBit( xorTappingBits() );
    }

    public boolean getClockBit() {
        return register.get( clockBitIndex );
    }

    public boolean get( int bitIndex ) {
        return register.get( bitIndex );
    }

    public boolean getLastBit() {
        return register.get( length - 1 );
    }

    private boolean xorTappingBits() {
        boolean result = false;
        for ( int i : tappingBitsIndices ) {
            result ^= register.get( i );
        }
        return result;
    }

    private boolean pushBit( boolean bit ) {
        boolean discardedBit = rightShift();
        register.set( 0, bit );
        return discardedBit;
    }

    private boolean rightShift() {
        boolean discardedBit = get( length - 1 );
        for ( int i = length - 1; i > 0; --i ) {
            register.set( i, get( i - 1 ) );
        }
        register.set( 0, false );
        return discardedBit;
    }

    @Override
    public String toString() {
        return register.toString();
    }
}