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uulib/crc32.c  view on Meta::CPAN

   *   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 */