/* crc32.c -- calculate POSIX CRC32 checksums
   Copyright (C) 2009, R.N. Bokhorst, reinoud.bokhorst@bmtargoss.com

   This program is free software: you can redistribute it and/or modify
   it under the terms of the GNU General Public License as published by
   the Free Software Foundation, either version 3 of the License, or
   (at your option) any later version.

   This program is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   GNU General Public License for more details.

   You should have received a copy of the GNU General Public License
   along with this program.  If not, see <http://www.gnu.org/licenses/>.
*/
/*
   The checksum algorithm and calculation of crctab was originally written
   by Q. Frank Xia for the cksum utility, part of the GNU coreutils package
   (v7.4). It was modified by R.N. Bokhorst for use in wgrib2.

   The code calculates the crctab when the cksum function is called for the
   first time. The calculated crctab should be identical to the one given in
   the code segment between "#ifdef STATIC_CRCTAB" and "#else". It was done
   this way to limit the number of 'static' bytes in the final binary. The
   static table can be used instead by defining STATIC_CRCTAB.

   This software is NOT compatible with either the System V or the BSD
   `sum' program. Nor will it produce the same result as most typical CRC32
   implementations that use the reverse representation of the polynomial
   (0xEDB88320 ). It is supposed to conform to POSIX, except perhaps
   for foreign language support.
*/

#include <stdio.h>
#include <inttypes.h>    /* C99 portable types */
#include "wgrib2.h"


//#define STATIC_CRCTAB

#ifdef STATIC_CRCTAB

static uint32_t crctab[256] = {
    0x00000000,
    0x04c11db7, 0x09823b6e, 0x0d4326d9, 0x130476dc, 0x17c56b6b,
    0x1a864db2, 0x1e475005, 0x2608edb8, 0x22c9f00f, 0x2f8ad6d6,
    0x2b4bcb61, 0x350c9b64, 0x31cd86d3, 0x3c8ea00a, 0x384fbdbd,
    0x4c11db70, 0x48d0c6c7, 0x4593e01e, 0x4152fda9, 0x5f15adac,
    0x5bd4b01b, 0x569796c2, 0x52568b75, 0x6a1936c8, 0x6ed82b7f,
    0x639b0da6, 0x675a1011, 0x791d4014, 0x7ddc5da3, 0x709f7b7a,
    0x745e66cd, 0x9823b6e0, 0x9ce2ab57, 0x91a18d8e, 0x95609039,
    0x8b27c03c, 0x8fe6dd8b, 0x82a5fb52, 0x8664e6e5, 0xbe2b5b58,
    0xbaea46ef, 0xb7a96036, 0xb3687d81, 0xad2f2d84, 0xa9ee3033,
    0xa4ad16ea, 0xa06c0b5d, 0xd4326d90, 0xd0f37027, 0xddb056fe,
    0xd9714b49, 0xc7361b4c, 0xc3f706fb, 0xceb42022, 0xca753d95,
    0xf23a8028, 0xf6fb9d9f, 0xfbb8bb46, 0xff79a6f1, 0xe13ef6f4,
    0xe5ffeb43, 0xe8bccd9a, 0xec7dd02d, 0x34867077, 0x30476dc0,
    0x3d044b19, 0x39c556ae, 0x278206ab, 0x23431b1c, 0x2e003dc5,
    0x2ac12072, 0x128e9dcf, 0x164f8078, 0x1b0ca6a1, 0x1fcdbb16,
    0x018aeb13, 0x054bf6a4, 0x0808d07d, 0x0cc9cdca, 0x7897ab07,
    0x7c56b6b0, 0x71159069, 0x75d48dde, 0x6b93dddb, 0x6f52c06c,
    0x6211e6b5, 0x66d0fb02, 0x5e9f46bf, 0x5a5e5b08, 0x571d7dd1,
    0x53dc6066, 0x4d9b3063, 0x495a2dd4, 0x44190b0d, 0x40d816ba,
    0xaca5c697, 0xa864db20, 0xa527fdf9, 0xa1e6e04e, 0xbfa1b04b,
    0xbb60adfc, 0xb6238b25, 0xb2e29692, 0x8aad2b2f, 0x8e6c3698,
    0x832f1041, 0x87ee0df6, 0x99a95df3, 0x9d684044, 0x902b669d,
    0x94ea7b2a, 0xe0b41de7, 0xe4750050, 0xe9362689, 0xedf73b3e,
    0xf3b06b3b, 0xf771768c, 0xfa325055, 0xfef34de2, 0xc6bcf05f,
    0xc27dede8, 0xcf3ecb31, 0xcbffd686, 0xd5b88683, 0xd1799b34,
    0xdc3abded, 0xd8fba05a, 0x690ce0ee, 0x6dcdfd59, 0x608edb80,
    0x644fc637, 0x7a089632, 0x7ec98b85, 0x738aad5c, 0x774bb0eb,
    0x4f040d56, 0x4bc510e1, 0x46863638, 0x42472b8f, 0x5c007b8a,
    0x58c1663d, 0x558240e4, 0x51435d53, 0x251d3b9e, 0x21dc2629,
    0x2c9f00f0, 0x285e1d47, 0x36194d42, 0x32d850f5, 0x3f9b762c,
    0x3b5a6b9b, 0x0315d626, 0x07d4cb91, 0x0a97ed48, 0x0e56f0ff,
    0x1011a0fa, 0x14d0bd4d, 0x19939b94, 0x1d528623, 0xf12f560e,
    0xf5ee4bb9, 0xf8ad6d60, 0xfc6c70d7, 0xe22b20d2, 0xe6ea3d65,
    0xeba91bbc, 0xef68060b, 0xd727bbb6, 0xd3e6a601, 0xdea580d8,
    0xda649d6f, 0xc423cd6a, 0xc0e2d0dd, 0xcda1f604, 0xc960ebb3,
    0xbd3e8d7e, 0xb9ff90c9, 0xb4bcb610, 0xb07daba7, 0xae3afba2,
    0xaafbe615, 0xa7b8c0cc, 0xa379dd7b, 0x9b3660c6, 0x9ff77d71,
    0x92b45ba8, 0x9675461f, 0x8832161a, 0x8cf30bad, 0x81b02d74,
    0x857130c3, 0x5d8a9099, 0x594b8d2e, 0x5408abf7, 0x50c9b640,
    0x4e8ee645, 0x4a4ffbf2, 0x470cdd2b, 0x43cdc09c, 0x7b827d21,
    0x7f436096, 0x7200464f, 0x76c15bf8, 0x68860bfd, 0x6c47164a,
    0x61043093, 0x65c52d24, 0x119b4be9, 0x155a565e, 0x18197087,
    0x1cd86d30, 0x029f3d35, 0x065e2082, 0x0b1d065b, 0x0fdc1bec,
    0x3793a651, 0x3352bbe6, 0x3e119d3f, 0x3ad08088, 0x2497d08d,
    0x2056cd3a, 0x2d15ebe3, 0x29d4f654, 0xc5a92679, 0xc1683bce,
    0xcc2b1d17, 0xc8ea00a0, 0xd6ad50a5, 0xd26c4d12, 0xdf2f6bcb,
    0xdbee767c, 0xe3a1cbc1, 0xe760d676, 0xea23f0af, 0xeee2ed18,
    0xf0a5bd1d, 0xf464a0aa, 0xf9278673, 0xfde69bc4, 0x89b8fd09,
    0x8d79e0be, 0x803ac667, 0x84fbdbd0, 0x9abc8bd5, 0x9e7d9662,
    0x933eb0bb, 0x97ffad0c, 0xafb010b1, 0xab710d06, 0xa6322bdf,
    0xa2f33668, 0xbcb4666d, 0xb8757bda, 0xb5365d03, 0xb1f740b4
};

#else

/*
    Calculate the crctab once on init.

    The generating polynomial is

        32   26   23   22   16   12   11   10   8   7   5   4   2   1
    G(X)=X  + X  + X  + X  + X  + X  + X  + X  + X + X + X + X + X + X + 1

    The i bit in GEN is set if X^i is a summand of G(X) except X^32.

    #define GEN    (BIT (26) | BIT (23) | BIT (22) | BIT (16) | BIT (12) \
                    | BIT (11) | BIT (10) | BIT (8) | BIT (7) | BIT (5) \
                    | BIT (4) | BIT (2) | BIT (1) | BIT (0))

    which produces 0x04c11db7
*/


#define BIT(x)  ((uint32_t) 1 << (x))
#define SBIT    BIT(31)
#define GEN     (uint32_t)0x04C11DB7

static uint32_t crctab[256];

static uint32_t crc_remainder(int m, uint32_t *r)
{
    uint32_t rem = 0;
    int i;

    for (i = 0; i < 8; i++) {
        if (BIT (i) & m) {
            rem ^= r[i];
        }
    }

    return rem & 0xFFFFFFFF;  /* Make it run on 64-bit machine */
}

static int init_crctab(void)
{
    int i;
    uint32_t r[8];

    /* Fill r */
    r[0] = GEN;
    for (i = 1; i < 8; i++) {
        r[i] = (r[i - 1] << 1) ^ ((r[i - 1] & SBIT) ? GEN : 0);
    }

    crctab[0] = 0x00000000;
    for (i = 1; i < 256; i++) {
        crctab[i] = crc_remainder(i, r);
    }

    return 1;
}


#endif /* ! STATIC_CRCTAB */


/*
  Calculate and return the CRC32 checksum of buffer with length in bytes

  Note: cksum needs to be 32-bit+ unsigned integer
*/

unsigned int cksum(unsigned char const *buf, size_t length)
{
  uint32_t crc = 0;
  size_t bytes = length;

#ifndef STATIC_CRCTAB
  static short inited = 0;
  if (!inited) inited = init_crctab();
#endif

  while (bytes--) {
    crc = (crc << 8) ^ crctab[((crc >> 24) ^ *buf++) & 0xFF];
  }

  for (; length; length >>= 8) {
    crc = (crc << 8) ^ crctab[((crc >> 24) ^ length) & 0xFF];
  }

  crc = ~crc & 0xFFFFFFFF;

  return crc;
}

