src/include/utils/pg_crc.h

   1 /*
   2  * pg_crc.h
   3  *
   4  * PostgreSQL CRC support
   5  *
   6  * See Ross Williams' excellent introduction
   7  * A PAINLESS GUIDE TO CRC ERROR DETECTION ALGORITHMS, available from
   8  * http://ross.net/crc/ or several other net sites.
   9  *
  10  * We have three slightly different variants of a 32-bit CRC calculation:
  11  * CRC-32C (Castagnoli polynomial), CRC-32 (Ethernet polynomial), and a legacy
  12  * CRC-32 version that uses the lookup table in a funny way. They all consist
  13  * of four macros:
  14  *
  15  * INIT_<variant>(crc)
  16  *              Initialize a CRC accumulator
  17  *
  18  * COMP_<variant>(crc, data, len)
  19  *              Accumulate some (more) bytes into a CRC
  20  *
  21  * FIN_<variant>(crc)
  22  *              Finish a CRC calculation
  23  *
  24  * EQ_<variant>(c1, c2)
  25  *              Check for equality of two CRCs.
  26  *
  27  * The CRC-32C variant is in port/pg_crc32c.h.
  28  *
  29  * Portions Copyright (c) 1996-2023, PostgreSQL Global Development Group
  30  * Portions Copyright (c) 1994, Regents of the University of California
  31  *
  32  * src/include/utils/pg_crc.h
  33  */
  34 #ifndef PG_CRC_H
  35 #define PG_CRC_H
  36
  37 typedef uint32 pg_crc32;
  38
  39 /*
  40  * CRC-32, the same used e.g. in Ethernet.
  41  *
  42  * This is currently only used in ltree and hstore contrib modules. It uses
  43  * the same lookup table as the legacy algorithm below. New code should
  44  * use the Castagnoli version instead.
  45  */
  46 #define INIT_TRADITIONAL_CRC32(crc) ((crc) = 0xFFFFFFFF)
  47 #define FIN_TRADITIONAL_CRC32(crc)      ((crc) ^= 0xFFFFFFFF)
  48 #define COMP_TRADITIONAL_CRC32(crc, data, len)  \
  49         COMP_CRC32_NORMAL_TABLE(crc, data, len, pg_crc32_table)
  50 #define EQ_TRADITIONAL_CRC32(c1, c2) ((c1) == (c2))
  51
  52 /* Sarwate's algorithm, for use with a "normal" lookup table */
  53 #define COMP_CRC32_NORMAL_TABLE(crc, data, len, table)                    \
  54 do {                                                                                                                      \
  55         const unsigned char *__data = (const unsigned char *) (data); \
  56         uint32          __len = (len); \
  57 \
  58         while (__len-- > 0) \
  59         { \
  60                 int             __tab_index = ((int) (crc) ^ *__data++) & 0xFF; \
  61                 (crc) = table[__tab_index] ^ ((crc) >> 8); \
  62         } \
  63 } while (0)
  64
  65 /*
  66  * The CRC algorithm used for WAL et al in pre-9.5 versions.
  67  *
  68  * This closely resembles the normal CRC-32 algorithm, but is subtly
  69  * different. Using Williams' terms, we use the "normal" table, but with
  70  * "reflected" code. That's bogus, but it was like that for years before
  71  * anyone noticed. It does not correspond to any polynomial in a normal CRC
  72  * algorithm, so it's not clear what the error-detection properties of this
  73  * algorithm actually are.
  74  *
  75  * We still need to carry this around because it is used in a few on-disk
  76  * structures that need to be pg_upgradeable. It should not be used in new
  77  * code.
  78  */
  79 #define INIT_LEGACY_CRC32(crc) ((crc) = 0xFFFFFFFF)
  80 #define FIN_LEGACY_CRC32(crc)   ((crc) ^= 0xFFFFFFFF)
  81 #define COMP_LEGACY_CRC32(crc, data, len)       \
  82         COMP_CRC32_REFLECTED_TABLE(crc, data, len, pg_crc32_table)
  83 #define EQ_LEGACY_CRC32(c1, c2) ((c1) == (c2))
  84
  85 /*
  86  * Sarwate's algorithm, for use with a "reflected" lookup table (but in the
  87  * legacy algorithm, we actually use it on a "normal" table, see above)
  88  */
  89 #define COMP_CRC32_REFLECTED_TABLE(crc, data, len, table) \
  90 do {                                                                                                                      \
  91         const unsigned char *__data = (const unsigned char *) (data); \
  92         uint32          __len = (len); \
  93 \
  94         while (__len-- > 0) \
  95         { \
  96                 int             __tab_index = ((int) ((crc) >> 24) ^ *__data++) & 0xFF; \
  97                 (crc) = table[__tab_index] ^ ((crc) << 8); \
  98         } \
  99 } while (0)
 100
 101 /*
 102  * Constant table for the CRC-32 polynomials. The same table is used by both
 103  * the normal and traditional variants.
 104  */
 105 extern PGDLLIMPORT const uint32 pg_crc32_table[256];
 106
 107 #endif                                                  /* PG_CRC_H */