XrdOucCRC32C.hh File Reference

#include <cstddef>
#include <cstdint>

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Functions
uint32_t	crc32c (uint32_t crc, void const *buf, size_t len)
uint32_t	crc32c_combine (uint32_t crcA, uint32_t crcB, size_t lengthB)
	merge two CRC32 such that result = crc32(dataB, lengthB, crc32(dataA, lengthA)) More...
uint32_t	crc32c_sw (uint32_t crc, void const *buf, size_t len)

Function Documentation

crc32c()

uint32_t crc32c	(	uint32_t	crc,
		void const *	buf,
		size_t	len
	)

Definition at line 277 of file XrdOucCRC32C.cc.

                                                           {
    return crc32c_sw(crc, buf, len);
}

References crc32c_sw().

Referenced by XrdEc::ObjCfg::ObjCfg(), XrdOucCRC::Calc32C(), XrdPfc::Info::CalcCksumStore(), XrdPfc::Info::CalcCksumSyncedAndAStats(), XrdOssCsiPages::FetchRangeUnaligned_postblock(), XrdOssCsiPages::FetchRangeUnaligned_preblock(), XrdOssCsiPages::StoreRangeUnaligned_postblock(), XrdOssCsiPages::StoreRangeUnaligned_preblock(), XrdOssCsiPages::truncate(), XrdOssCsiPages::UpdateRangeHoleUntilPage(), and XrdOucCRC::Ver32C().

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crc32c_combine()

uint32_t crc32c_combine	(	uint32_t	crcA,
		uint32_t	crcB,
		size_t	lengthB
	)

merge two CRC32 such that result = crc32(dataB, lengthB, crc32(dataA, lengthA))

CRC32 => 32 bits

Definition at line 437 of file XrdOucCRC32C.cc.

{
  // Original source from Stephan Brumme
  // https://github.com/stbrumme/crc32
  // https://create.stephan-brumme.com/crc32/
  // based on Mark Adler's crc_combine from
  // https://github.com/madler/pigz/blob/master/pigz.c
 
  // main idea:
  // - if you have two equally-sized blocks A and B,
  //   then you can create a block C = A ^ B
  //   which has the property crc(C) = crc(A) ^ crc(B)
  // - if you append length(B) zeros to A and call it A' (think of it as AAAA000)
  //   and   prepend length(A) zeros to B and call it B' (think of it as 0000BBB)
  //   then exists a C' = A' ^ B'
  // - remember: if you XOR someting with zero, it remains unchanged: X ^ 0 = X
  // - that means C' = A concat B so that crc(A concat B) = crc(C') = crc(A') ^ crc(B')
  // - the trick is to compute crc(A') based on crc(A)
  //                       and crc(B') based on crc(B)
  // - since B' starts with many zeros, the crc of those initial zeros is still zero
  // - that means crc(B') = crc(B)
  // - unfortunately the trailing zeros of A' change the crc, so usually crc(A') != crc(A)
  // - the following code is a fast algorithm to compute crc(A')
  // - starting with crc(A) and appending length(B) zeros, needing just log2(length(B)) iterations
  // - the details are explained by the original author at
  //   https://stackoverflow.com/questions/23122312/crc-calculation-of-a-mostly-static-data-stream/23126768
  //
  // notes:
  // - I squeezed everything into one function to keep global namespace clean (original code two helper functions)
  // - most original comments are still in place, I added comments where these helper functions where made inline code
  // - performance-wise there isn't any differenze to the original zlib/pigz code
 
  // degenerated case
  if (lengthB == 0)
    return crcA;
 
  const uint32_t CrcBits = 32;
 
  uint32_t odd [CrcBits]; // odd-power-of-two  zeros operator
  uint32_t even[CrcBits]; // even-power-of-two zeros operator
 
  // put operator for one zero bit in odd
  odd[0] = POLY;          // CRC-32 polynomial
  for (int i = 1; i < (int)CrcBits; i++)
    odd[i] = 1 << (i - 1);
 
  // put operator for two zero bits in even
  // same as gf2_matrix_square(even, odd);
  for (int i = 0; i < (int)CrcBits; i++)
  {
    uint32_t vec = odd[i];
    even[i] = 0;
    for (int j = 0; vec != 0; j++, vec >>= 1)
      if (vec & 1)
        even[i] ^= odd[j];
  }
  // put operator for four zero bits in odd
  // same as gf2_matrix_square(odd, even);
  for (int i = 0; i < (int)CrcBits; i++)
  {
    uint32_t vec = even[i];
    odd[i] = 0;
    for (int j = 0; vec != 0; j++, vec >>= 1)
      if (vec & 1)
        odd[i] ^= even[j];
  }
 
  // the following loop becomes much shorter if I keep swapping even and odd
  uint32_t* a = even;
  uint32_t* b = odd;
  // apply secondLength zeros to firstCrc32
  for (; lengthB > 0; lengthB >>= 1)
  {
    // same as gf2_matrix_square(a, b);
    for (int i = 0; i < (int)CrcBits; i++)
    {
      uint32_t vec = b[i];
      a[i] = 0;
      for (int j = 0; vec != 0; j++, vec >>= 1)
        if (vec & 1)
          a[i] ^= b[j];
    }
 
    // apply zeros operator for this bit
    if (lengthB & 1)
    {
      // same as firstCrc32 = gf2_matrix_times(a, firstCrc32);
      uint32_t sum = 0;
      for (int i = 0; crcA != 0; i++, crcA >>= 1)
        if (crcA & 1)
          sum ^= a[i];
      crcA = sum;
    }
 
    // switch even and odd
    uint32_t* t = a; a = b; b = t;
  }
 
  // return combined crc
  return crcA ^ crcB;
}

References POLY.

Referenced by XrdOucCRC::Combine32C().

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crc32c_sw()

uint32_t crc32c_sw	(	uint32_t	crc,
		void const *	buf,
		size_t	len
	)

Definition at line 424 of file XrdOucCRC32C.cc.

                                                              {
    static int const little = 1;
    if (*(char const *)&little)
        return crc32c_sw_little(crc, buf, len);
    else
        return crc32c_sw_big(crc, buf, len);
}

References crc32c_sw_big(), and crc32c_sw_little().

Referenced by crc32c().

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