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/*
 * Copyright 2013 The Netty Project
 *
 * The Netty Project licenses this file to you under the Apache License,
 * version 2.0 (the "License"); you may not use this file except in compliance
 * with the License. You may obtain a copy of the License at:
 *
 *   http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
 * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
 * License for the specific language governing permissions and limitations
 * under the License.
 */
package io.netty.handler.codec.compression;

import java.util.zip.Checksum;

/**
 * Implements CRC32-C as defined in:
 * "Optimization of Cyclic Redundancy-CHeck Codes with 24 and 32 Parity Bits",
 * IEEE Transactions on Communications 41(6): 883-892 (1993).
 *
 * The implementation of this class has been sourced from the Appendix of RFC 3309,
 * but with masking due to Java not being able to support unsigned types.
 */
class Crc32c implements Checksum {
    private static final int[] CRC_TABLE = {
            0x00000000, 0xF26B8303, 0xE13B70F7, 0x1350F3F4,
            0xC79A971F, 0x35F1141C, 0x26A1E7E8, 0xD4CA64EB,
            0x8AD958CF, 0x78B2DBCC, 0x6BE22838, 0x9989AB3B,
            0x4D43CFD0, 0xBF284CD3, 0xAC78BF27, 0x5E133C24,
            0x105EC76F, 0xE235446C, 0xF165B798, 0x030E349B,
            0xD7C45070, 0x25AFD373, 0x36FF2087, 0xC494A384,
            0x9A879FA0, 0x68EC1CA3, 0x7BBCEF57, 0x89D76C54,
            0x5D1D08BF, 0xAF768BBC, 0xBC267848, 0x4E4DFB4B,
            0x20BD8EDE, 0xD2D60DDD, 0xC186FE29, 0x33ED7D2A,
            0xE72719C1, 0x154C9AC2, 0x061C6936, 0xF477EA35,
            0xAA64D611, 0x580F5512, 0x4B5FA6E6, 0xB93425E5,
            0x6DFE410E, 0x9F95C20D, 0x8CC531F9, 0x7EAEB2FA,
            0x30E349B1, 0xC288CAB2, 0xD1D83946, 0x23B3BA45,
            0xF779DEAE, 0x05125DAD, 0x1642AE59, 0xE4292D5A,
            0xBA3A117E, 0x4851927D, 0x5B016189, 0xA96AE28A,
            0x7DA08661, 0x8FCB0562, 0x9C9BF696, 0x6EF07595,
            0x417B1DBC, 0xB3109EBF, 0xA0406D4B, 0x522BEE48,
            0x86E18AA3, 0x748A09A0, 0x67DAFA54, 0x95B17957,
            0xCBA24573, 0x39C9C670, 0x2A993584, 0xD8F2B687,
            0x0C38D26C, 0xFE53516F, 0xED03A29B, 0x1F682198,
            0x5125DAD3, 0xA34E59D0, 0xB01EAA24, 0x42752927,
            0x96BF4DCC, 0x64D4CECF, 0x77843D3B, 0x85EFBE38,
            0xDBFC821C, 0x2997011F, 0x3AC7F2EB, 0xC8AC71E8,
            0x1C661503, 0xEE0D9600, 0xFD5D65F4, 0x0F36E6F7,
            0x61C69362, 0x93AD1061, 0x80FDE395, 0x72966096,
            0xA65C047D, 0x5437877E, 0x4767748A, 0xB50CF789,
            0xEB1FCBAD, 0x197448AE, 0x0A24BB5A, 0xF84F3859,
            0x2C855CB2, 0xDEEEDFB1, 0xCDBE2C45, 0x3FD5AF46,
            0x7198540D, 0x83F3D70E, 0x90A324FA, 0x62C8A7F9,
            0xB602C312, 0x44694011, 0x5739B3E5, 0xA55230E6,
            0xFB410CC2, 0x092A8FC1, 0x1A7A7C35, 0xE811FF36,
            0x3CDB9BDD, 0xCEB018DE, 0xDDE0EB2A, 0x2F8B6829,
            0x82F63B78, 0x709DB87B, 0x63CD4B8F, 0x91A6C88C,
            0x456CAC67, 0xB7072F64, 0xA457DC90, 0x563C5F93,
            0x082F63B7, 0xFA44E0B4, 0xE9141340, 0x1B7F9043,
            0xCFB5F4A8, 0x3DDE77AB, 0x2E8E845F, 0xDCE5075C,
            0x92A8FC17, 0x60C37F14, 0x73938CE0, 0x81F80FE3,
            0x55326B08, 0xA759E80B, 0xB4091BFF, 0x466298FC,
            0x1871A4D8, 0xEA1A27DB, 0xF94AD42F, 0x0B21572C,
            0xDFEB33C7, 0x2D80B0C4, 0x3ED04330, 0xCCBBC033,
            0xA24BB5A6, 0x502036A5, 0x4370C551, 0xB11B4652,
            0x65D122B9, 0x97BAA1BA, 0x84EA524E, 0x7681D14D,
            0x2892ED69, 0xDAF96E6A, 0xC9A99D9E, 0x3BC21E9D,
            0xEF087A76, 0x1D63F975, 0x0E330A81, 0xFC588982,
            0xB21572C9, 0x407EF1CA, 0x532E023E, 0xA145813D,
            0x758FE5D6, 0x87E466D5, 0x94B49521, 0x66DF1622,
            0x38CC2A06, 0xCAA7A905, 0xD9F75AF1, 0x2B9CD9F2,
            0xFF56BD19, 0x0D3D3E1A, 0x1E6DCDEE, 0xEC064EED,
            0xC38D26C4, 0x31E6A5C7, 0x22B65633, 0xD0DDD530,
            0x0417B1DB, 0xF67C32D8, 0xE52CC12C, 0x1747422F,
            0x49547E0B, 0xBB3FFD08, 0xA86F0EFC, 0x5A048DFF,
            0x8ECEE914, 0x7CA56A17, 0x6FF599E3, 0x9D9E1AE0,
            0xD3D3E1AB, 0x21B862A8, 0x32E8915C, 0xC083125F,
            0x144976B4, 0xE622F5B7, 0xF5720643, 0x07198540,
            0x590AB964, 0xAB613A67, 0xB831C993, 0x4A5A4A90,
            0x9E902E7B, 0x6CFBAD78, 0x7FAB5E8C, 0x8DC0DD8F,
            0xE330A81A, 0x115B2B19, 0x020BD8ED, 0xF0605BEE,
            0x24AA3F05, 0xD6C1BC06, 0xC5914FF2, 0x37FACCF1,
            0x69E9F0D5, 0x9B8273D6, 0x88D28022, 0x7AB90321,
            0xAE7367CA, 0x5C18E4C9, 0x4F48173D, 0xBD23943E,
            0xF36E6F75, 0x0105EC76, 0x12551F82, 0xE03E9C81,
            0x34F4F86A, 0xC69F7B69, 0xD5CF889D, 0x27A40B9E,
            0x79B737BA, 0x8BDCB4B9, 0x988C474D, 0x6AE7C44E,
            0xBE2DA0A5, 0x4C4623A6, 0x5F16D052, 0xAD7D5351,
    };

    private static final long LONG_MASK = 0xFFFFFFFFL;
    private static final int BYTE_MASK = 0xFF;

    private int crc = ~0;

    @Override
    public void update(int b) {
        crc = crc32c(crc, b);
    }

    @Override
    public void update(byte[] buffer, int offset, int length) {
        for (int i = offset; i < offset + length; i++) {
            crc = crc32c(crc, buffer[i]);
        }
    }

    @Override
    public long getValue() {
        return (crc ^ LONG_MASK) & LONG_MASK;
    }

    @Override
    public void reset() {
        crc = ~0;
    }

    private static int crc32c(int crc, int b) {
        return crc >>> 8 ^ CRC_TABLE[(crc ^ b & BYTE_MASK) & BYTE_MASK];
    }
}