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<title>2020/giles - Most phony</title>
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<img src="../../png/ioccc.png" alt="IOCCC image by Matt Zucker" width=300 height=110>
<h1>The International Obfuscated C Code Contest</h1>
<h2>2020/giles - Most phony</h2>
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<a class="Left" href="../ferguson2/index.html">&larr; ferguson2</a>
<a class="Left" href="../index.html">&uarr; 2020 &uarr;</a>
<a class="Left" href="../kurdyukov1/index.html">kurdyukov1 &rarr;</a>
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<h2 id="author"><a name="author"></a>Author:</h2>
<ul>
<li>Name: <a href="../../authors.html#Edward_Giles">Edward Giles</a><br>
Location: <a href="../../location.html#AU">AU</a> - <em>Commonwealth of Australia</em> (<em>Australia</em>)
<br><br></li>
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<h2 id="to-build">To build:</h2>
<div class="sourceCode" id="cb1"><pre class="sourceCode sh"><code class="sourceCode bash"><span id="cb1-1"><a href="#cb1-1" aria-hidden="true" tabindex="-1"></a><span class="fu">make</span></span></code></pre></div>
<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#2020_giles">2020 giles bugs</a>.</p>
<h2 id="to-use">To use:</h2>
<div class="sourceCode" id="cb3"><pre class="sourceCode sh"><code class="sourceCode bash"><span id="cb3-1"><a href="#cb3-1" aria-hidden="true" tabindex="-1"></a><span class="ex">./prog</span> <span class="op">&lt;</span> dtmf.wav</span>
<span id="cb3-2"><a href="#cb3-2" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb3-3"><a href="#cb3-3" aria-hidden="true" tabindex="-1"></a><span class="ex">./prog</span> <span class="pp">[</span><span class="ss">digits</span><span class="pp">]</span> <span class="op">&gt;</span> digits.wav</span></code></pre></div>
<p>where <code>dtmf.wav</code> is a WAV file of the dialling of a number being dialled with
<a href="https://en.wikipedia.org/wiki/Dual-tone_multi-frequency_signaling">DTMF (Dual-tone multi
frequency)</a>
(more commonly known as Touch-Tone) and <code>[digits]</code> is a sequence of digits that
will be converted to the DTMF sound!</p>
<h2 id="try">Try:</h2>
<div class="sourceCode" id="cb4"><pre class="sourceCode sh"><code class="sourceCode bash"><span id="cb4-1"><a href="#cb4-1" aria-hidden="true" tabindex="-1"></a><span class="ex">./try.sh</span></span>
<span id="cb4-2"><a href="#cb4-2" aria-hidden="true" tabindex="-1"></a><span class="ex">./prog</span> <span class="op">&lt;</span> pi.wav</span>
<span id="cb4-3"><a href="#cb4-3" aria-hidden="true" tabindex="-1"></a></span>
<span id="cb4-4"><a href="#cb4-4" aria-hidden="true" tabindex="-1"></a><span class="co"># See</span></span>
<span id="cb4-5"><a href="#cb4-5" aria-hidden="true" tabindex="-1"></a><span class="ex">./prog</span> 867-5309 <span class="op">&gt;</span> jenny.wav</span></code></pre></div>
<p>If the script <a href="try.sh">try.sh</a> did not play a WAV file, it means you do not have
SoX installed; see <a href="../../faq.html#sox">FAQ 3.10: How do I compile and use an IOCCC entry that
requires sound?</a>). In this case install it and try it again or
play the files <code>jenny.wav</code> and <code>ioccc28.wav</code> in a program that can play WAV
files.</p>
<p>NOTE: the phone number of <code>jenny.wav</code> comes from
<a href="https://en.wikipedia.org/wiki/867-5309/Jenny" class="uri">https://en.wikipedia.org/wiki/867-5309/Jenny</a>.</p>
<h2 id="judges-remarks">Judges’ remarks:</h2>
<p>A cross platform utility for both encoding and decoding touch tones.</p>
<h2 id="authors-remarks">Author’s remarks:</h2>
<h3 id="introduction-and-usage">Introduction and Usage</h3>
<p>This program encodes and decodes <a href="https://en.wikipedia.org/wiki/Dual-tone_multi-frequency_signaling">dual-tone multiple
frequency</a>
signals used in telephone systems, more commonly known as Touch-Tones.</p>
<p>If the program is executed with no arguments, it will read a WAV file from
standard input, decode the touch-tones in the file, and output the corresponding
digits to standard output. This program only supports WAV files that have
exactly 16 bits per sample, but it allows any sample rate and any number of
audio channels.</p>
<div class="sourceCode" id="cb5"><pre class="sourceCode sh"><code class="sourceCode bash"><span id="cb5-1"><a href="#cb5-1" aria-hidden="true" tabindex="-1"></a><span class="ex">$</span> ./prog <span class="op">&lt;</span> pi.wav</span>
<span id="cb5-2"><a href="#cb5-2" aria-hidden="true" tabindex="-1"></a><span class="ex">31415926</span></span></code></pre></div>
<p>If the program is executed with a command-line argument, it will generate the
tones corresponding to the specified characters, writing them to standard output
as a WAV file.</p>
<div class="sourceCode" id="cb6"><pre class="sourceCode sh"><code class="sourceCode bash"><span id="cb6-1"><a href="#cb6-1" aria-hidden="true" tabindex="-1"></a><span class="ex">$</span> ./prog 867-5309 <span class="kw">|</span> <span class="fu">aplay</span></span></code></pre></div>
<h3 id="interesting-features">Interesting Features</h3>
<ul>
<li>When compiled with <code>gcc -pedantic -Wall -Wextra</code> there are no compiler
warnings.</li>
<li>This program can be successfully compiled and run on Windows, using both
Microsoft’s C compiler and <a href="https://bellard.org/tcc/">TCC</a>.</li>
<li>This program resamples the incoming audio, so that it can accept audio data at
any sample rate.</li>
<li>The code that generates the output sine-waves is the same as the code that
looks for sine-waves in the input. (see Algorithm Details for roughly how this
works).</li>
<li>Despite handling lots of sine waves, the program only uses basic arithmetic
operations. It does not depend on <code>libm</code>.</li>
<li>All type punning is done with <code>memcpy(3)</code> so there are no strict-aliasing
violations.</li>
<li>The size of the source code using the IOCCC size tool is 1963, the year the
first push-button telephones were made available to customers.</li>
<li>The source code (not counting the C pre-processor) is roughly in the shape of
a telephone handset.</li>
</ul>
<h3 id="assumptions">Assumptions</h3>
<ul>
<li>Code is compiled using the C99 or C11 standard.</li>
<li><code>CHAR_BIT == 8</code>.</li>
<li>Machine is little-endian.</li>
<li><code>int16_t</code>, <code>uint16_t</code>, and <code>uint32_t</code> exist.</li>
<li><code>int16_t</code> is 2’s-complement.</li>
<li><code>double</code> is an IEEE 754 binary64 floating-point type.</li>
<li><code>sizeof(double) == 8</code>.</li>
</ul>
<h3 id="obfuscations">Obfuscations</h3>
<ul>
<li>DSP code is inherently somewhat difficult to understand.</li>
<li>Lots of identifier reuse. Each identifier is used for about 2.2 different
tasks, on average. For example, <code>aa</code> is both a variable and a macro for error
handling, and <code>memcpy(3)</code> is both a library function and a <code>oto</code> label.</li>
<li>Most of the identifiers are only 1 or 2 characters long, and are unrelated to
their actual function.</li>
<li>The <code>f</code> macro is used to unroll some of the loops.</li>
<li>Some numerical constants are replaced with character literals, so the actual
value is unclear.</li>
<li>All of the strings used by the program have been combined into a single string
literal.</li>
<li>Most of the values used for the actual signal-processing all reside in one
array, named <code>l</code>. This array is usually accessed at index <code>z</code> plus a constant.
Accesses of this type have been rewritten from, e.g. <code>l[z+17]</code> to <code>17[l+z]</code>.
<code>[l+z]</code> ends up occurring quite often, so I created the macro <code>a</code> as a
shorthand.</li>
</ul>
<h3 id="algorithm-details">Algorithm Details</h3>
<p>Each tone in a DTMF signal is a combination of two frequencies. The lower
frequency determines which row the digit is in, and the higher frequency
determines the column. There are 4 possible row frequencies and 4 possible
column frequencies.</p>
<pre><code>| | **1209 Hz** | **1336 Hz** | **1477 Hz** | **1633 Hz** |
|------------|-------------|-------------|-------------|-------------|
| **697 Hz** | 1 | 2 abc | 3 def | A |
| **770 Hz** | 4 ghi | 5 jkl | 6 mno | B |
| **852 Hz** | 7 pqrs | 8 tuv | 9 wxyz | C |
| **941 Hz** | * | 0 | # | D |</code></pre>
<p>This program determines which frequencies are present in the input by passing it
through a set of 8 virtual resonators, each tuned to one of the frequencies used
for DTMF. These can be implemented using only basic arithmetic, and they are
able to ‘select’ a specific frequency from the input sound, blocking all other
frequencies from reaching the output. The loudness of the sound output from each
resonator can therefore be used as a measure of how much of a particular
frequency was present in the input. The program then decides, over the course of
one tone, which row frequency and which column frequency were most prominent.</p>
<p>Tones are generated using those same resonators by providing <a href="https://en.wikipedia.org/wiki/Kronecker_delta#Digital_signal_processing">an
impulse</a>
as input. An impulse can be thought of as consisting of sine-waves at all
frequencies, and when it is put through one of the resonators, the result is a
single pure sine wave.</p>
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<h1 id="inventory-for-2020giles"><a name="inventory"></a>Inventory for 2020/giles</h1>
<p><a href="#top">Jump to top</a></p>
<h2 id="primary-files">Primary files</h2>
<ul>
<li><a href="https://github.com/ioccc-src/temp-test-ioccc/blob/master/2020/giles/prog.c">prog.c</a> - entry source</li>
<li><a href="https://github.com/ioccc-src/temp-test-ioccc/blob/master/2020/giles/Makefile">Makefile</a> - entry Makefile</li>
<li><a href="https://github.com/ioccc-src/temp-test-ioccc/blob/master/2020/giles/prog.orig.c">prog.orig.c</a> - original source</li>
<li><a href="https://github.com/ioccc-src/temp-test-ioccc/blob/master/2020/giles/try.sh">try.sh</a> - how to run</li>
<li><a href="pi.wav">pi.wav</a> - touch tone for leading decimal digits of pi</li>
<li><a href="2020_giles.tar.bz2">2020_giles.tar.bz2</a> - download entry source</li>
</ul>
<h2 id="secondary-files">Secondary files</h2>
<ul>
<li><a href="index.html">index.html</a> - entry home page</li>
<li><a href="https://github.com/ioccc-src/temp-test-ioccc/blob/master/2020/giles/README.md">README.md</a> - markdown source for index.html</li>
<li><a href="dtmf.bin">dtmf.bin</a> - binary support file</li>
<li><a href="https://github.com/ioccc-src/temp-test-ioccc/blob/master/2020/giles/.entry.json">.entry.json</a> - description of entry in JSON</li>
<li><a href="https://github.com/ioccc-src/temp-test-ioccc/blob/master/2020/giles/.gitignore">.gitignore</a> - list of files that should not be committed under git</li>
<li><a href=".path">.path</a> - directory path from top directory</li>
</ul>
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