Author:

• Name: John Dalbec
  Location: US - United States of America (United States)

To build:

    make all

To use:

    ./dalbec

Try:

    ./try.sh

Why does the output end with 121?

Judges’ remarks:

The output is mis-leading … why does it also print 2047?

For a hint see https://web.archive.org/web/20000510191341/https://www.utm.edu/research/primes/prove2.html.

… and then try to understand the source! :-)

We (the judges) recommend that you take the time needed to understand how this program works. The source is small enough to make the effort reasonable, and complex enough to make it interesting.

Author’s remarks:

This program assumes that your terminal wraps lines automatically. If this is not the case, you may need to change the space in the “printf” format string to a newline character.

An extra feature of this program is that it allows you (in most cases) to estimate the stack space available to programs on your system. Simply allow the program to run to completion. The stack space is roughly proportional to the last number printed.

(On SunOS/cc, the proportionality constant is about 48 bytes.)

On completion, the program produces a core file that may be used to refine this estimate. (Or you could try typing limit if you’re running csh.) In rare cases the stack space may exceed 24*INT_MAX bytes, in which case the program will produce an inaccurate estimate.

This program is obfuscated by:

1. The use of easily-confused variable names.

2. Using the implicit !=0 in conditional expressions.

3. Substituting expressions such as !a for 0 where a!=0 and !a for 1 where a==0.

4. The fact that main() is a single expression and so beautifies poorly.

5. The fact that main() is used:

   • to compute powers in modular arithmetic.
   • to recursively call itself in the main loop.

Primary files

• dalbec.c - entry source code
• Makefile - entry Makefile
• dalbec.orig.c - original source code
• try.sh - script to try entry

Secondary files

• 1996_dalbec.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

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