## To build:

``` <!---sh-->
    make all
```


## To use:

``` <!---sh-->
    ./tomtorfs filename bitwidth polynom reflected init xor
```

where:

- `filename`
    * file to calculate the CRC of
- `bitwidth`
    * width of the CRC in bits (decimal)
- `polynom`
    * polynomial used for the CRC (hexadecimal)
- `reflected`
    * 0 if the CRC is not reflected, 1 if it is
- `init`
    * initial value for the CRC (hexadecimal)
- `xor`
    * value to xor the final CRC with (hexadecimal)


## Try:

``` <!---sh-->
    ./try.sh
```


## Judges' remarks:

If you don't believe the program is correct, try and compare the output
of the program with the value of CRCs computed by PKZIP or any other
archiver that uses the CRC-32 algorithm.


## Author's remarks:

Usage:

``` <!---sh-->
    ./tomtorfs filename bitwidth polynom reflected init xor
```

where:

- `filename`
    * file to calculate the CRC of
- `bitwidth`
    * width of the CRC in bits (decimal)
- `polynom`
    * polynomial used for the CRC (hexadecimal)
- `reflected`
    * 0 if the CRC is not reflected, 1 if it is
- `init`
    * initial value for the CRC (hexadecimal)
- `xor`
    * value to xor the final CRC with (hexadecimal)


### Examples


#### CRC-16:

``` <!---sh-->
    ./tomtorfs filename 16 1021 0 FFFF 0000
```


#### CRC-32:


``` <!---sh-->
    ./tomtorfs filename 32 04C11DB7 1 FFFFFFFF FFFFFFFF
```


#### XMODEM:

``` <!---sh-->
    ./tomtorfs filename 16 8408 1 0000 0000
```


#### ARC:

``` <!---sh-->
    ./tomtorfs filename 16 8005 1 0000 0000
```


### Portability notes

The program assumes `CHAR_BIT==8`. Results are undefined if `bitwidth >
sizeof(unsigned long) * 8`. It is also assumed that `EOF` is `-1` and `1` is a
valid exit value to indicate failure (replace them with `EOF` resp.
`EXIT_FAILURE` if the assumptions are false). Otherwise ANSI/ISO C.

No special build options required.


### Special notes

Count the number of non-whitespace characters in the source
file to find out what dark force drove me to write this ;-).


### NOTICE to those who wish for a greater challenge:

**If you want a greater challenge, don't read any further**:
just try to understand the program via the source.

If you get stuck, come back and read below for additional hints and information.


### How this entry works:

First of all, if you run a C beautifier on the program (thereby of course losing
the nice code shape I worked so hard on ;-) ) the code will become a bit more
readable.

The variable names are not very descriptive. They have the
following meanings:

- `a`
    * argc
- `A`
    * argv
- `b`
    * array of `unsigned long` variables:
    - `b[0]`
        * bit-width of the CRC (`argv[2]`)
    * `b[1]`
        - CRC polynomial (`argv[3]`)
    * `b[2]`
        - nonzero if CRC is reflected (argv[4])
    * `b[3]`
        - initial value of CRC (argv[5])
    * `b[4]`
        - value final CRC is XORed with (argv[6])
    * `b[5]`
        - CRC value
    * `b[6]`
        - byte from file
    * `b[7]`
        - counter

- `B`
    * `FILE *` through which the file `argv[1]` is opened

- `C`
    - `typedef` for `unsigned long`

The first `for` loop simply reads the command-line arguments
in the `b[]` array. The third parameter to `strtoul(3)` causes
all arguments to be in base-16 (hex) except the first one,
which will be in base-10 (decimal).

The rest of the program "simply" calculates the CRC in `b[5]`
and then prints it to `stdout`. The bit reflection loops may
appear a bit tricky at first, but if you have a good look
at them they should become obvious also.

###  Acknowledgments:

Ross N. Williams, whose CRC guide was very useful to me
for writing this. You can find it in [crc.txt](crc.txt).


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