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<div class="header">
  <img src="../../png/ioccc.png" alt="IOCCC image by Matt Zucker" width=300 height=110>
  <h1>The International Obfuscated C Code Contest</h1>
  <h2>1992/vern - Best of Show</h2>
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  <a class="Left" href="../nathan/index.html">&larr; nathan</a>
  <a class="Left" href="../index.html">&uarr; 1992 &uarr;</a>
  <a class="Left" href="../westley/index.html">westley &rarr;</a>
  <a class="Right" href="https://github.com/ioccc-src/temp-test-ioccc/1992/vern/vern.c">C code</a>
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<h2 id="author"><a name="author"></a>Author:</h2>
<ul>
<li>Name: <a href="../../authors.html#Vern_Paxson">Vern Paxson</a><br>
Location: <a href="../../location.html#US">US</a> - <em>United States of America</em> (<em>United States</em>)</li>
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<h2 id="to-build">To build:</h2>
<pre class="&lt;!---sh--&gt;"><code>    make all</code></pre>
<h3 id="bugs-and-misfeatures">Bugs and (Mis)features:</h3>
<p>The current status of this entry is:</p>
<pre><code>    STATUS: INABIAF - please **DO NOT** fix</code></pre>
<p>For more detailed information see <a href="../../bugs.html#1992_vern">1992 vern bugs</a>.</p>
<h2 id="to-use">To use:</h2>
<pre class="&lt;!---sh--&gt;"><code>    ./vern 3    # &lt;-- default is 2</code></pre>
<h2 id="try">Try:</h2>
<p>You might start off by giving the following input:</p>
<pre><code>    63 43
    76 55
    71 52
    67 57               # &lt;-- this may take a while</code></pre>
<h2 id="judges-remarks">Judges’ remarks:</h2>
<p>The judges assume no responsibility for the obfuscated opening
suggested above. :-)</p>
<p>For a real quick game try:</p>
<pre class="&lt;!---sh--&gt;"><code>    ./vern

    65 55
    66 46</code></pre>
<p>NOTE: Move values must be restricted to the range 00 to 77.</p>
<p>NOTE: Because some mailers have problems with the original winning source,
a slightly modified version with shorter lines has been provided.</p>
<h2 id="authors-remarks">Author’s remarks:</h2>
<p>This program plays chess. You play the white pieces and the program the black
pieces. Moves are entered as a two-digit number specifying the coordinates of
the piece to be moved followed by another two-digit number specifying the
coordinates of where to move it. Rows and columns are numbered starting with
<code>(0,0)</code> for the upper-left-hand corner (black’s queen’s rook) and going to
<code>(7,7)</code> for the lower-right-hand corner (white’s king’s rook). For example, the
<code>PK4</code> opening for white is indicated as <code>"64 44" (without the quotes)</code>. Moves
are read using <code>scanf()</code> so care should be taken to not enter the wrong number
of fields.</p>
<p>If the move you enter is illegal then you are just prompted again to
move. If the move is legal then the new position is displayed and the
program starts computing its response. For every 64 board positions it
examines a “.” is printed. Once it has chosen its move it simply
displays the board updated to reflect the move.</p>
<p>Your move is prompted for using a string like <code>"1351 9 2 &gt;"</code>. The first
number is how many board positions the program examined when computing
its previous response. The second number is its anticipated value for
the move it just made (with larger numbers meaning an ultimately better
board position for it), and the third number is the evaluation it
assigns to the your current position (larger numbers meaning better
positions for you). A <code>&gt;</code> indicates that you are not in check; for
check it is changed to <code>!</code>.</p>
<p>There are a few limitations on play:</p>
<ul>
<li>castling can only be done on the king’s side.</li>
<li>prohibitions against castling out of or through check are not enforced.</li>
<li>prohibitions against castling if the king or rook have moved are not enforced.</li>
<li>pawns are always promoted to queens.</li>
<li>en passant is not allowed.</li>
<li>the board-position-repeated-three-times and fifty-moves stalemates are not
recognized.</li>
</ul>
<p>If you checkmate the computer it prints “Rats!” and exits. If your own
checkmate is imminent it prints “Har har.” but doesn’t exit so it can
rub your nose in defeat (which may be a move or two away if you’re
playing at a high “ply” - see the next paragraph). Stalemates are not
recognized per se, but when selecting its move the program assigns a
stalemate position a value of 0 so it will try for a stalemate if its
other options are worse.</p>
<p>The program takes one optional argument indicating the ply (lookahead) it should
use. The default is 2, which is enough to keep it from making flaming blunders.
The higher the ply, the better the play, and the longer you have to wait. For
example, at a ply of 2, a response to the <code>PK4</code> opening takes about 1.8 CPU
seconds on a SPARC ELC when compiled <code>-O</code> with Sun <code>cc</code>. A ply of 3 takes 13
CPU seconds (and results in a different response to <code>PK4</code>) and a ply of 4 takes
a little under 4 CPU minutes (and for <code>PK4</code> comes up with the same response as a
ply of 3). A ply of 3 is in general good enough to avoid not only flaming
blunders but also immediately stupid moves altogether. A ply of 4 will find all
mate-in-two’s.</p>
<h3 id="obfuscation">Obfuscation</h3>
<p>This program is obfuscated in a number of ways. First, it abuses the
contest rule regarding ‘;’, ‘{’, and ‘}’ in the source not counting
against the non-whitespace character limit if followed by whitespace.
As can be seen from the build file, certain combinations of these
characters followed by a particular whitespace character are expanded
at build time into much-needed text. Without this hack the program
wouldn’t pass the character limit. A nice side effect is that the
initial source is gloriously difficult to peruse. In particular, the Makefile
does:</p>
<pre class="&lt;!---sh--&gt;"><code>    sed &lt;vern.c &#39;s/{ /(/g;s/} /)/g;s/;       /#define /&#39; | \
        sed &#39;s/}      /=/g;s/{        /i/g&#39; &gt;vern.tmp.c</code></pre>
<p>and compiles <code>vern.tmp.c</code>.</p>
<p>Second, the rather ho-hum step has been taking of naming each variable
with a one or two letter identifier to render it meaningless (and save
space). This tactic is rather effective in a complex program.</p>
<p>Third, the board is represented not as an array of pieces but rather as
an array of function pointers (with a separate array representing
whether a piece is white or black). This obfuscates the program’s
central data structure. There is one function per type of piece; when
called they return 1 if the current move is illegal for that piece and
0 if it is legal. The functions also as a side effect set a global
indicating the type of the piece (though the type can also be determined
by comparing the function pointer with the function itself).</p>
<p>Fourth, there are no independent routines for generating potential moves.
Instead, the program generates its moves by brute force: for every piece it
controls on the board, it attempts to move it to every square on the board.
Those moves that are legal it then explores to see their effects (using
<a href="https://en.wikipedia.org/wiki/Alpha-beta_pruning">alpha-beta search</a>). This
tactic somewhat obfuscates the algorithm used by the program.</p>
<p>Finally, there are three key constants that occur throughout the program: <code>64</code>,
<code>8</code>, and <code>3</code> (<code>lg 8</code>). Rather than making these available at compile-time,
which provides a hint as to what the program is up to, they are computed
probabilistically at run-time. An instance of the <a href="https://en.wikipedia.org/wiki/Renewal_theory#Inspection_paradox">“Inspection
Paradox”</a> is
used which happens to produce a value that on average is close to <code>.64</code>.
<code>10000</code> instances of this value are computed, added up, and then divided by
<code>100</code>. Sometimes the value produced will be <code>63</code> or <code>65</code> instead of <code>64</code> (but
I’ve never observed any other values), so the result is then rounded to the
nearest multiple of <code>4</code>, and then the other constants are derived from it.</p>
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<h1 id="inventory-for-1992vern"><a name="inventory"></a>Inventory for 1992/vern</h1>
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<h2 id="primary-files">Primary files</h2>
<ul>
<li><a href="https://github.com/ioccc-src/temp-test-ioccc/blob/master/1992/vern/vern.c">vern.c</a> - entry source</li>
<li><a href="https://github.com/ioccc-src/temp-test-ioccc/blob/master/1992/vern/Makefile">Makefile</a> - entry Makefile</li>
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