我需要对存储在分布式 FS 上的相当大的文件进行哈希处理。我能够以比整个文件更好的性能处理文件的各个部分，因此我希望能够计算各个部分的哈希值，然后对其求和。

我正在考虑将 CRC64 作为哈希算法，但我不知道如何使用其理论上的“线性函数”属性，因此我可以对文件的各个部分求和 CRC。有什么推荐吗？我在这里遗漏了什么吗？

我查看 CRC64 的附加说明:

• 我可以控制文件 block ，但由于应用程序的性质，它们需要具有不同的大小(最多 1 个字节，不可能有任何固定 block )。
• 我知道 CRC32 实现 (zlib)，它包括对部分 CRC 求和的方法，但我想要更广泛的东西。 8 个字节对我来说很不错。
• 我知道 CRC 非常快。我想从中获利，因为文件可能非常大(最多几 Gb)。

最佳答案

认为这通常足够有用，可以编写并提供:

/* crc64.c -- compute CRC-64
 * Copyright (C) 2013 Mark Adler
 * Version 1.4  16 Dec 2013  Mark Adler
 */

/*
  This software is provided 'as-is', without any express or implied
  warranty.  In no event will the author be held liable for any damages
  arising from the use of this software.

  Permission is granted to anyone to use this software for any purpose,
  including commercial applications, and to alter it and redistribute it
  freely, subject to the following restrictions:

  1. The origin of this software must not be misrepresented; you must not
     claim that you wrote the original software. If you use this software
     in a product, an acknowledgment in the product documentation would be
     appreciated but is not required.
  2. Altered source versions must be plainly marked as such, and must not be
     misrepresented as being the original software.
  3. This notice may not be removed or altered from any source distribution.

  Mark Adler
  madler@alumni.caltech.edu
 */

/* Compute CRC-64 in the manner of xz, using the ECMA-182 polynomial,
   bit-reversed, with one's complement pre and post processing.  Provide a
   means to combine separately computed CRC-64's. */

/* Version history:
   1.0  13 Dec 2013  First version
   1.1  13 Dec 2013  Fix comments in test code
   1.2  14 Dec 2013  Determine endianess at run time
   1.3  15 Dec 2013  Add eight-byte processing for big endian as well
                     Make use of the pthread library optional
   1.4  16 Dec 2013  Make once variable volatile for limited thread protection
 */

#include <stdio.h>
#include <inttypes.h>
#include <assert.h>

/* The include of pthread.h below can be commented out in order to not use the
   pthread library for table initialization.  In that case, the initialization
   will not be thread-safe.  That's fine, so long as it can be assured that
   there is only one thread using crc64(). */
#include <pthread.h>            /* link with -lpthread */

/* 64-bit CRC polynomial with these coefficients, but reversed:
    64, 62, 57, 55, 54, 53, 52, 47, 46, 45, 40, 39, 38, 37, 35, 33, 32,
    31, 29, 27, 24, 23, 22, 21, 19, 17, 13, 12, 10, 9, 7, 4, 1, 0 */
#define POLY UINT64_C(0xc96c5795d7870f42)

/* Tables for CRC calculation -- filled in by initialization functions that are
   called once.  These could be replaced by constant tables generated in the
   same way.  There are two tables, one for each endianess.  Since these are
   static, i.e. local, one should be compiled out of existence if the compiler
   can evaluate the endianess check in crc64() at compile time. */
static uint64_t crc64_little_table[8][256];
static uint64_t crc64_big_table[8][256];

/* Fill in the CRC-64 constants table. */
static void crc64_init(uint64_t table[][256])
{
    unsigned n, k;
    uint64_t crc;

    /* generate CRC-64's for all single byte sequences */
    for (n = 0; n < 256; n++) {
        crc = n;
        for (k = 0; k < 8; k++)
            crc = crc & 1 ? POLY ^ (crc >> 1) : crc >> 1;
        table[0][n] = crc;
    }

    /* generate CRC-64's for those followed by 1 to 7 zeros */
    for (n = 0; n < 256; n++) {
        crc = table[0][n];
        for (k = 1; k < 8; k++) {
            crc = table[0][crc & 0xff] ^ (crc >> 8);
            table[k][n] = crc;
        }
    }
}

/* This function is called once to initialize the CRC-64 table for use on a
   little-endian architecture. */
static void crc64_little_init(void)
{
    crc64_init(crc64_little_table);
}

/* Reverse the bytes in a 64-bit word. */
static inline uint64_t rev8(uint64_t a)
{
    uint64_t m;

    m = UINT64_C(0xff00ff00ff00ff);
    a = ((a >> 8) & m) | (a & m) << 8;
    m = UINT64_C(0xffff0000ffff);
    a = ((a >> 16) & m) | (a & m) << 16;
    return a >> 32 | a << 32;
}

/* This function is called once to initialize the CRC-64 table for use on a
   big-endian architecture. */
static void crc64_big_init(void)
{
    unsigned k, n;

    crc64_init(crc64_big_table);
    for (k = 0; k < 8; k++)
        for (n = 0; n < 256; n++)
            crc64_big_table[k][n] = rev8(crc64_big_table[k][n]);
}

/* Run the init() function exactly once.  If pthread.h is not included, then
   this macro will use a simple static state variable for the purpose, which is
   not thread-safe.  The init function must be of the type void init(void). */
#ifdef PTHREAD_ONCE_INIT
#  define ONCE(init) \
    do { \
        static pthread_once_t once = PTHREAD_ONCE_INIT; \
        pthread_once(&once, init); \
    } while (0)
#else
#  define ONCE(init) \
    do { \
        static volatile int once = 1; \
        if (once) { \
            if (once++ == 1) { \
                init(); \
                once = 0; \
            } \
            else \
                while (once) \
                    ; \
        } \
    } while (0)
#endif

/* Calculate a CRC-64 eight bytes at a time on a little-endian architecture. */
static inline uint64_t crc64_little(uint64_t crc, void *buf, size_t len)
{
    unsigned char *next = buf;

    ONCE(crc64_little_init);
    crc = ~crc;
    while (len && ((uintptr_t)next & 7) != 0) {
        crc = crc64_little_table[0][(crc ^ *next++) & 0xff] ^ (crc >> 8);
        len--;
    }
    while (len >= 8) {
        crc ^= *(uint64_t *)next;
        crc = crc64_little_table[7][crc & 0xff] ^
              crc64_little_table[6][(crc >> 8) & 0xff] ^
              crc64_little_table[5][(crc >> 16) & 0xff] ^
              crc64_little_table[4][(crc >> 24) & 0xff] ^
              crc64_little_table[3][(crc >> 32) & 0xff] ^
              crc64_little_table[2][(crc >> 40) & 0xff] ^
              crc64_little_table[1][(crc >> 48) & 0xff] ^
              crc64_little_table[0][crc >> 56];
        next += 8;
        len -= 8;
    }
    while (len) {
        crc = crc64_little_table[0][(crc ^ *next++) & 0xff] ^ (crc >> 8);
        len--;
    }
    return ~crc;
}

/* Calculate a CRC-64 eight bytes at a time on a big-endian architecture. */
static inline uint64_t crc64_big(uint64_t crc, void *buf, size_t len)
{
    unsigned char *next = buf;

    ONCE(crc64_big_init);
    crc = ~rev8(crc);
    while (len && ((uintptr_t)next & 7) != 0) {
        crc = crc64_big_table[0][(crc >> 56) ^ *next++] ^ (crc << 8);
        len--;
    }
    while (len >= 8) {
        crc ^= *(uint64_t *)next;
        crc = crc64_big_table[0][crc & 0xff] ^
              crc64_big_table[1][(crc >> 8) & 0xff] ^
              crc64_big_table[2][(crc >> 16) & 0xff] ^
              crc64_big_table[3][(crc >> 24) & 0xff] ^
              crc64_big_table[4][(crc >> 32) & 0xff] ^
              crc64_big_table[5][(crc >> 40) & 0xff] ^
              crc64_big_table[6][(crc >> 48) & 0xff] ^
              crc64_big_table[7][crc >> 56];
        next += 8;
        len -= 8;
    }
    while (len) {
        crc = crc64_big_table[0][(crc >> 56) ^ *next++] ^ (crc << 8);
        len--;
    }
    return ~rev8(crc);
}

/* Return the CRC-64 of buf[0..len-1] with initial crc, processing eight bytes
   at a time.  This selects one of two routines depending on the endianess of
   the architecture.  A good optimizing compiler will determine the endianess
   at compile time if it can, and get rid of the unused code and table.  If the
   endianess can be changed at run time, then this code will handle that as
   well, initializing and using two tables, if called upon to do so. */
uint64_t crc64(uint64_t crc, void *buf, size_t len)
{
    uint64_t n = 1;

    return *(char *)&n ? crc64_little(crc, buf, len) :
                         crc64_big(crc, buf, len);
}

#define GF2_DIM 64      /* dimension of GF(2) vectors (length of CRC) */

static uint64_t gf2_matrix_times(uint64_t *mat, uint64_t vec)
{
    uint64_t sum;

    sum = 0;
    while (vec) {
        if (vec & 1)
            sum ^= *mat;
        vec >>= 1;
        mat++;
    }
    return sum;
}

static void gf2_matrix_square(uint64_t *square, uint64_t *mat)
{
    unsigned n;

    for (n = 0; n < GF2_DIM; n++)
        square[n] = gf2_matrix_times(mat, mat[n]);
}

/* Return the CRC-64 of two sequential blocks, where crc1 is the CRC-64 of the
   first block, crc2 is the CRC-64 of the second block, and len2 is the length
   of the second block. */
uint64_t crc64_combine(uint64_t crc1, uint64_t crc2, uintmax_t len2)
{
    unsigned n;
    uint64_t row;
    uint64_t even[GF2_DIM];     /* even-power-of-two zeros operator */
    uint64_t odd[GF2_DIM];      /* odd-power-of-two zeros operator */

    /* degenerate case */
    if (len2 == 0)
        return crc1;

    /* put operator for one zero bit in odd */
    odd[0] = POLY;              /* CRC-64 polynomial */
    row = 1;
    for (n = 1; n < GF2_DIM; n++) {
        odd[n] = row;
        row <<= 1;
    }

    /* put operator for two zero bits in even */
    gf2_matrix_square(even, odd);

    /* put operator for four zero bits in odd */
    gf2_matrix_square(odd, even);

    /* apply len2 zeros to crc1 (first square will put the operator for one
       zero byte, eight zero bits, in even) */
    do {
        /* apply zeros operator for this bit of len2 */
        gf2_matrix_square(even, odd);
        if (len2 & 1)
            crc1 = gf2_matrix_times(even, crc1);
        len2 >>= 1;

        /* if no more bits set, then done */
        if (len2 == 0)
            break;

        /* another iteration of the loop with odd and even swapped */
        gf2_matrix_square(odd, even);
        if (len2 & 1)
            crc1 = gf2_matrix_times(odd, crc1);
        len2 >>= 1;

        /* if no more bits set, then done */
    } while (len2 != 0);

    /* return combined crc */
    crc1 ^= crc2;
    return crc1;
}

/* Test crc64() on vector[0..len-1] which should have CRC-64 crc.  Also test
   crc64_combine() on vector[] split in two. */
static void crc64_test(void *vector, size_t len, uint64_t crc)
{
    uint64_t crc1, crc2;

    /* test crc64() */
    crc1 = crc64(0, vector, len);
    if (crc1 ^ crc)
        printf("mismatch: %" PRIx64 ", should be %" PRIx64 "\n", crc1, crc);

    /* test crc64_combine() */
    crc1 = crc64(0, vector, (len + 1) >> 1);
    crc2 = crc64(0, vector + ((len + 1) >> 1), len >> 1);
    crc1 = crc64_combine(crc1, crc2, len >> 1);
    if (crc1 ^ crc)
        printf("mismatch: %" PRIx64 ", should be %" PRIx64 "\n", crc1, crc);
}

/* Test vectors. */
#define TEST1 "123456789"
#define TESTLEN1 9
#define TESTCRC1 UINT64_C(0x995dc9bbdf1939fa)
#define TEST2 "This is a test of the emergency broadcast system."
#define TESTLEN2 49
#define TESTCRC2 UINT64_C(0x27db187fc15bbc72)

int main(void)
{
    crc64_test(TEST1, TESTLEN1, TESTCRC1);
    crc64_test(TEST2, TESTLEN2, TESTCRC2);
    return 0;
}

关于java - 如何进行CRC64分布式计算(利用其线性特性)？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/20562546/