2004/omoikane - Best utility (original) (raw)

To build:

    make

To use:

    ./omoikane infile layout outfile

Try:

    ./try.sh

For extra credit, spot the checksum in the source file itself. What makes this one particularly interesting is that the checksum, strictly speaking, isn’t being computed at all. Rather, the file is modified to have the desired checksum! This does, we suppose, solve the problem that inserting a checksum would change the file’s checksum; if you have to change it, you might as well change it!

This program was submitted under the name Rinia, so the Author’s remarks refer to it as such.

Rinia (1/14/04)

Rinia is a tool for embedding CRCs in text files. No need to assign strange file names or package .SFVs with your files! Rinia will insert a human-readable checksum string in the text itself! ^_^

Usage

    ./omoikane <infile> <layout> <outfile>

All three arguments may reference the same file. All files is assumed to be of ASCII character set.

Input

Any file larger than 8 bytes is valid. Upper limit on file size is limited by memory.

If input file can’t be loaded for any reason, Rinia will say “bad input” and exit immediately.

Layout

The layout file contains a map of where Rinia can patch. The content consists of marker characters (where Rinia is allowed to change in the original text) and other characters (where the original text will stay as is). Rinia will select the marker character using these heuristics:

  1. Ignore all space/control/extended characters.
  2. Select characters that appeared 8 times in a row.
  3. Of those selected characters, pick one with the lowest frequency.

One way to create the layout file is to make a copy of the original file, replace where you want Rinia to match with a rare character, then replace the remaining ones with space. Often it’s possible to write the original file in such a way that it can be used as a layout.

Example file/layout:

    file:                layout:
       crc = ########       ... . XXXXXXXX
       patch = ######       ..... . XXXXXX
       other text here      ..... .... ....

And Rinia may generate a file similar to this:

    crc = 1bad73cf
    patch = #FR-QQ
    other text here

Note that the original file by itself is a valid layout file, sinceRinia will recognize that only # marks enough space to hold the checksum, and has the lowest frequency.

If layout file can’t be used for any reason, Rinia will say “bad layout” and exit immediately.

Output m

If Rinia finds a usable checksum, she will say “OK.” and write to the specified output file. Otherwise the exit message is “fail” and nothing is written.

Chances of success and how long it takes to run to completion is completely dominated by the layout file design. In general, Riniaruns faster if the checksum is placed before the other marker characters rather than after, e.g.:

    a: X X XXX      b: XXXXXXXX     c: XXXXXXXX
       XXXXXXXX        X X XXX         XXXXX

a usually takes the longest time, while c usually is the fastest.

That said, each run is randomized with respect to system time, so it’s not always certain whether Rinia will succeed in a short time or not. In general, 5 marker characters are sufficient, and takes a few minutes.

For reference, rinia.c was produced on a 1.7GHz machine (the checksum is at line 17). Out of 25 runs, the shortest was 2 seconds while the longest took 5 minutes. Average was about a minute. Checksum for this file (b7a81524) took about 4 minutes.

Algorithm

Inserting a checksum in the file you are computing the checksum of isn’t quite trivial, since changing the file content would change the final checksum value.

To insert the checksum, Rinia first assumes a random final checksum, inserts that string, then modifies other parts to compensate for the change in content.

The compensation bytes are very easy to compute for binary files with enough consecutive scratch areas, but for text files with non-consecutive areas and limited degrees of freedom, Rinia has to try out all character combinations. This is done in a hierarchical fashion, computing only smallest partial checksum after each change.

For computers a few years old, this is probably still too slow to be acceptable… but at the speed the IOCCC judges run the contest these days, I am sure you have enough patience for it _^

Source code

rinia.c should compile anywhere (but with a few warnings). I don’t have a big-endian machine to test but Rinia should work there as well. ASCII character set required though.

Code is formatted to the shape of “Rinia” from the anime/game “Moekko Company”. Of course at 90x50 it’s hard to recognize anything^_^

Rinia is the slow but hardworking android. On some jobs she does very well, on others she would spend lots of time and effort but still fail…

Inventory for 2004/omoikane

Primary files

Secondary files

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