/* Compute CRC32 from file or stdin. */

#include<stdint.h>
#include<stdio.h>

#ifdef _WIN32
   #include<fcntl.h>
   #include<io.h>
#endif

/* Table for updating CRC one byte at a time. */
static uint32_t crc_table[256];

/* Read buffer. */
static uint8_t read_buffer[4096];

/* Initialize crc_table. */
static void InitCrcTable(void)
{
   static const uint32_t kPoly = 0xedb88320U;
   for(int i = 0; i < 256; i++)
   {
      uint32_t crc = i;
      for(int j = 8; j > 0; j--)
      {
         if( (crc & 1) != 0 )
            crc = (crc >> 1) ^ kPoly;
         else
            crc >>= 1;
      }
      crc_table[i] = crc;
   }
}

/* Checksum input until end of file. */
static void Crc32(FILE *infile)
{
   size_t read_size;
   uint32_t crc = ~0;

   while( (read_size = fread(read_buffer, 1, sizeof(read_buffer), infile)) > 0 )
   {
      for(size_t i = 0; i < read_size; i++)
      {
         crc = ((crc >> 8) & 0xffffff) ^
               crc_table[(crc ^ read_buffer[i]) & 0xff];
      }
      if( read_size < sizeof(read_buffer) )
         break;
   }
   printf("%08x\n", ~crc);
}

int main(int argc, char **argv)
{
   FILE *infile = NULL;

   if( argc > 2 )
      return printf("%s [input]\n", *argv);

   #ifdef _WIN32
      setmode(STDIN_FILENO, O_BINARY);
   #endif

   /* Open file specified on command line for reading, unless it's "-". */
   if( argc == 2 && (argv[1][0] != '-' || argv[1][1] != '\0') )
   {
      if( (infile = fopen(argv[1], "rb")) == NULL )
         return printf("Error opening %s\n", argv[1]);
   }

   InitCrcTable();
   if( infile != NULL )
   {
      Crc32(infile);
      return fclose(infile);
   }
   Crc32(stdin);
   return 0;
}