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The Bouncy Castle Java APIs for the OpenPGP Protocol. The APIs are designed primarily to be used in conjunction with the BC FIPS provider. The APIs may also be used with other providers although if being used in a FIPS context it is the responsibility of the user to ensure that any other providers used are FIPS certified and used appropriately.

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package org.bouncycastle.bcpg;

public class FastCRC24
    extends CRC24
{
    private static final int[] TABLE = getTable();

    /**
     * Fast CRC-24 implementation using a lookup table to handle multiple bits at a time.
     * 
     * Compare: Sarwate, Dilip V. "Computation of cyclic redundancy checks via table look-up."
     */
    public FastCRC24()
    {
    }

    public void update(int b)
    {
        // b^(crc>>16) mod 256
        int index = (b ^ (crc >> 16)) & ~-256;
        crc = (crc << 8) ^ TABLE[index];
    }

    /**
     * Lazily init and return the lookup table.
     *
     * @return lookup table
     */
    private static int[] getTable()
    {
        int[] TABLE = new int[256];
        int crc = 0x800000;
        int i = 1;
        while (i != 256)
        {
            if ((crc & 0x800000) > 0)
            {
                crc = (crc << 1) ^ CRC24_POLY;
            }
            else
            {
                crc <<= 1;
            }

            for (int j = 0; j < i; j++)
            {
                TABLE[i + j] = crc ^ TABLE[j];
            }
            i <<= 1;
        }
        return TABLE;
    }
}