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<!DOCTYPE html>
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<title>Generating Binary Inputs — Fuzzing with Fandango</title>
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<div id="jb-print-docs-body" class="onlyprint">
<h1>Generating Binary Inputs</h1>
<!-- Table of contents -->
<div id="print-main-content">
<div id="jb-print-toc">
<div>
<h2> Contents </h2>
</div>
<nav aria-label="Page">
<ul class="visible nav section-nav flex-column">
<li class="toc-h2 nav-item toc-entry"><a class="reference internal nav-link" href="#checksums">Checksums</a></li>
<li class="toc-h2 nav-item toc-entry"><a class="reference internal nav-link" href="#characters-and-bytes">Characters and Bytes</a><ul class="nav section-nav flex-column">
<li class="toc-h3 nav-item toc-entry"><a class="reference internal nav-link" href="#text-files-and-binary-files">Text Files and Binary Files</a></li>
<li class="toc-h3 nav-item toc-entry"><a class="reference internal nav-link" href="#bytes-and-regular-expressions">Bytes and Regular Expressions</a></li>
</ul>
</li>
<li class="toc-h2 nav-item toc-entry"><a class="reference internal nav-link" href="#length-encodings">Length Encodings</a><ul class="nav section-nav flex-column">
<li class="toc-h3 nav-item toc-entry"><a class="reference internal nav-link" href="#encoding-lengths-with-constraints">Encoding Lengths with Constraints</a></li>
<li class="toc-h3 nav-item toc-entry"><a class="reference internal nav-link" href="#encoding-lengths-with-repetitions">Encoding Lengths with Repetitions</a></li>
</ul>
</li>
<li class="toc-h2 nav-item toc-entry"><a class="reference internal nav-link" href="#converting-values-to-binary-formats">Converting Values to Binary Formats</a></li>
</ul>
</nav>
</div>
</div>
</div>
<div id="searchbox"></div>
<article class="bd-article">
<section class="tex2jax_ignore mathjax_ignore" id="generating-binary-inputs">
<span id="sec-binary"></span><h1>Generating Binary Inputs<a class="headerlink" href="#generating-binary-inputs" title="Link to this heading">#</a></h1>
<p>Creating <em>binary</em> inputs with Fandango is a bit more challenging than creating human-readable inputs.
This is because they have a few special features, such as <em>checksums</em> and <em>length encodings</em>.
Fortunately, we can address all of them with dedicated constraints.</p>
<section id="checksums">
<h2>Checksums<a class="headerlink" href="#checksums" title="Link to this heading">#</a></h2>
<aside class="margin sidebar">
<p class="sidebar-title"></p>
<p>Strictly speaking, this only holds for context-free grammars Fandango uses.
<em>Context-sensitive</em> and <em>universal</em> grammars can perform arithmetic computations, but someone would have to implement them all.</p>
</aside>
<p>Our first challenge is <em>checksums</em>.
Binary input formats frequently use checksums to ensure integrity.
The problem is that checksums cannot be expressed in a grammar alone, as grammars lack the arithmetic functions required to compute and check checksums.
In Fandango, though, we can express the computation of a checksum in a dedicated function, which is then used in a dedicated constraint.</p>
<p>As an example for checksums, let’s have a look at <em>credit card numbers</em>.
These are definitely very human-readable and not binary at all, but for an example, they will do fine.
A credit card number consists of a series of digits, where the last one is a <em>check digit</em>.
Here is a grammar that expresses the structure for 16-digit credit card numbers:</p>
<div class="cell tag_remove-input docutils container">
<div class="cell_output docutils container">
<div class="output stream highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span><start> ::= <credit_card_number>
<credit_card_number> ::= <number> <check_digit>
<number> ::= <digit>{15} # for 16-digit numbers
<check_digit> ::= <digit>
</pre></div>
</div>
</div>
</div>
<p>All credit cards use <a class="reference external" href="https://en.wikipedia.org/wiki/Luhn_algorithm">Luhn’s algorithm</a> to compute the check digit.
Here is an implementation, adapted from the <a class="reference external" href="https://github.com/joke2k/faker/blob/master/faker/providers/credit_card/__init__.py#L99">Faker library</a>.
The function <code class="docutils literal notranslate"><span class="pre">credit_card_check_digit()</span></code> gets all numbers of a credit card (except the last digit) and returns the computed check digit.</p>
<div class="cell tag_remove-input docutils container">
<div class="cell_output docutils container">
<div class="output stream highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span>def credit_card_check_digit(number: str) -> str:
"""Create a check digit for the credit card number `number`."""
luhn_lookup = {
"0": 0,
"1": 2,
"2": 4,
"3": 6,
"4": 8,
"5": 1,
"6": 3,
"7": 5,
"8": 7,
"9": 9,
}
# Calculate sum
length = len(number) + 1
reverse = number[::-1]
tot = 0
pos = 0
while pos < length - 1:
tot += luhn_lookup[reverse[pos]]
if pos != (length - 2):
tot += int(reverse[pos + 1])
pos += 2
# Calculate check digit
check_digit = (10 - (tot % 10)) % 10
return str(check_digit)
</pre></div>
</div>
</div>
</div>
<p>We can easily make use of <code class="docutils literal notranslate"><span class="pre">credit_card_check_digit()</span></code> in a constraint that ties <code class="docutils literal notranslate"><span class="pre"><check_digit></span></code> and <code class="docutils literal notranslate"><span class="pre"><number></span></code>:</p>
<div class="cell tag_remove-input docutils container">
<div class="cell_output docutils container">
<div class="output stream highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span>where <check_digit> == credit_card_check_digit(str(<number>))
</pre></div>
</div>
</div>
</div>
<p>All of this can go into a single <code class="docutils literal notranslate"><span class="pre">.fan</span></code> file: <a class="reference download internal" download="" href="_downloads/2312c346764e20c82908283cac3c76ae/credit_card.fan"><span class="xref download myst"><code class="docutils literal notranslate"><span class="pre">credit_card.fan</span></code></span></a> joins the above grammar, the <code class="docutils literal notranslate"><span class="pre">credit_card_check_digit()</span></code> definition, and the above constraint into a single file.</p>
<aside class="margin sidebar">
<p class="sidebar-title"></p>
<p>Do not use such numbers to test third-party systems.</p>
</aside>
<p>We can now use <code class="docutils literal notranslate"><span class="pre">credit-card.fan</span></code> to produce valid credit card numbers:</p>
<div class="highlight-shell notranslate"><div class="highlight"><pre><span></span>$<span class="w"> </span>fandango<span class="w"> </span>fuzz<span class="w"> </span>-f<span class="w"> </span>credit_card.fan<span class="w"> </span>-n<span class="w"> </span><span class="m">10</span>
</pre></div>
</div>
<div class="cell tag_remove-input docutils container">
<div class="cell_output docutils container">
<div class="output stream highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span>📦 Checking for Fandango updates... (set FANDANGO_DISABLE_UPDATE_CHECK=1 to disable)
</pre></div>
</div>
<div class="output stream highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span>9915935767030457
8910995245555829
2832286487843257
8125546893974183
</pre></div>
</div>
<div class="output stream highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span>0931746150079194
9725883329378628
5023701024640182
0098963071069605
</pre></div>
</div>
<div class="output stream highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span>4680561526575444
9076092408822390
</pre></div>
</div>
</div>
</div>
<p>We can also use the grammar to <em>parse</em> and <em>check</em> numbers.
This credit card number should be valid:</p>
<div class="highlight-shell notranslate"><div class="highlight"><pre><span></span>$<span class="w"> </span><span class="nb">echo</span><span class="w"> </span>-n<span class="w"> </span><span class="m">4931633575526870</span><span class="w"> </span><span class="p">|</span><span class="w"> </span>fandango<span class="w"> </span>parse<span class="w"> </span>-f<span class="w"> </span>credit_card.fan
$<span class="w"> </span><span class="nb">echo</span><span class="w"> </span><span class="nv">$?</span><span class="w"> </span><span class="c1"># print exit code</span>
<span class="m">0</span>
</pre></div>
</div>
<div class="cell tag_remove-input docutils container">
</div>
<p>Adding a <code class="docutils literal notranslate"><span class="pre">1</span></code> digit to this number should make it <em>invalid</em>:</p>
<div class="highlight-shell notranslate"><div class="highlight"><pre><span></span>$<span class="w"> </span><span class="nb">echo</span><span class="w"> </span>-n<span class="w"> </span><span class="m">4931633575526871</span><span class="w"> </span><span class="p">|</span><span class="w"> </span>fandango<span class="w"> </span>parse<span class="w"> </span>-f<span class="w"> </span>credit_card.fan
</pre></div>
</div>
<div class="cell tag_remove-input docutils container">
<div class="cell_output docutils container">
<div class="output stream highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span>FandangoParseError: Did not match the following constraints: <check_digit> == credit_card_check_digit(str(<number>))
FandangoParseError: Did not match the following constraints: <check_digit> == credit_card_check_digit(str(<number>))
Traceback (most recent call last):
File <span class=" -Color -Color-Magenta">"/home/runner/work/fandango/fandango/.venv/bin/fandango"</span>, line <span class=" -Color -Color-Magenta">10</span>, in <span class=" -Color -Color-Magenta"><module></span>
sys.exit(<span class=" -Color -Color-Red">main</span><span class=" -Color -Color-Bold -Color-Bold-Red">()</span>)
<span class=" -Color -Color-Red">~~~~</span><span class=" -Color -Color-Bold -Color-Bold-Red">^^</span>
File <span class=" -Color -Color-Magenta">"<@beartype(fandango.cli.main) at 0x7f5798fd1440>"</span>, line <span class=" -Color -Color-Magenta">68</span>, in <span class=" -Color -Color-Magenta">main</span>
File <span class=" -Color -Color-Magenta">"/home/runner/work/fandango/fandango/src/fandango/cli/__init__.py"</span>, line <span class=" -Color -Color-Magenta">51</span>, in <span class=" -Color -Color-Magenta">main</span>
last_status = run(command, args)
File <span class=" -Color -Color-Magenta">"<@beartype(fandango.cli.commands.run) at 0x7f57985165c0>"</span>, line <span class=" -Color -Color-Magenta">51</span>, in <span class=" -Color -Color-Magenta">run</span>
File <span class=" -Color -Color-Magenta">"/home/runner/work/fandango/fandango/src/fandango/cli/commands.py"</span>, line <span class=" -Color -Color-Magenta">521</span>, in <span class=" -Color -Color-Magenta">run</span>
<span class=" -Color -Color-Red">print_exception</span><span class=" -Color -Color-Bold -Color-Bold-Red">(e)</span>
<span class=" -Color -Color-Red">~~~~~~~~~~~~~~~</span><span class=" -Color -Color-Bold -Color-Bold-Red">^^^</span>
File <span class=" -Color -Color-Magenta">"<@beartype(fandango.logger.print_exception) at 0x7f5799d445e0>"</span>, line <span class=" -Color -Color-Magenta">54</span>, in <span class=" -Color -Color-Magenta">print_exception</span>
File <span class=" -Color -Color-Magenta">"/home/runner/work/fandango/fandango/src/fandango/logger.py"</span>, line <span class=" -Color -Color-Magenta">47</span>, in <span class=" -Color -Color-Magenta">print_exception</span>
raise e
File <span class=" -Color -Color-Magenta">"/home/runner/work/fandango/fandango/src/fandango/cli/commands.py"</span>, line <span class=" -Color -Color-Magenta">519</span>, in <span class=" -Color -Color-Magenta">run</span>
<span class=" -Color -Color-Red">command</span><span class=" -Color -Color-Bold -Color-Bold-Red">(args)</span>
<span class=" -Color -Color-Red">~~~~~~~</span><span class=" -Color -Color-Bold -Color-Bold-Red">^^^^^^</span>
File <span class=" -Color -Color-Magenta">"<@beartype(fandango.cli.commands.parse_command) at 0x7f5798515d00>"</span>, line <span class=" -Color -Color-Magenta">33</span>, in <span class=" -Color -Color-Magenta">parse_command</span>
File <span class=" -Color -Color-Magenta">"/home/runner/work/fandango/fandango/src/fandango/cli/commands.py"</span>, line <span class=" -Color -Color-Magenta">283</span>, in <span class=" -Color -Color-Magenta">parse_command</span>
<span class=" -Color -Color-Red">print_exception</span><span class=" -Color -Color-Bold -Color-Bold-Red">(e)</span>
<span class=" -Color -Color-Red">~~~~~~~~~~~~~~~</span><span class=" -Color -Color-Bold -Color-Bold-Red">^^^</span>
File <span class=" -Color -Color-Magenta">"<@beartype(fandango.logger.print_exception) at 0x7f5799d445e0>"</span>, line <span class=" -Color -Color-Magenta">54</span>, in <span class=" -Color -Color-Magenta">print_exception</span>
File <span class=" -Color -Color-Magenta">"/home/runner/work/fandango/fandango/src/fandango/logger.py"</span>, line <span class=" -Color -Color-Magenta">47</span>, in <span class=" -Color -Color-Magenta">print_exception</span>
raise e
File <span class=" -Color -Color-Magenta">"/home/runner/work/fandango/fandango/src/fandango/cli/commands.py"</span>, line <span class=" -Color -Color-Magenta">280</span>, in <span class=" -Color -Color-Magenta">parse_command</span>
tree = parse_file(fd, args, grammar, constraints, settings)
File <span class=" -Color -Color-Magenta">"<@beartype(fandango.cli.utils.parse_file) at 0x7f5798e296c0>"</span>, line <span class=" -Color -Color-Magenta">136</span>, in <span class=" -Color -Color-Magenta">parse_file</span>
File <span class=" -Color -Color-Magenta">"/home/runner/work/fandango/fandango/src/fandango/cli/utils.py"</span>, line <span class=" -Color -Color-Magenta">318</span>, in <span class=" -Color -Color-Magenta">parse_file</span>
raise FandangoParseError(
f"Did not match the following constraints: {', '.join(failed_constraints)}"
)
<span class=" -Color -Color-Bold -Color-Bold-Magenta">fandango.errors.FandangoParseError</span>: <span class=" -Color -Color-Magenta">Did not match the following constraints: <check_digit> == credit_card_check_digit(str(<number>))</span>
</pre></div>
</div>
</div>
</div>
<div class="highlight-shell notranslate"><div class="highlight"><pre><span></span>$<span class="w"> </span><span class="nb">echo</span><span class="w"> </span><span class="nv">$?</span><span class="w"> </span><span class="c1"># print exit code</span>
<span class="m">1</span>
</pre></div>
</div>
<aside class="margin sidebar">
<p class="sidebar-title"></p>
<p>You can also simply do an Internet search for a Python implementation of the respective algorithm.
Or ask your favorite AI assistant.</p>
</aside>
<p>Similarly, you can define any kind of checksum function and then use it in a constraint.
In Python, it is likely that someone has already implemented the specific checksum function, so you can also <em>import</em> it:</p>
<ul class="simple">
<li><p><a class="reference external" href="https://docs.python.org/3/library/hashlib.html">The <code class="docutils literal notranslate"><span class="pre">hashlib</span></code> module</a> provides hash functions such as MD5 or SHA-256.</p></li>
<li><p><a class="reference external" href="https://docs.python.org/3/library/binascii.html">The <code class="docutils literal notranslate"><span class="pre">binascii</span></code> module</a> offers CRC checks.</p></li>
<li><p><a class="reference external" href="https://docs.python.org/3/library/zlib.html">The <code class="docutils literal notranslate"><span class="pre">zlib</span></code> module</a> provides CRC32 and ADLER32 checks used in zip files.</p></li>
</ul>
</section>
<section id="characters-and-bytes">
<h2>Characters and Bytes<a class="headerlink" href="#characters-and-bytes" title="Link to this heading">#</a></h2>
<p>The second set of features one frequently encounters in binary formats is, well, <em>bytes</em>.
So far, we have seen Fandango operates on strings of Unicode <em>characters</em>, which use UTF-8 encoding.
This clashes with a byte interpretation as soon as the produced string contains a UTF-8 prefix byte, such as <code class="docutils literal notranslate"><span class="pre">\xc2</span></code> or <code class="docutils literal notranslate"><span class="pre">\xe0</span></code>, which mark the beginning of a two- and three-byte UTF-8 sequence, respectively.</p>
<p>To ensure bytes will be interpreted as bytes (and as bytes only), place a <code class="docutils literal notranslate"><span class="pre">b</span></code> (binary) prefix in front of them.
This ensures that a byte <code class="docutils literal notranslate"><span class="pre">b'\xc2'</span></code> will always be interpreted as a single byte, whereas <code class="docutils literal notranslate"><span class="pre">💃</span></code> will be interpreted as a single character (despite occupying multiple bytes).</p>
<div class="admonition tip">
<p class="admonition-title">Tip</p>
<p>Fandango provides a <code class="docutils literal notranslate"><span class="pre"><byte></span></code> symbol by default, which expands into all bytes <code class="docutils literal notranslate"><span class="pre">b'\x00'..b'\xff'</span></code>.</p>
</div>
<section id="text-files-and-binary-files">
<h3>Text Files and Binary Files<a class="headerlink" href="#text-files-and-binary-files" title="Link to this heading">#</a></h3>
<p>By default, Fandango will read and write files in <code class="docutils literal notranslate"><span class="pre">text</span></code> mode, meaning that characters will be read in using UTF-8 encoding.
However, if a grammar can produce bytes (or <a class="reference internal" href="Bits.html#sec-bits"><span class="std std-ref">bits</span></a>), the associated files will be read and written in <code class="docutils literal notranslate"><span class="pre">binary</span></code> mode, reading and writing <em>bytes</em> instead of (encoded) characters.
If your grammar contains bytes <em>and</em> strings, then the strings will be written in UTF-8 encoding into the binary file.</p>
<p>You can enforce a specific behavior using the Fandango <code class="docutils literal notranslate"><span class="pre">--file-mode</span></code> flag for the <code class="docutils literal notranslate"><span class="pre">fuzz</span></code> and <code class="docutils literal notranslate"><span class="pre">parse</span></code> commands:</p>
<ul class="simple">
<li><p><code class="docutils literal notranslate"><span class="pre">fuzz</span> <span class="pre">--file-mode=text</span></code> opens all files in <code class="docutils literal notranslate"><span class="pre">text</span></code> mode. Strings and bytes will be written in UTF-8 encoding.</p></li>
<li><p><code class="docutils literal notranslate"><span class="pre">fuzz</span> <span class="pre">--file-mode=binary</span></code> opens all files in <code class="docutils literal notranslate"><span class="pre">binary</span></code> mode. Strings will be written in UTF-8 encoding; bytes will be written as is.</p></li>
</ul>
<p>The default is <code class="docutils literal notranslate"><span class="pre">fuzz</span> <span class="pre">--file=mode=auto</span></code> (default), which will use <code class="docutils literal notranslate"><span class="pre">binary</span></code> or <code class="docutils literal notranslate"><span class="pre">text</span></code> mode as described above.</p>
<div class="admonition tip">
<p class="admonition-title">Tip</p>
<p>Avoid mixing non-ASCII strings with bits and bytes in a single grammar.</p>
</div>
</section>
<section id="bytes-and-regular-expressions">
<span id="sec-byte-regexes"></span><h3>Bytes and Regular Expressions<a class="headerlink" href="#bytes-and-regular-expressions" title="Link to this heading">#</a></h3>
<p>Fandango also supports <a class="reference internal" href="Regexes.html"><span class="std std-doc">regular expressions</span></a> over bytes.
To obtain a regular expression over a byte string, use both <code class="docutils literal notranslate"><span class="pre">r</span></code> and <code class="docutils literal notranslate"><span class="pre">b</span></code> prefixes.
This is especially useful for character classes.</p>
<p>Here is an example: <a class="reference download internal" download="" href="_downloads/7e045c29e0d6adedac754e3744fcba5c/binfinity.fan"><span class="xref download myst"><code class="docutils literal notranslate"><span class="pre">binfinity.fan</span></code></span></a> produces strings of five bytes <em>outside</em> the range <code class="docutils literal notranslate"><span class="pre">\x80-\xff</span></code>:</p>
<div class="cell tag_remove-input docutils container">
<div class="cell_output docutils container">
<div class="output stream highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span><start> ::= rb"[^\x80-\xff]{5}"
</pre></div>
</div>
</div>
</div>
<p>This is what we get:</p>
<div class="highlight-shell notranslate"><div class="highlight"><pre><span></span>$<span class="w"> </span>fandango<span class="w"> </span>fuzz<span class="w"> </span>-f<span class="w"> </span>binfinity.fan<span class="w"> </span>-n<span class="w"> </span><span class="m">10</span>
</pre></div>
</div>
<div class="cell tag_remove-input docutils container">
<div class="cell_output docutils container">
<div class="output stream highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span>d3'VT
7sok;
8Z Bk
odK?U
Y+aJO
!ig2<
AjnWw
#3"Z1
q>O5s
RqYY]
</pre></div>
</div>
</div>
</div>
</section>
</section>
<section id="length-encodings">
<h2>Length Encodings<a class="headerlink" href="#length-encodings" title="Link to this heading">#</a></h2>
<p>The third set of features one frequently encounters in binary formats is <em>length encodings</em> - that is, a particular field holds a value that represents the length of one or more fields that follow.
Here is a simple grammar that expresses this characteristic: A <code class="docutils literal notranslate"><span class="pre"><field></span></code> has a two-byte length, followed by the actual (byte) content (of length <code class="docutils literal notranslate"><span class="pre"><length></span></code>).</p>
<div class="cell tag_remove-input docutils container">
<div class="cell_output docutils container">
<div class="output stream highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span><start> ::= <field>
<field> ::= <length> <content>
<length> ::= <byte> <byte>
<content> ::= <byte>+
</pre></div>
</div>
</div>
</div>
<section id="encoding-lengths-with-constraints">
<h3>Encoding Lengths with Constraints<a class="headerlink" href="#encoding-lengths-with-constraints" title="Link to this heading">#</a></h3>
<p>The relationship between <code class="docutils literal notranslate"><span class="pre"><length></span></code> and <code class="docutils literal notranslate"><span class="pre"><content></span></code> can again be expressed using a constraint.
Let us assume that <code class="docutils literal notranslate"><span class="pre"><length></span></code> comes as a two-byte (16-bit) unsigned integer with <em>little-endian</em> encoding - that is, the low byte comes first, and the high byte follows.
The value 258 (hexadecimal <code class="docutils literal notranslate"><span class="pre">0x0102</span></code>) would thus be represented as the two bytes <code class="docutils literal notranslate"><span class="pre">\x02</span></code> and <code class="docutils literal notranslate"><span class="pre">\x01</span></code>.</p>
<p>We can define a function <code class="docutils literal notranslate"><span class="pre">uint16()</span></code> that takes an integer and converts it to a two-byte string according to these rules.
The Python method <code class="docutils literal notranslate"><span class="pre">N.to_bytes(LENGTH,</span> <span class="pre">ENDIANNESS)</span></code> converts the integer <code class="docutils literal notranslate"><span class="pre">N</span></code> into a bytes string of length <code class="docutils literal notranslate"><span class="pre">LENGTH</span></code>. <code class="docutils literal notranslate"><span class="pre">ENDIANNESS</span></code> is either <code class="docutils literal notranslate"><span class="pre">'big'</span></code> (default) or <code class="docutils literal notranslate"><span class="pre">'little'</span></code>.</p>
<div class="cell tag_remove-input docutils container">
<div class="cell_output docutils container">
<div class="output stream highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span>def uint16(n: int) -> bytes:
return n.to_bytes(2, 'little')
</pre></div>
</div>
</div>
</div>
<p>Using <code class="docutils literal notranslate"><span class="pre">uint16()</span></code>, we can now define how the value of <code class="docutils literal notranslate"><span class="pre"><length></span></code> is related to the length of <code class="docutils literal notranslate"><span class="pre"><content></span></code>:</p>
<div class="cell tag_remove-input docutils container">
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<div class="output stream highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span>where <length> == uint16(len(bytes(<content>)))
</pre></div>
</div>
</div>
</div>
<div class="admonition tip">
<p class="admonition-title">Tip</p>
<p>Having a derived value (like <code class="docutils literal notranslate"><span class="pre"><length></span></code>) isolated on the left-hand side of an equality equation makes it easy for Fandango to first compute the content and then compute and assign the derived value.</p>
</div>
<p>Again, all of this goes into a single <code class="docutils literal notranslate"><span class="pre">.fan</span></code> file: <a class="reference download internal" download="" href="_downloads/4c57f6c2c62644bc4b94f1498df6e8f0/binary.fan"><span class="xref download myst"><code class="docutils literal notranslate"><span class="pre">binary.fan</span></code></span></a> holds the grammar, the <code class="docutils literal notranslate"><span class="pre">uint16()</span></code> definition, and the constraint.
Let us produce a single output using <code class="docutils literal notranslate"><span class="pre">binary.fan</span></code> and view its (binary) contents, using <code class="docutils literal notranslate"><span class="pre">od</span> <span class="pre">-c</span></code>:</p>
<div class="highlight-shell notranslate"><div class="highlight"><pre><span></span>$<span class="w"> </span>fandango<span class="w"> </span>fuzz<span class="w"> </span>-n<span class="w"> </span><span class="m">1</span><span class="w"> </span>-f<span class="w"> </span>binary.fan<span class="w"> </span><span class="p">|</span><span class="w"> </span>hexdump<span class="w"> </span>-C
</pre></div>
</div>
<div class="cell tag_remove-input docutils container">
<div class="cell_output docutils container">
<div class="output stream highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span>00000000 0f 00 c3 84 c3 a4 c3 b9 1e 29 c2 b1 c2 98 73 c2 |.........)....s.|
</pre></div>
</div>
<div class="output stream highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span>00000010 8e 79 c2 97 c3 8e c2 ad 0d 5c 0a |.y.......\.|
0000001b
</pre></div>
</div>
</div>
</div>
<p>The hexadecimal dump shows that the first two bytes encode the length of the string of digits that follows.
The format is correct - we have successfully produced a length encoding.</p>
</section>
<section id="encoding-lengths-with-repetitions">
<h3>Encoding Lengths with Repetitions<a class="headerlink" href="#encoding-lengths-with-repetitions" title="Link to this heading">#</a></h3>
<p>Another way to implement length constraints is by using <em>repetitions</em>.
In Fandango, repetitions <code class="docutils literal notranslate"><span class="pre">{}</span></code> can also contain <em>expressions</em>, and like constraints, these can also refer to nonterminals that have already been parsed or produced.
Hence, we can specify a rule</p>
<div class="highlight-python notranslate"><div class="highlight"><pre><span></span><span class="o"><</span><span class="n">content</span><span class="o">></span> <span class="p">:</span><span class="o">:=</span> <span class="o"><</span><span class="n">byte</span><span class="o">></span><span class="p">{</span><span class="n">f</span><span class="p">(</span><span class="o"><</span><span class="n">length</span><span class="o">></span><span class="p">)}</span>
</pre></div>
</div>
<p>where <code class="docutils literal notranslate"><span class="pre">f()</span></code> is a function that computes the number of <code class="docutils literal notranslate"><span class="pre"><byte></span></code> repetitions based on <code class="docutils literal notranslate"><span class="pre"><length></span></code>.</p>
<p>Let us define a variant <a class="reference download internal" download="" href="_downloads/06beaf77b5ef8c1261e4ce6aecb7754b/binary-rep.fan"><span class="xref download myst"><code class="docutils literal notranslate"><span class="pre">binary-rep.fan</span></code></span></a> that makes use of this.
Here, we specify that <code class="docutils literal notranslate"><span class="pre"><content></span></code> consists of <code class="docutils literal notranslate"><span class="pre">N</span></code> bytes, where <code class="docutils literal notranslate"><span class="pre">N</span></code> is given as follows:</p>
<aside class="margin sidebar">
<p class="sidebar-title"></p>
<p>We use a generator expression <code class="docutils literal notranslate"><span class="pre">:=</span> <span class="pre">VALUE</span></code> to prevent generated values from getting too large.</p>
</aside>
<div class="cell tag_remove-input docutils container">
<div class="cell_output docutils container">
<div class="output stream highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span><start> ::= <field>
<field> ::= <length> <content>
<length> ::= <byte> <byte> := b'\x00\x04'
<content> ::= <byte>{from_uint16(bytes(<length>))}
</pre></div>
</div>
</div>
</div>
<p>The method <code class="docutils literal notranslate"><span class="pre"><length>.value()</span></code> returns the bytes string value of the <code class="docutils literal notranslate"><span class="pre"><length></span></code> element.
The function <code class="docutils literal notranslate"><span class="pre">from_uint16()</span></code> is defined as follows:</p>
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<div class="cell_output docutils container">
<div class="output stream highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span>def from_uint16(n: bytes) -> int:
return n[0] << 8 | n[1]
</pre></div>
</div>
</div>
</div>
<p>With this, we can easily produce length-encoded inputs:</p>
<div class="highlight-shell notranslate"><div class="highlight"><pre><span></span>$<span class="w"> </span>fandango<span class="w"> </span>fuzz<span class="w"> </span>-n<span class="w"> </span><span class="m">1</span><span class="w"> </span>-f<span class="w"> </span>binary-rep.fan<span class="w"> </span><span class="p">|</span><span class="w"> </span>hexdump<span class="w"> </span>-C
</pre></div>
</div>
<div class="cell tag_remove-input docutils container">
<div class="cell_output docutils container">
<div class="output stream highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span>00000000 00 04 5c 4f c2 9b 51 0a |..\O..Q.|
00000008
</pre></div>
</div>
</div>
</div>
<div class="admonition tip">
<p class="admonition-title">Tip</p>
<p>When <a class="reference internal" href="Parsing.html#sec-parsing"><span class="std std-ref">parsing</span></a> inputs, computed repetitions are much more efficient than constraints.</p>
</div>
</section>
</section>
<section id="converting-values-to-binary-formats">
<h2>Converting Values to Binary Formats<a class="headerlink" href="#converting-values-to-binary-formats" title="Link to this heading">#</a></h2>
<p>Instead of implementing <code class="docutils literal notranslate"><span class="pre">uint16()</span></code> manually, we can also use the Python <a class="reference external" href="https://docs.python.org/3/library/struct.html"><code class="docutils literal notranslate"><span class="pre">struct</span></code> module</a>, which offers several functions to convert data into binary formats.
Using <code class="docutils literal notranslate"><span class="pre">struct</span></code>, we can redefine <code class="docutils literal notranslate"><span class="pre">uint16()</span></code> as</p>
<div class="cell tag_remove-input docutils container">
<div class="cell_output docutils container">
<div class="output stream highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span>from struct import pack
def uint16(n: int) -> str:
return pack('<H', n) # convert to bytes
</pre></div>
</div>
</div>
</div>
<p>and obtain the same result:</p>
<div class="cell tag_remove-input docutils container">
<div class="cell_output docutils container">
<div class="output stream highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span>0000000 \a \0 4 8 2 6 9 4 6 \n
0000012
</pre></div>
</div>
</div>
</div>
<p>Note that the return value of <code class="docutils literal notranslate"><span class="pre">struct.pack()</span></code> has the type <code class="docutils literal notranslate"><span class="pre">bytes</span></code> (byte string), which is different from the <code class="docutils literal notranslate"><span class="pre">str</span></code> Unicode strings that Fandango uses:</p>
<div class="cell tag_remove-input docutils container">
</div>
<div class="cell docutils container">
<div class="cell_input docutils container">
<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="n">pack</span><span class="p">(</span><span class="s1">'<H'</span><span class="p">,</span> <span class="mi">20</span><span class="p">)</span>
</pre></div>
</div>
</div>
<div class="cell_output docutils container">
<div class="output text_plain highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span>b'\x14\x00'
</pre></div>
</div>
</div>
</div>
<div class="cell docutils container">
<div class="cell_input docutils container">
<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="nb">type</span><span class="p">(</span><span class="n">pack</span><span class="p">(</span><span class="s1">'<H'</span><span class="p">,</span> <span class="mi">20</span><span class="p">))</span>
</pre></div>
</div>
</div>
<div class="cell_output docutils container">
<div class="output text_plain highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span>bytes
</pre></div>
</div>
</div>
</div>
<p>In Python, comparisons of different types always return <code class="docutils literal notranslate"><span class="pre">False</span></code>:</p>
<div class="cell docutils container">
<div class="cell_input docutils container">
<div class="highlight-ipython3 notranslate"><div class="highlight"><pre><span></span><span class="c1"># Left hand is byte string, right hand is Unicode string</span>
<span class="sa">b</span><span class="s1">'</span><span class="se">\x14\x00</span><span class="s1">'</span> <span class="o">==</span> <span class="s1">'</span><span class="se">\x14\x00</span><span class="s1">'</span>
</pre></div>
</div>
</div>
<div class="cell_output docutils container">
<div class="output text_plain highlight-myst-ansi notranslate"><div class="highlight"><pre><span></span>False
</pre></div>
</div>
</div>
</div>
<p>Hence, a constraint that compares a Fandango symbol against a byte string <em>will always fail</em>.</p>
<div class="admonition important">
<p class="admonition-title">Important</p>
<p>When comparing symbols against values, always be sure to convert the values to the appropriate type first.</p>
</div>
<div class="admonition tip">
<p class="admonition-title">Tip</p>
<p>Using the <code class="docutils literal notranslate"><span class="pre">'iso8859-1'</span></code> encoding (also known as <code class="docutils literal notranslate"><span class="pre">'latin-1'</span></code>) allows a 1:1 conversion of byte strings with values <code class="docutils literal notranslate"><span class="pre">'\x00'..'\xff'</span></code> into Unicode <code class="docutils literal notranslate"><span class="pre">str</span></code> strings without further interpretation.</p>
</div>
<div class="admonition tip">
<p class="admonition-title">Tip</p>
<p>Adding <em>type annotations</em> to functions in <code class="docutils literal notranslate"><span class="pre">.fan</span></code> files allows for future static type checking and further optimizations.</p>
</div>
<p>Check out the <a class="reference external" href="https://docs.python.org/3/library/struct.html"><code class="docutils literal notranslate"><span class="pre">struct</span></code> module</a> for additional encodings, including float types, long and short integers, and many more.</p>
</section>
</section>
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<li class="toc-h2 nav-item toc-entry"><a class="reference internal nav-link" href="#checksums">Checksums</a></li>
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