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unzip-6.0/crc32.c  view on Meta::CPAN

  x^32 mod p is p+x^32 = x^26+...+1), and the register is multiplied mod p by
  x (which is shifting right by one and adding x^32 mod p if the bit shifted
  out is a one).  We start with the highest power (least significant bit) of
  q and repeat for all eight bits of q.

  The first (or only) table is simply the CRC of all possible eight bit values.
  This is all the information needed to generate CRC's on data a byte-at-a-time
  for all combinations of CRC register values and incoming bytes.
  The remaining 3 tables (if IZ_CRCOPTIM_UNFOLDTBL is enabled) allow for
  word-at-a-time CRC calculation, where a word is four bytes.
*/

#ifdef DYNAMIC_CRC_TABLE

/* =========================================================================
 * Make the crc table. This function is needed only if you want to compute
 * the table dynamically.
 */

local void make_crc_table OF((void));

#if (defined(DYNALLOC_CRCTAB) && defined(REENTRANT))
   error: Dynamic allocation of CRC table not safe with reentrant code.
#endif /* DYNALLOC_CRCTAB && REENTRANT */

#ifdef DYNALLOC_CRCTAB
   local ulg near *crc_table = NULL;
# if 0          /* not used, since sizeof("near *") <= sizeof(int) */
   /* Use this section when access to a "local int" is faster than access to
      a "local pointer" (e.g.: i86 16bit code with far pointers). */
   local int crc_table_empty = 1;
#  define CRC_TABLE_IS_EMPTY    (crc_table_empty != 0)
#  define MARK_CRCTAB_FILLED    crc_table_empty = 0
#  define MARK_CRCTAB_EMPTY     crc_table_empty = 1
# else
   /* Use this section on systems where the size of pointers and ints is
      equal (e.g.: all 32bit systems). */
#  define CRC_TABLE_IS_EMPTY    (crc_table == NULL)
#  define MARK_CRCTAB_FILLED    crc_table = crctab_p
#  define MARK_CRCTAB_EMPTY     crc_table = NULL
# endif
#else /* !DYNALLOC_CRCTAB */
   local ulg near crc_table[CRC_TBLS*256];
   local int crc_table_empty = 1;
#  define CRC_TABLE_IS_EMPTY    (crc_table_empty != 0)
#  define MARK_CRCTAB_FILLED    crc_table_empty = 0
#endif /* ?DYNALLOC_CRCTAB */


local void make_crc_table()
{
  ulg c;                /* crc shift register */
  int n;                /* counter for all possible eight bit values */
  int k;                /* byte being shifted into crc apparatus */
#ifdef DYNALLOC_CRCTAB
  ulg near *crctab_p;   /* temporary pointer to allocated crc_table area */
#else /* !DYNALLOC_CRCTAB */
# define crctab_p crc_table
#endif /* DYNALLOC_CRCTAB */

#ifdef COMPUTE_XOR_PATTERN
  /* This piece of code has been left here to explain how the XOR pattern
   * used in the creation of the crc_table values can be recomputed.
   * For production versions of this function, it is more efficient to
   * supply the resultant pattern at compile time.
   */
  ulg xor;              /* polynomial exclusive-or pattern */
  /* terms of polynomial defining this crc (except x^32): */
  static ZCONST uch p[] = {0,1,2,4,5,7,8,10,11,12,16,22,23,26};

  /* make exclusive-or pattern from polynomial (0xedb88320L) */
  xor = 0L;
  for (n = 0; n < sizeof(p)/sizeof(uch); n++)
    xor |= 1L << (31 - p[n]);
#else
# define xor 0xedb88320L
#endif

#ifdef DYNALLOC_CRCTAB
  crctab_p = (ulg near *) nearmalloc (CRC_TBLS*256*sizeof(ulg));
  if (crctab_p == NULL) {
    ziperr(ZE_MEM, "crc_table allocation");
  }
#endif /* DYNALLOC_CRCTAB */

  /* generate a crc for every 8-bit value */
  for (n = 0; n < 256; n++) {
    c = (ulg)n;
    for (k = 8; k; k--)
      c = c & 1 ? xor ^ (c >> 1) : c >> 1;
    crctab_p[n] = REV_BE(c);
  }

#ifdef IZ_CRCOPTIM_UNFOLDTBL
  /* generate crc for each value followed by one, two, and three zeros */
  for (n = 0; n < 256; n++) {
      c = crctab_p[n];
      for (k = 1; k < 4; k++) {
          c = CRC32(c, 0, crctab_p);
          crctab_p[k*256+n] = c;
      }
  }
#endif /* IZ_CRCOPTIM_UNFOLDTBL */

  MARK_CRCTAB_FILLED;
}

#else /* !DYNAMIC_CRC_TABLE */

#ifdef DYNALLOC_CRCTAB
   error: Inconsistent flags, DYNALLOC_CRCTAB without DYNAMIC_CRC_TABLE.
#endif

/* ========================================================================
 * Table of CRC-32's of all single-byte values (made by make_crc_table)
 */
local ZCONST ulg near crc_table[CRC_TBLS*256] = {
# ifdef IZ_CRC_BE_OPTIMIZ
    0x00000000L, 0x96300777L, 0x2c610eeeL, 0xba510999L, 0x19c46d07L,
    0x8ff46a70L, 0x35a563e9L, 0xa395649eL, 0x3288db0eL, 0xa4b8dc79L,
    0x1ee9d5e0L, 0x88d9d297L, 0x2b4cb609L, 0xbd7cb17eL, 0x072db8e7L,