Author:

• Name: Jens Schweikhardt
  Location: DE - Federal Republic of Germany (Germany)

To build:

    make all

The author stated:

In the remote event that the input has more than 8192 files with the same size (on systems where sizeof (char *) == 4, or 4096 when sizeof (char *) == 8), increase the manifest constant 32767 on line 31.

An alternate version allows one to fix this; see Alternate code below.

To use:

If dir is the directory (or a directory tree) where you keep all your favorite pictures off the Internet or from alt.binaries.pictures.*, do

    find dir -type f -print | ./samefile

Maybe you will not need to buy another 10 GB disk to store them. :-)

Try:

If you’re in this winning entry’s directory:

    ./try.sh

Notice that the tool finds some files that are duplicates.

Alternate code:

If you run into the problem that the author described (above) you can redefine the size as described; this was done with the SZ macro defined in the Makefile which if (in the code) is unset will be set to the default, 32767.

Alternate build:

If you wish to change the size to say, 55555:

    make clobber SIZE=55555 alt

or whatever you wish to redefine it to.

Judges’ remarks:

This program is not as smart as to detect identity of JPEG files with the same picture but with different compression levels. On the other hand compression can make a thing look/sound/feel different so these probably aren’t duplicates anyway. :-)

Some time after this entry won, the Internet Systems Consortium (ISC) added the samefile utility to its treasure of software gems! See the author’s page about the tool as well at http://www.schweikhardt.net/samefile.

Author’s remarks:

The source is expected to conform to IEEE Std 1003.1-1990 (“POSIX”). Thank God the IEEE does not standardize a coding style…

What this program does

This is best explained in the man page. The troff source for this man page can be found in the file samefile.1. To render try:

    `man ./samefile.1

Otherwise:

SAMEFILE(1) User Manuals SAMEFILE(1)

NAME

samefile - find identical files

SYNOPSIS

    ./samefile

DESCRIPTION

samefile reads a list of file names (one file name per line) from stdin. For each file name pair with identical contents, a line consisting of six tab separated fields is output: The size in bytes, two file names, the character = if the two files are on the same device, X otherwise, and the link counts of the two files. The output is sorted in reverse order by size.

samefile uses two stages to give optimum performance.

In the first stage, all non-plain files are silently ignored (directories, devices, FIFOs, sockets, symbolic links) as well as files for which stat(2) or open(2) fails. The result of the first stage (the file names) is written into a binary tree with one node for every file size. Each node is in turn a linked list of file names along with inode and device information. It is also at this first stage where checks for hard links are done. For any inode only one filename will be added to the binary tree.

In the second stage all files having the same size are compared against each other. The rules of mathematical logic are applied to reduce work and output noise: if files A, B, and C have the same size and samefile finds that A = B and A = C then it will not compare B against C (and will not output a line for B and C) but only for A = B and A = C. The algorithm will detect equality across arbitrarily long chains. Note however, that because only the first filename per inode gets into the first stage, the output for a group of identical files with different inode numbers is also minimized. Suppose you have six identical files of size 100 in an inode group consisting of the three inodes with numbers 10, 20 and 30:

    $ ls -i   # output edited for readability:
      10 file1     20 file4     30 file6
      10 file2     20 file5
      10 file3
    $ ls | ./samefile
    100     file1   file4   =       3       2
    100     file1   file6   =       3       1

The sum of the sizes in the first column is the amount of disk space you could gain by making all 6 files links to only one file or remove all but one of the files. To be precise, disk space is allocated in blocks - you will probably gain two blocks here, rather than 200 bytes. Note that it is not enough to just remove file4 and file6 (you would gain only 100 bytes because file5 still exists.)

LIMITATIONS

samefile was written with no limits in mind. The number of input lines is unlimited. The size of the actual files is only limited by available virtual memory needed to compare one pair of files. The only hard limit is the constraint that there should not be more than about 8192 files having the same size. Experience has shown that there are rarely more than a couple dozen files of the same size.

EXAMPLES

For everybody:

What are the duplicates under my home directory?

    find $HOME | ./samefile

For the sysadmin folks:

Report all duplicate files under /usr larger than 16k:

    find /usr -size +16384c -a -type f | ./samefile

For the ftp and WWW admins: How much space is wasted below our site’s /pub directory?

    find /pub -type f | samefile | awk '{sum += $1} END {print sum}'

EXIT > STATUS

If samefile runs out of memory the exit status is 1, zero otherwise.

SEE ALSO

find(1), ln(1), rm(1)

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.

Why I think it is obfuscated

• The code is commented but this gives only an extremely faint clue.
• The layout does not reflect the program structure, or does it?
• The data structure the program builds is an ordered binary tree where each node consists of a linked list. This is probably too heavy to comprehend for N percent of all programmers ;-)
• The tree is built and traversed by recursive function calls.
• Why does this program look at dev_t and ino_t? Heavy wizardry.
• Can you find out how the algorithm works that detects identity of files across arbitrarily long chains? In my private unobfuscated source there are 17 lines describing just that. A classical case for the well known /* You're not supposed to understand this */ comment.
• Would Dijkstra consider this goto harmful?
• The only string literal is split in more than a dozen substrings.
• For the task at hand the code is pretty terse. Some dynamic memory is even freed as soon as it’s no longer needed. I have tried to make the preprocessor directives as short as possible. This means no spaces after #include and no spaces after some of the macro names.

Are you an algorithms guru? If so you should know why the output appears sorted without any use of qsort(3) or system("sort").

Rewrite (hah!) the program to use memory mapping of files instead of read(2). Does this affect performance noticeably?

It’s easy to write a script that takes samefile’s output and hard links (or symlinks) identical files. Writing such a script is left as an exercise for the deobfuscator.

When you run samefile on some directory tree, try to estimate how much you think it will find. Is the actual result anywhere near your estimate? How many copies of the GPL COPYING file are there on your filesystems? :-)

In the remote event that the input has more than 8192 files with the same size (on systems where sizeof (char *) == 4, or 4096 when sizeof (char *) == 8), increase the manifest constant 32767 on line 31.

Primary files

• schweikh3.c - entry source code
• Makefile - entry Makefile
• schweikh3.alt.c - alternate source code
• schweikh3.orig.c - original source code
• samefile.1.dupe - copy of samefile man page
• samefile.1 - samefile man page
• try.sh - script to try entry

Secondary files

• 1998_schweikh3.tar.bz2 - download entry tarball
• README.md - markdown source for this web page
• .entry.json - entry summary and manifest in JSON
• .gitignore - list of files that should not be committed under git
• .path - directory path from top level directory
• index.html - this web page

Jump to: top