/* crc32.c -- compute the CRC-32 of a data stream

 * Copyright (C) 1995-2003 Mark Adler

 * For conditions of distribution and use, see copyright notice in zlib.h

 *

 * Thanks to Rodney Brown <rbrown64@csc.com.au> for his contribution of faster

 * CRC methods: exclusive-oring 32 bits of data at a time, and pre-computing

 * tables for updating the shift register in one step with three exclusive-ors

 * instead of four steps with four exclusive-ors.  This results about a factor

 * of two increase in speed on a Power PC G4 (PPC7455) using gcc -O3.

 */

/* @(#) $Id: crc32.c,v 1.1 2003/12/07 05:29:20 icculus Exp $ */

#ifdef MAKECRCH

#  include <stdio.h>

#  ifndef DYNAMIC_CRC_TABLE

#    define DYNAMIC_CRC_TABLE

#  endif /* !DYNAMIC_CRC_TABLE */

#endif /* MAKECRCH */

#include "zutil.h"      /* for STDC and FAR definitions */

#define local static

/* Find a four-byte integer type for crc32_little() and crc32_big(). */

#ifndef NOBYFOUR

#  ifdef STDC           /* need ANSI C limits.h to determine sizes */

#    include <limits.h>

#    define BYFOUR

#    if (UINT_MAX == 0xffffffffUL)

       typedef unsigned int u4;

#    else

#      if (ULONG_MAX == 0xffffffffUL)

         typedef unsigned long u4;

#      else

#        if (USHRT_MAX == 0xffffffffUL)

           typedef unsigned short u4;

#        else

#          undef BYFOUR     /* can't find a four-byte integer type! */

#        endif

#      endif

#    endif

#  endif /* STDC */

#endif /* !NOBYFOUR */

/* Definitions for doing the crc four data bytes at a time. */

#ifdef BYFOUR

#  define REV(w) (((w)>>24)+(((w)>>8)&0xff00)+ \

                (((w)&0xff00)<<8)+(((w)&0xff)<<24))

   local unsigned long crc32_little OF((unsigned long,

                        const unsigned char FAR *, unsigned));

   local unsigned long crc32_big OF((unsigned long,

                        const unsigned char FAR *, unsigned));

#  define TBLS 8

#else

#  define TBLS 1

#endif /* BYFOUR */

#ifdef DYNAMIC_CRC_TABLE

local int crc_table_empty = 1;

local unsigned long FAR crc_table[TBLS][256];

local void make_crc_table OF((void));

#ifdef MAKECRCH

   local void write_table OF((FILE *, const unsigned long FAR *));

#endif /* MAKECRCH */

/*

  Generate tables for a byte-wise 32-bit CRC calculation on the polynomial:

  x^32+x^26+x^23+x^22+x^16+x^12+x^11+x^10+x^8+x^7+x^5+x^4+x^2+x+1.

  Polynomials over GF(2) are represented in binary, one bit per coefficient,

  with the lowest powers in the most significant bit.  Then adding polynomials

  is just exclusive-or, and multiplying a polynomial by x is a right shift by

  one.  If we call the above polynomial p, and represent a byte as the

  polynomial q, also with the lowest power in the most significant bit (so the

  byte 0xb1 is the polynomial x^7+x^3+x+1), then the CRC is (q*x^32) mod p,

  where a mod b means the remainder after dividing a by b.

  This calculation is done using the shift-register method of multiplying and

  taking the remainder.  The register is initialized to zero, and for each

  incoming bit, x^32 is added mod p to the register if the bit is a one (where

  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 table is simply the CRC of all possible eight bit values.  This is

  all the information needed to generate CRCs on data a byte at a time for all

  combinations of CRC register values and incoming bytes.  The remaining tables

  allow for word-at-a-time CRC calculation for both big-endian and little-

  endian machines, where a word is four bytes.

*/

local void make_crc_table()

{

    unsigned long c;

    int n, k;

    unsigned long poly;            /* polynomial exclusive-or pattern */

    /* terms of polynomial defining this crc (except x^32): */

    static const unsigned char p[] = {0,1,2,4,5,7,8,10,11,12,16,22,23,26};

    /* make exclusive-or pattern from polynomial (0xedb88320UL) */

    poly = 0UL;

    for (n = 0; n < sizeof(p)/sizeof(unsigned char); n++)

        poly |= 1UL << (31 - p[n]);

    /* generate a crc for every 8-bit value */

    for (n = 0; n < 256; n++) {

        c = (unsigned long)n;

        for (k = 0; k < 8; k++)

            c = c & 1 ? poly ^ (c >> 1) : c >> 1;

        crc_table[0][n] = c;

    }

#ifdef BYFOUR

    /* generate crc for each value followed by one, two, and three zeros, and

       then the byte reversal of those as well as the first table */

    for (n = 0; n < 256; n++) {

        c = crc_table[0][n];

        crc_table[4][n] = REV(c);

        for (k = 1; k < 4; k++) {

            c = crc_table[0][c & 0xff] ^ (c >> 8);

            crc_table[k][n] = c;

            crc_table[k + 4][n] = REV(c);

        }

    }

#endif /* BYFOUR */

  crc_table_empty = 0;

#ifdef MAKECRCH

    /* write out CRC tables to crc32.h */

    {

        FILE *out;

        out = fopen("crc32.h", "w");

        if (out == NULL) return;

        fprintf(out, "/* crc32.h -- tables for rapid CRC calculation\n");

        fprintf(out, " * Generated automatically by crc32.c\n */\n\n");

        fprintf(out, "local const unsigned long FAR ");

        fprintf(out, "crc_table[TBLS][256] =\n{\n  {\n");

        write_table(out, crc_table[0]);

#  ifdef BYFOUR

        fprintf(out, "#ifdef BYFOUR\n");

        for (k = 1; k < 8; k++) {

            fprintf(out, "  },\n  {\n");

            write_table(out, crc_table[k]);

        }

        fprintf(out, "#endif\n");

#  endif /* BYFOUR */

        fprintf(out, "  }\n};\n");

        fclose(out);

    }

#endif /* MAKECRCH */

}

#ifdef MAKECRCH

local void write_table(out, table)

    FILE *out;

    const unsigned long FAR *table;

{

    int n;

    for (n = 0; n < 256; n++)

        fprintf(out, "%s0x%08lxUL%s", n % 5 ? "" : "    ", table[n],

                n == 255 ? "\n" : (n % 5 == 4 ? ",\n" : ", "));

}

#endif /* MAKECRCH */

#else /* !DYNAMIC_CRC_TABLE */

/* ========================================================================

 * Tables of CRC-32s of all single-byte values, made by make_crc_table().

 */

#include "crc32.h"

#endif /* DYNAMIC_CRC_TABLE */

/* =========================================================================

 * This function can be used by asm versions of crc32()

 */

const unsigned long FAR * ZEXPORT get_crc_table()

{

#ifdef DYNAMIC_CRC_TABLE

  if (crc_table_empty) make_crc_table();

#endif /* DYNAMIC_CRC_TABLE */

  return (const unsigned long FAR *)crc_table;

}

/* ========================================================================= */

#define DO1 crc = crc_table[0][((int)crc ^ (*buf++)) & 0xff] ^ (crc >> 8)

#define DO8 DO1; DO1; DO1; DO1; DO1; DO1; DO1; DO1

/* ========================================================================= */

unsigned long ZEXPORT crc32(crc, buf, len)

    unsigned long crc;

    const unsigned char FAR *buf;

    unsigned len;

{

    if (buf == Z_NULL) return 0UL;

#ifdef DYNAMIC_CRC_TABLE

    if (crc_table_empty)

        make_crc_table();

#endif /* DYNAMIC_CRC_TABLE */

#ifdef BYFOUR

    if (sizeof(void *) == sizeof(ptrdiff_t)) {

        u4 endian;

        endian = 1;

        if (*((unsigned char *)(&endian)))

            return crc32_little(crc, buf, len);

        else

            return crc32_big(crc, buf, len);

    }

#endif /* BYFOUR */

    crc = crc ^ 0xffffffffUL;

    while (len >= 8) {

        DO8;

        len -= 8;

    }

    if (len) do {

        DO1;

    } while (--len);

    return crc ^ 0xffffffffUL;

}

#ifdef BYFOUR

/* ========================================================================= */

#define DOLIT4 c ^= *buf4++; \

        c = crc_table[3][c & 0xff] ^ crc_table[2][(c >> 8) & 0xff] ^ \

            crc_table[1][(c >> 16) & 0xff] ^ crc_table[0][c >> 24]

#define DOLIT32 DOLIT4; DOLIT4; DOLIT4; DOLIT4; DOLIT4; DOLIT4; DOLIT4; DOLIT4

/* ========================================================================= */

local unsigned long crc32_little(crc, buf, len)

    unsigned long crc;

    const unsigned char FAR *buf;

    unsigned len;

{

    register u4 c;

    register const u4 FAR *buf4;

    c = (u4)crc;

    c = ~c;

    while (len && ((ptrdiff_t)buf & 3)) {

        c = crc_table[0][(c ^ *buf++) & 0xff] ^ (c >> 8);

        len--;

    }

    buf4 = (const u4 FAR *)buf;

    while (len >= 32) {

        DOLIT32;

        len -= 32;

    }

    while (len >= 4) {

        DOLIT4;

        len -= 4;

    }

    buf = (const unsigned char FAR *)buf4;

    if (len) do {

        c = crc_table[0][(c ^ *buf++) & 0xff] ^ (c >> 8);

    } while (--len);

    c = ~c;

    return (unsigned long)c;

}

/* ========================================================================= */

#define DOBIG4 c ^= *++buf4; \

        c = crc_table[4][c & 0xff] ^ crc_table[5][(c >> 8) & 0xff] ^ \

            crc_table[6][(c >> 16) & 0xff] ^ crc_table[7][c >> 24]

#define DOBIG32 DOBIG4; DOBIG4; DOBIG4; DOBIG4; DOBIG4; DOBIG4; DOBIG4; DOBIG4

/* ========================================================================= */

local unsigned long crc32_big(crc, buf, len)

    unsigned long crc;

    const unsigned char FAR *buf;

    unsigned len;

{

    register u4 c;

    register const u4 FAR *buf4;

    c = REV((u4)crc);

    c = ~c;

    while (len && ((ptrdiff_t)buf & 3)) {

        c = crc_table[4][(c >> 24) ^ *buf++] ^ (c << 8);

        len--;

    }

    buf4 = (const u4 FAR *)buf;

    buf4--;

    while (len >= 32) {

        DOBIG32;

        len -= 32;

    }

    while (len >= 4) {

        DOBIG4;

        len -= 4;

    }

    buf4++;

    buf = (const unsigned char FAR *)buf4;

    if (len) do {

        c = crc_table[4][(c >> 24) ^ *buf++] ^ (c << 8);

    } while (--len);

    c = ~c;

    return (unsigned long)(REV(c));

}

#endif /* BYFOUR */