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<h1>The International Obfuscated C Code Contest</h1>
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<h2>2024/endoh2 - Prize in solid body physics</h2>
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<h3>rotating rigid body simulation</h3>
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<a class="Left" href="../endoh1/index.html">← 2024/endoh1</a>
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<a class="Right" href="https://github.com/ioccc-src/winner/blob/master/2024/endoh2/prog.orig.c">Original C</a>
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<a class="Right" href="https://validator.w3.org/nu/?doc=https%3A%2F%2Fwww.ioccc.org%2F2024%2Fendoh2%2Findex.html">✓</a>
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<h2 id="author">Author:</h2>
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<ul>
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<li>Name: <a href="../../authors.html#Yusuke_Endoh">Yusuke Endoh</a><br>
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Location: <a href="../../location.html#JP">JP</a> - <em>Japan</em></li>
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<h2 id="award-presentation">Award presentation:</h2>
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||
<p>Watch the <a href="https://www.youtube.com/@OurFavoriteUniverse">Our Favorite Universe</a>
|
||
YouTube show for this entry:</p>
|
||
<blockquote>
|
||
<p><a href="https://www.youtube.com/watch?v=KT9Fgg4t7gY">IOCCC28 - 2024/endoh2 - Prize in solid body physics</a></p>
|
||
</blockquote>
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||
<h2 id="to-build">To build:</h2>
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<pre><code> make all</code></pre>
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<h2 id="to-use">To use:</h2>
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<pre><code> ./prog < body.txt</code></pre>
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<h2 id="try">Try:</h2>
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<pre><code> ./try.sh</code></pre>
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<h2 id="judges-remarks">Judges’ remarks:</h2>
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<p>Playing with this entry, we were able to experience joy akin to
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<a href="https://repository.aip.org/wolfgang-pauli-and-niels-bohr-demonstrating-tippe-top-toy">what Niels Bohr and Wolfgang Pauli could have experienced when they played with the Tippe-Top</a>.</p>
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<p>We invite you to experiment beyond the <a href="https://github.com/ioccc-src/winner/blob/master/2024/endoh2/try.sh">try.sh</a>, and test with these “<em>body.txt</em>” files:</p>
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<ul>
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<li>bottle.txt</li>
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<li>cylinder.txt</li>
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<li>egg.txt</li>
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<li>halo.txt</li>
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<li>stick.txt</li>
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<li>tippe-top.txt</li>
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<li>tippe-top2.txt</li>
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<li>tippe-top3.txt</li>
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<li>top.txt</li>
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<li>top2.txt</li>
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<li>top3.txt</li>
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<li>top4.txt</li>
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<li>umbrella.txt</li>
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<li>vase.txt</li>
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</ul>
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<p>For example:</p>
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<pre><code> ./prog < vase.txt</code></pre>
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<p>Moreover, you should experiment with various simulation parameters. On the make line you may set:</p>
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<pre><code> | Perameter | Default | Meaning
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|-------------|-----------------------------------------------------|
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| WIDTH | 120 | Screen width |
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| | | |
|
||
| HEIGHT | 24 | Screen height |
|
||
| | | |
|
||
| TIMESTEP | 0.01 | Simulation time step |
|
||
| | | a smaller dt |
|
||
| | | increases precision |
|
||
| | | down the simulation |
|
||
| | | |
|
||
| RESTITUTION | 0.5 | Coefficient of restitution |
|
||
| | | |
|
||
| FRICTION | 0.5 | Coefficient of friction |
|
||
| | | |
|
||
| RPM | 750 | Initial rotation speed in RPM |
|
||
| | | |
|
||
| FASTRPM | 4000 | Tipple top initial rotation speed in RPM |
|
||
| | | used by ./tipple, not ./prog |
|
||
| | | |
|
||
| TILT | 5 | Initial tilt angle |
|
||
| | | in degrees |
|
||
| | | 0 is upright |
|
||
| | | positive values tilt to the right |</code></pre>
|
||
<p>For example, to increase the “<em>Coefficient of friction</em>” to <strong>0.75</strong>,
|
||
the “<em>Initial rotation speed in RPM</em>” to <strong>600</strong>, and the “<em>Initial
|
||
tilt angle</em>” to <strong>15</strong>:</p>
|
||
<pre><code> make clobber all FRICTION=0.75 RPM=600 TILT=15
|
||
|
||
./prog < top.txt</code></pre>
|
||
<p>Now we would like to see if this entry can be hacked to demonstrate the
|
||
<a href="https://en.wikipedia.org/wiki/Tennis_racket_theorem">Dzhanibekov effect</a>.</p>
|
||
<h2 id="authors-remarks">Author’s remarks:</h2>
|
||
<h3 id="a-spinning-top-simulator">A Spinning Top Simulator</h3>
|
||
<pre><code> @@
|
||
@@
|
||
@@@@@@@@@@@@
|
||
@@ @@@@@@@@@@@@@@@@
|
||
@@@@ @@@@@@@@@@@@@@@@
|
||
@@@@@@@ @@@@@@@@@@@ @
|
||
@@@@@@@@@@@ @@@
|
||
@@@@@@@@@@@@@@@@@@@@@@@@
|
||
@@@@@@@@@@@@@@@@@@@@@@@@
|
||
@@@@@@@@@@@@@@@@@@@@@
|
||
@@@@@@@@@@@@@@@@@@@@
|
||
@@@@@@@@@@@@@@@@
|
||
@@@@@@@@@@@@@
|
||
@@@@@@@@@
|
||
@@@@
|
||
@@
|
||
@@
|
||
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
|
||
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
|
||
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@</code></pre>
|
||
<p>This is a physics engine that simulates the motion of a rotating rigid body, such as a <a href="https://en.wikipedia.org/wiki/Top">top</a>.</p>
|
||
<h4 id="how-to-run">How to run</h4>
|
||
<p>Compilation requires specifying several <code>-D</code> options:</p>
|
||
<pre><code>$ gcc -O3 -DW=120 -DH=24 -Ddt=0.01 -De=0.5 -Du=0.5 -Drpm=1000 -Dtilt=5 -o prog prog.c</code></pre>
|
||
<p>These options define the following parameters:</p>
|
||
<ul>
|
||
<li><code>-DW=120</code>: Screen width.</li>
|
||
<li><code>-DH=24</code>: Screen height.</li>
|
||
<li><code>-Ddt=0.01</code>: Simulation time step (a smaller <code>dt</code> increases precision but slows down the simulation).</li>
|
||
<li><code>-De=0.5</code>: Coefficient of restitution.</li>
|
||
<li><code>-Du=0.5</code>: Coefficient of friction.</li>
|
||
<li><code>-Drpm=1000</code>: Initial rotation speed (in RPM).</li>
|
||
<li><code>-Dtilt=5</code>: Initial tilt angle (in degrees; 0 is upright; positive values tilt to the right).</li>
|
||
</ul>
|
||
<p>Then, run the program like this:</p>
|
||
<pre><code>$ ./prog < top.txt</code></pre>
|
||
<h4 id="input-file-format">Input file format</h4>
|
||
<p>The input file defines the shape of a rigid body. Here’s an example:</p>
|
||
<pre><code>// A typical spinning top
|
||
|
||
// Use `-Drpm=1000 -Dtilt=5`
|
||
|
||
.
|
||
.
|
||
. // Twirl here
|
||
.
|
||
.
|
||
.
|
||
...
|
||
#####
|
||
##
|
||
# // Spin!
|
||
##
|
||
#####
|
||
..
|
||
.
|
||
.
|
||
. // Looks painful</code></pre>
|
||
<p>This figure is rotated around its left edge to create a 3D object (a solid of revolution).
|
||
Only solids of revolution are supported by this simulator.</p>
|
||
<p>Internally, the rigid body is modeled as a collection of particles (point masses). Each character in the input file has the following meaning:</p>
|
||
<ul>
|
||
<li>A full-stop (or any printable character whose LSB is 0): Represents light particle.</li>
|
||
<li>A hash (or any character whose LSB is 1): Represents heavy particle.</li>
|
||
<li>A space: Represents no particle (empty space).</li>
|
||
<li>A slash: Indicates the start of a comment; everything after it on the line is ignored.</li>
|
||
</ul>
|
||
<h4 id="algorithm-overview">Algorithm overview</h4>
|
||
<p>The core algorithm works as follows:</p>
|
||
<ol type="1">
|
||
<li>Compute the inertia tensor</li>
|
||
<li>Compute an inverse of the tensor</li>
|
||
<li>For each particle in contact with the floor:
|
||
3-1. Compute the normal force
|
||
3-2. Compute friction
|
||
3-3. Convert these into the resultant force and torque acting on the body’s center of mass</li>
|
||
<li>Update the position, velocity, angular velocity, and orientation of the body</li>
|
||
</ol>
|
||
<p>For the detailed theory, refer to a physics textbook.</p>
|
||
<p>To keep code size minimal, this simulator uses <a href="https://en.wikipedia.org/wiki/Quaternion">quaternions</a> to represent 3D vectors and matrices.
|
||
Quaternion multiplication is powerful, allowing the inertia tensor and its inverse to be calculated concisely, each requiring only about 100 characters of code.
|
||
Because <code>math.h</code> doesn’t natively support quaternions for these operations, it’s not used at all. This also means linking with <code>-lm</code> is unnecessary.</p>
|
||
<p>Motion blur is used in the rendering. You can observe it more clearly by pausing the animation (e.g., with Ctrl+S in some terminals).</p>
|
||
<p>Distant points appear as colons (<code>:</code>), while closer ones appear as at-signs (<code>@</code>).</p>
|
||
<h4 id="included-data">Included data</h4>
|
||
<p>A few shape files are provided. The top*.txt files are standard spinning top shapes.</p>
|
||
<pre><code>$ gcc -O3 -DW=120 -DH=24 -Ddt=0.01 -De=0.5 -Du=0.5 -Drpm=1000 -Dtilt=5 -o prog prog.c && ./prog < top.txt
|
||
|
||
$ gcc -O3 -DW=120 -DH=24 -Ddt=0.01 -De=0.5 -Du=0.5 -Drpm=500 -Dtilt=5 -o prog prog.c && ./prog < top2.txt
|
||
|
||
$ gcc -O3 -DW=120 -DH=24 -Ddt=0.01 -De=0.5 -Du=0.5 -Drpm=500 -Dtilt=5 -o prog prog.c && ./prog < top3.txt
|
||
|
||
$ gcc -O3 -DW=120 -DH=24 -Ddt=0.01 -De=0.5 -Du=0.5 -Drpm=500 -Dtilt=5 -o prog prog.c && ./prog < top4.txt</code></pre>
|
||
<p>The <a href="https://en.wikipedia.org/wiki/Tippe_top">tippe top</a> is a top that flips itself over when spun quickly.</p>
|
||
<pre><code>$ gcc -O3 -DW=120 -DH=24 -Ddt=0.01 -De=0.5 -Du=0.5 -Drpm=2200 -Dtilt=5 -o prog prog.c && ./prog < tippe-top.txt
|
||
|
||
$ gcc -O3 -DW=120 -DH=24 -Ddt=0.01 -De=0.5 -Du=0.5 -Drpm=4000 -Dtilt=5 -o prog prog.c && ./prog < tippe-top2.txt
|
||
|
||
$ gcc -O3 -DW=120 -DH=24 -Ddt=0.01 -De=0.5 -Du=0.5 -Drpm=4000 -Dtilt=5 -o prog prog.c && ./prog < tippe-top3.txt</code></pre>
|
||
<p>Spinning a boiled egg? Try this with <code>-Dtilt=90</code> to start it from a lying position.</p>
|
||
<pre><code>$ gcc -O3 -DW=120 -DH=24 -Ddt=0.01 -De=0.5 -Du=0.5 -Drpm=1300 -Dtilt=90 -o prog prog.c && ./prog < egg.txt</code></pre>
|
||
<p>Spin everyday items – no risk of breaking them:</p>
|
||
<pre><code>$ gcc -O3 -DW=120 -DH=24 -Ddt=0.01 -De=0.5 -Du=0.5 -Drpm=1000 -Dtilt=175 -o prog prog.c && ./prog < bottle.txt
|
||
|
||
$ gcc -O3 -DW=120 -DH=24 -Ddt=0.01 -De=0.5 -Du=0.5 -Drpm=1000 -Dtilt=5 -o prog prog.c && ./prog < bottle.txt
|
||
|
||
$ gcc -O3 -DW=120 -DH=24 -Ddt=0.01 -De=0.5 -Du=0.5 -Drpm=1000 -Dtilt=5 -o prog prog.c && ./prog < umbrella.txt
|
||
|
||
$ gcc -O3 -DW=120 -DH=24 -Ddt=0.01 -De=0.5 -Du=0.5 -Drpm=2000 -Dtilt=5 -o prog prog.c && ./prog < vase.txt
|
||
|
||
$ gcc -O3 -DW=120 -DH=24 -Ddt=0.01 -De=0.5 -Du=0.5 -Drpm=2000 -Dtilt=20 -o prog prog.c && ./prog < vase.txt</code></pre>
|
||
<p>More shapes: <code>stick.txt</code>, <code>cylinder.txt</code>, <code>halo.txt</code>, and even <code>prog.c</code>.</p>
|
||
<p>Yes, <code>prog.c</code> itself can be used as input. Try it:</p>
|
||
<pre><code>$ gcc -O3 -DW=120 -DH=24 -Ddt=0.01 -De=0.5 -Du=0.5 -Drpm=1000 -Dtilt=5 -o prog prog.c && ./prog < prog.c</code></pre>
|
||
<!--
|
||
|
||
Copyright © 1984-2025 by Landon Curt Noll and Leonid A. Broukhis. All Rights Reserved.
|
||
|
||
You are free to share and adapt this file under the terms of this license:
|
||
|
||
Creative Commons Attribution-ShareAlike 4.0 International (CC BY-SA 4.0)
|
||
|
||
For more information, see:
|
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|
||
https://creativecommons.org/licenses/by-sa/4.0/
|
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|
||
<div id="inventory">
|
||
<h1 id="inventory-for-2024endoh2">Inventory for 2024/endoh2</h1>
|
||
</div>
|
||
<h2 id="primary-files">Primary files</h2>
|
||
<ul>
|
||
<li><a href="https://github.com/ioccc-src/winner/blob/master/2024/endoh2/prog.c">prog.c</a> - entry source code</li>
|
||
<li><a href="https://github.com/ioccc-src/winner/blob/master/2024/endoh2/Makefile">Makefile</a> - entry Makefile</li>
|
||
<li><a href="https://github.com/ioccc-src/winner/blob/master/2024/endoh2/prog.orig.c">prog.orig.c</a> - original source code</li>
|
||
<li><a href="https://github.com/ioccc-src/winner/blob/master/2024/endoh2/bottle.txt">bottle.txt</a> - 1/2 of a wine bottle</li>
|
||
<li><a href="https://github.com/ioccc-src/winner/blob/master/2024/endoh2/cylinder.txt">cylinder.txt</a> - 1/2 of a cylinder</li>
|
||
<li><a href="https://github.com/ioccc-src/winner/blob/master/2024/endoh2/egg.txt">egg.txt</a> - 1/2 of an egg</li>
|
||
<li><a href="https://github.com/ioccc-src/winner/blob/master/2024/endoh2/halo.txt">halo.txt</a> - 1/2 of a halo</li>
|
||
<li><a href="https://github.com/ioccc-src/winner/blob/master/2024/endoh2/stick.txt">stick.txt</a> - 1/2 of a stick</li>
|
||
<li><a href="https://github.com/ioccc-src/winner/blob/master/2024/endoh2/tippe-top2.txt">tippe-top2.txt</a> - 1/2 of a tippe top style 2</li>
|
||
<li><a href="https://github.com/ioccc-src/winner/blob/master/2024/endoh2/tippe-top3.txt">tippe-top3.txt</a> - 1/2 of a tippe top style 3</li>
|
||
<li><a href="https://github.com/ioccc-src/winner/blob/master/2024/endoh2/tippe-top.txt">tippe-top.txt</a> - 1/2 of a tippe top style 1</li>
|
||
<li><a href="https://github.com/ioccc-src/winner/blob/master/2024/endoh2/top2.txt">top2.txt</a> - 1/2 of a classic top style 2</li>
|
||
<li><a href="https://github.com/ioccc-src/winner/blob/master/2024/endoh2/top3.txt">top3.txt</a> - 1/2 of a classic top style 3</li>
|
||
<li><a href="https://github.com/ioccc-src/winner/blob/master/2024/endoh2/top4.txt">top4.txt</a> - 1/2 of a classic top style 4</li>
|
||
<li><a href="https://github.com/ioccc-src/winner/blob/master/2024/endoh2/top.txt">top.txt</a> - 1/2 of a classic top style 1</li>
|
||
<li><a href="https://github.com/ioccc-src/winner/blob/master/2024/endoh2/try.sh">try.sh</a> - script to try entry</li>
|
||
<li><a href="https://github.com/ioccc-src/winner/blob/master/2024/endoh2/umbrella.txt">umbrella.txt</a> - 1/2 of an umbrella</li>
|
||
<li><a href="https://github.com/ioccc-src/winner/blob/master/2024/endoh2/vase.txt">vase.txt</a> - 1/2 of a flower vase</li>
|
||
</ul>
|
||
<h2 id="secondary-files">Secondary files</h2>
|
||
<ul>
|
||
<li><a href="2024_endoh2.tar.bz2">2024_endoh2.tar.bz2</a> - download entry tarball</li>
|
||
<li><a href="https://github.com/ioccc-src/winner/blob/master/2024/endoh2/README.md">README.md</a> - markdown source for this web page</li>
|
||
<li><a href="https://github.com/ioccc-src/winner/blob/master/2024/endoh2/.entry.json">.entry.json</a> - entry summary and manifest in JSON</li>
|
||
<li><a href="https://github.com/ioccc-src/winner/blob/master/2024/endoh2/.gitignore">.gitignore</a> - list of files that should not be committed under git</li>
|
||
<li><a href=".path">.path</a> - directory path from top level directory</li>
|
||
<li><a href="index.html">index.html</a> - this web page</li>
|
||
</ul>
|
||
<hr style="width:10%;text-align:left;margin-left:0">
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<h4>
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Jump to: <a href="#">top</a>
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</h4>
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<!--
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Copyright © 1984-2025 by Landon Curt Noll. All Rights Reserved.
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You are free to share and adapt this file under the terms of this license:
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Creative Commons Attribution-ShareAlike 4.0 International (CC BY-SA 4.0)
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For more information, see:
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https://creativecommons.org/licenses/by-sa/4.0/
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-->
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<div class="footer">
|
||
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||
<div id="copyright"><h3>Copyright © 1984-2025 by Landon Curt Noll:
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<a href="https://creativecommons.org/faq/#what-does-some-rights-reserved-mean"
|
||
target="_blank"
|
||
rel="license noopener noreferrer">Some Rights Reserved</a></h3>
|
||
<p>
|
||
This work is <b>licensed by Landon Curt Noll</b> under
|
||
<b><a href="https://creativecommons.org/licenses/by-sa/4.0/?ref=chooser-v1"
|
||
target="_blank"
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rel="license noopener noreferrer"
|
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style="display:inline-block;">CC BY-SA 4.0</a></b>.
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<img style="height:22px!important;margin-left:3px;vertical-align:text-bottom;"
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src="../../png/cc.png"
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<img style="height:22px!important;margin-left:3px;vertical-align:text-bottom;"
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src="../../png/by.png"
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alt="person inside circle symbol">
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<img style="height:22px!important;margin-left:3px;vertical-align:text-bottom;"
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|
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alt="arrow looping back on itself inside circle symbol"><br>
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You should <b>carefully review</b> the
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||
You may wish to review the
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<b><a href="../../license.html"
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rel="license noopener noreferrer">highlights of some of the key features and terms</a></b>
|
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of <b>CC BY-SA 4.0</b>.<br>
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||
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|
||
|
||
<div id="coda"><h3>Coda</h3>
|
||
<p>
|
||
<a href="https://validator.w3.org/nu/?doc=https%3A%2F%2Fwww.ioccc.org%2F2024%2Fendoh2%2Findex.html" rel="nofollow">Nu HTML check this web page</a><br>
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<a href="#top">Jump to top</a> <a href="#content">Jump to Content</a> <a href="#inventory">Jump to Inventory</a><br>
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