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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 LTS provider but may also be used with other providers providing cryptographic services.

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package org.bouncycastle.bcpg;

public class FastCRC24
    extends CRC24
{
    private static final int[] TABLE0, TABLE8, TABLE16;

    static
    {
        int[] table0 = new int[256];
        int[] table8 = new int[256];
        int[] table16 = new int[256];

        int crc = 0x800000;
        for (int i = 1; i < 256; i <<= 1)
        {
            int carry = ((crc << 8) >> 31) & CRC24_POLY;
            crc = (crc << 1) ^ carry;

            for (int j = 0; j < i; ++j)
            {
                table0[i + j] = crc ^ table0[j];
            }
        }

        for (int i = 1; i < 256; ++i)
        {
            int crc0 = table0[i];
            int crc8 = ((crc0 & 0xFFFF) << 8) ^ table0[(crc0 >> 16) & 255];
            int crc16 = ((crc8 & 0xFFFF) << 8) ^ table0[(crc8 >> 16) & 255];

            table8[i] = crc8;
            table16[i] = crc16;
        }

        TABLE0 = table0;
        TABLE8 = table8;
        TABLE16 = table16;
    }

    /**
     * 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)
    {
        int index = (b ^ (crc >> 16)) & 255;
        crc = (crc << 8) ^ TABLE0[index];
    }

    public void update3(byte[] buf, int off)
    {
        crc = TABLE16[(buf[off + 0] ^ (crc >> 16)) & 255]
            ^ TABLE8[(buf[off + 1] ^ (crc >> 8)) & 255]
            ^ TABLE0[(buf[off + 2] ^ crc) & 255];
    }
}