прога странно работает=) кому интересно покопать?

#include<stdio.h>
#include<conio.h>
#include<stdlib.h>
#include<math.h>
 
/*
  Name  : CRC-16
  Poly  : 0x18005   x^16 + x^15 + x^2 + 1
  Init  : 0x0000
  Revert: true
  XorOut: 0x0000
  Check : 0x4B37 ("123456789")
  MaxLen: 4095 байт (32767 бит)*/
const unsigned short Crc16Table[256] = {
    0x0000, 0xC0C1, 0xC181, 0x0140, 0xC301, 0x03C0, 0x0280, 0xC241,
    0xC601, 0x06C0, 0x0780, 0xC741, 0x0500, 0xC5C1, 0xC481, 0x0440,
    0xCC01, 0x0CC0, 0x0D80, 0xCD41, 0x0F00, 0xCFC1, 0xCE81, 0x0E40,
    0x0A00, 0xCAC1, 0xCB81, 0x0B40, 0xC901, 0x09C0, 0x0880, 0xC841,
    0xD801, 0x18C0, 0x1980, 0xD941, 0x1B00, 0xDBC1, 0xDA81, 0x1A40,
    0x1E00, 0xDEC1, 0xDF81, 0x1F40, 0xDD01, 0x1DC0, 0x1C80, 0xDC41,
    0x1400, 0xD4C1, 0xD581, 0x1540, 0xD701, 0x17C0, 0x1680, 0xD641,
    0xD201, 0x12C0, 0x1380, 0xD341, 0x1100, 0xD1C1, 0xD081, 0x1040,
    0xF001, 0x30C0, 0x3180, 0xF141, 0x3300, 0xF3C1, 0xF281, 0x3240,
    0x3600, 0xF6C1, 0xF781, 0x3740, 0xF501, 0x35C0, 0x3480, 0xF441,
    0x3C00, 0xFCC1, 0xFD81, 0x3D40, 0xFF01, 0x3FC0, 0x3E80, 0xFE41,
    0xFA01, 0x3AC0, 0x3B80, 0xFB41, 0x3900, 0xF9C1, 0xF881, 0x3840,
    0x2800, 0xE8C1, 0xE981, 0x2940, 0xEB01, 0x2BC0, 0x2A80, 0xEA41,
    0xEE01, 0x2EC0, 0x2F80, 0xEF41, 0x2D00, 0xEDC1, 0xEC81, 0x2C40,
    0xE401, 0x24C0, 0x2580, 0xE541, 0x2700, 0xE7C1, 0xE681, 0x2640,
    0x2200, 0xE2C1, 0xE381, 0x2340, 0xE101, 0x21C0, 0x2080, 0xE041,
    0xA001, 0x60C0, 0x6180, 0xA141, 0x6300, 0xA3C1, 0xA281, 0x6240,
    0x6600, 0xA6C1, 0xA781, 0x6740, 0xA501, 0x65C0, 0x6480, 0xA441,
    0x6C00, 0xACC1, 0xAD81, 0x6D40, 0xAF01, 0x6FC0, 0x6E80, 0xAE41,
    0xAA01, 0x6AC0, 0x6B80, 0xAB41, 0x6900, 0xA9C1, 0xA881, 0x6840,
    0x7800, 0xB8C1, 0xB981, 0x7940, 0xBB01, 0x7BC0, 0x7A80, 0xBA41,
    0xBE01, 0x7EC0, 0x7F80, 0xBF41, 0x7D00, 0xBDC1, 0xBC81, 0x7C40,
    0xB401, 0x74C0, 0x7580, 0xB541, 0x7700, 0xB7C1, 0xB681, 0x7640,
    0x7200, 0xB2C1, 0xB381, 0x7340, 0xB101, 0x71C0, 0x7080, 0xB041,
    0x5000, 0x90C1, 0x9181, 0x5140, 0x9301, 0x53C0, 0x5280, 0x9241,
    0x9601, 0x56C0, 0x5780, 0x9741, 0x5500, 0x95C1, 0x9481, 0x5440,
    0x9C01, 0x5CC0, 0x5D80, 0x9D41, 0x5F00, 0x9FC1, 0x9E81, 0x5E40,
    0x5A00, 0x9AC1, 0x9B81, 0x5B40, 0x9901, 0x59C0, 0x5880, 0x9841,
    0x8801, 0x48C0, 0x4980, 0x8941, 0x4B00, 0x8BC1, 0x8A81, 0x4A40,
    0x4E00, 0x8EC1, 0x8F81, 0x4F40, 0x8D01, 0x4DC0, 0x4C80, 0x8C41,
    0x4400, 0x84C1, 0x8581, 0x4540, 0x8701, 0x47C0, 0x4680, 0x8641,
    0x8201, 0x42C0, 0x4380, 0x8341, 0x4100, 0x81C1, 0x8081, 0x4040
};
 
unsigned short Crc16(unsigned char * pcBlock, long long len);
 
main()
{
    clrscr();
    long long int i, len=0,len2=0,a=0;
    FILE *secret;
    unsigned char filename[256];
    unsigned char *str = NULL;
    char ch;
    for(i=0;i<256;i++)
    {
        filename[i]=NULL;
    }
    printf("\n\tGood Day! It is CRC-16 Programm\n");
    printf("\n\tEnter name of file: ");
    gets(filename);
    while((secret = fopen(filename, "rt"))== NULL)
    {
        fprintf(stderr, "\tError:Cannot open input file.\n");
        for(i=0;i<256;i++)
        {
            filename[i]=NULL;
        }
        printf("\n\tEnter name of file: ");
        gets(filename);
    }
    i=0;
    while(!feof(secret))
    {
        ch=fgetc(secret);
        i++;
    }
    len=i;
    len2=len;
    rewind(secret);
    str = (unsigned char *) calloc(len, sizeof(char));
 
    for(a=0;a<len;a++)
    {
        str[i]=NULL;
    }
    ch=a=0;
    i=0;
    while(!feof(secret))
    {
         if(a>=len)printf("\n\tError:something wrong with file\n");
         else
         {
        str[a]=fgetc(secret);
        a++;
         }
    }
    rewind(secret);
    printf("\n\tCRC = 0x%X\n", Crc16(str, len-1));
    printf("\n\tPress eny key to exit programm");
 
    fclose(secret);
    free(str);
    getch();
    return(0);
 
}
unsigned short Crc16(unsigned char * pcBlock,  long long len)
{
    unsigned short crc = 0;
 
    while (len>0)
    {
    crc = (crc >> 8) ^ Crc16Table[(crc & 0xFF) ^ *pcBlock++];
        len--;
    }
    
    return crc;
}