//Copyright 2011 Google Inc. All rights reserved.
package com.google.cloud;

import java.util.zip.Checksum;

/**
* This class generates a CRC32C checksum, defined by rfc3720 section B.4.
*
*
*/
public final class Crc32c implements Checksum {

private static final long[] 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 long BYTE_MASK = 0xff;

private long crc;

public Crc32c() {
 crc = 0;
}

/**
* Updates the checksum with a new byte.
* @param b the new byte.
*/
@Override
public void update(int b) {
 long newCrc = crc ^ LONG_MASK;
 newCrc = updateByte((byte) b, newCrc);
 crc = newCrc ^ LONG_MASK;
}

/**
* Updates the checksum with an array of bytes.
* @param bArray the array of bytes.
* @param off the offset into the array where the update should begin.
* @param len the length of data to examine.
*/
@Override
public void update(byte[] bArray, int off, int len) {
 long newCrc = crc ^ LONG_MASK;
 for (int i = off; i < off + len; i++) {
   newCrc = updateByte(bArray[i], newCrc);
 }
 crc = newCrc ^ LONG_MASK;
}

/**
* Returns the value of the checksum.
* @return the long representation of the checksum (high bits set to zero).
*/
@Override
public long getValue() {
 return crc;
}

/**
* Returns the value of the checksum.
* @return the 4-byte array representation of the checksum in network byte order (big endian).
*/
public byte[] getValueAsBytes() {
 long value = crc;
 byte[] result = new byte[4];
 for (int i = 3; i >= 0; i--) {
   result[i] = (byte) (value & 0xffL);
   value >>= 8;
 }
 return result;
}

/**
* Resets the crc.
*/
@Override
public void reset() {
 crc = 0;
}

private long updateByte(byte newByte, long crc) {
 byte b = (byte) (newByte & BYTE_MASK);
 int index = (int) ((crc ^ b) & BYTE_MASK);
 return (CRC_TABLE[index] ^ (crc >> 8)) & LONG_MASK;
}
}