#include <stddef.h>
#include <stdint.h>
#include "user.h"

uint32_t
crc32_for_byte(uint32_t r)
{
  for(int j = 0; j < 8; ++j)
    r = (r & 1 ? 0xEDB88320L ^ (r >> 1) : r >> 1);
  return r;
}

void
init_crc32_table(uint32_t table[256])
{
  for(int i = 0; i < 256; ++i)
    table[i] = crc32_for_byte(i);
}

uint32_t
crc32(uint8_t *data, size_t len, uint32_t crc, uint32_t table[256])
{
  for(size_t i = 0; i < len; ++i)
    crc = table[(crc ^ data[i]) & 0xFF] ^ (crc >> 8);
  return crc;
}

int
main(int argc, char *argv[])
{
  int      fd;
  uint8_t  buf[1024];
  size_t   n;
  uint32_t crc = 0xFFFFFFFF;
  uint32_t table[256];

  if(argc != 2) {
    fprintf(stderr, "Usage: %s <file>\n", argv[0]);
    exit(1);
  }

  if((fd = open(argv[1], 0)) < 0) {
    fprintf(stderr, "Couldnt open: %s\n", argv[1]);
    exit(1);
  }

  init_crc32_table(table);

  while((n = read(fd, buf, sizeof(buf))) > 0)
    crc = crc32(buf, n, crc, table);

  if(n < 0) {
    fprintf(stderr, "Couldnt read file...\n");
    close(fd);
    exit(1);
  }

  close(fd);

  crc ^= 0xFFFFFFFF;
  printf("%x\n", crc);

  return 0;
}