oppi.li
advent of code solutions
aoc.oppi.li
haskell
aoc
2025: add d6
Signed-off-by: oppiliappan <me@oppi.li>
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<span class="navlink-label">Up:</span> <a href="2-section-1.html" accesskey="u" rel="up">2025</a>
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<span class="navlink-label">Top:</span> <a href="index.html" accesskey="t" rel="top"></a>
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<span class="navlink-label">Previous:</span> <a href="2.3-day-5.html" accesskey="p" rel="previous">Day 5</a>
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<h2 data-number="2.4" id="day-6">Day 6</h2>
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<div class="row">
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<p>This day is largely string munging, which is quite boring, but
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also quite easy to do with Haskell!</p>
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<div class="code">
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</div>
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</div>
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<h3 data-number="2.4.1" id="part-1-5">Part 1</h3>
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<div class="row">
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<p>This part needs us to consider each operation on a column.
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<code>Data.List.transpose</code> fortunately makes this very
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simple. If we are able to massage our input into the form
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<code>["+", "1", "2", "3"]</code>, we can use this simple
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<code>solve</code> function to produce the result. The only
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motivation to use this format is the ease of pattern-matching over
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the start of the list.</p>
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<div class="code">
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<div class="sourceCode" id="cb1"><pre
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class="sourceCode haskell"><code class="sourceCode haskell"><span id="cb1-1"><a href="#cb1-1" aria-hidden="true" tabindex="-1"></a>solve (<span class="st">"+"</span> <span class="op">:</span> rest) <span class="ot">=</span> <span class="fu">sum</span> <span class="op">.</span> <span class="fu">map</span> <span class="fu">read</span> <span class="op">$</span> rest</span>
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<span id="cb1-2"><a href="#cb1-2" aria-hidden="true" tabindex="-1"></a>solve (<span class="st">"*"</span> <span class="op">:</span> rest) <span class="ot">=</span> <span class="fu">product</span> <span class="op">.</span> <span class="fu">map</span> <span class="fu">read</span> <span class="op">$</span> rest</span></code></pre></div>
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</div>
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</div>
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<div class="row">
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<p>Thus, to solve the first part, we can break up our input using
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<code>lines</code>. <code>rot</code> is a simple function that
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rotates a list forwards to move the last element to the first. By
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doing <code>rot . lines</code> upon our input, we have a list of
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the form <code>["+ *", "1 2 3", "4 5 6"]</code>. Next, we define
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an <code>align</code> function to line up the colums. This method
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simply breaks each item in the list by spaces, and then transposes
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them, so we end up with
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<code>[["+", "1", "2", "3"], ["*", "4", "5", "6"]]</code>:</p>
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<div class="code">
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<div class="sourceCode" id="cb2"><pre
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class="sourceCode haskell haskell-top"><code class="sourceCode haskell"><span id="cb2-1"><a href="#cb2-1" aria-hidden="true" tabindex="-1"></a><span class="kw">import</span> <span class="dt">Data.Char</span> (isSpace)</span>
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<span id="cb2-2"><a href="#cb2-2" aria-hidden="true" tabindex="-1"></a><span class="kw">import</span> <span class="dt">Data.List</span> (transpose)</span>
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<span id="cb2-3"><a href="#cb2-3" aria-hidden="true" tabindex="-1"></a><span class="kw">import</span> <span class="dt">Data.List.Split</span> (splitWhen)</span></code></pre></div>
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<div class="sourceCode" id="cb3"><pre
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class="sourceCode haskell"><code class="sourceCode haskell"><span id="cb3-1"><a href="#cb3-1" aria-hidden="true" tabindex="-1"></a>rot <span class="ot">=</span> liftA2 (<span class="op">:</span>) <span class="fu">last</span> <span class="fu">init</span></span>
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<span id="cb3-2"><a href="#cb3-2" aria-hidden="true" tabindex="-1"></a></span>
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<span id="cb3-3"><a href="#cb3-3" aria-hidden="true" tabindex="-1"></a>p1 <span class="ot">=</span> <span class="fu">sum</span> <span class="op">.</span> <span class="fu">map</span> solve <span class="op">.</span> align <span class="op">.</span> rot <span class="op">.</span> <span class="fu">lines</span></span>
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<span id="cb3-4"><a href="#cb3-4" aria-hidden="true" tabindex="-1"></a> <span class="kw">where</span></span>
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<span id="cb3-5"><a href="#cb3-5" aria-hidden="true" tabindex="-1"></a> align <span class="ot">=</span> transpose <span class="op">.</span> <span class="fu">map</span> <span class="fu">words</span></span></code></pre></div>
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</div>
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</div>
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<h3 data-number="2.4.2" id="part-2-4">Part 2</h3>
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<div class="row">
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<p>We are now tasked with lining up the digits in a columnar
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fashion to for a number:</p>
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<div class="code">
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</div>
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</div>
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<pre><code>123 ...
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45 ...
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6 ...
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* ...</code></pre>
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<div class="row">
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<p>Here, the numbers to operate on are 356, 24 and 1.
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Unfortunately, we can no longer employ <code>words</code> as it
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breaks on all whitespace. We must now transpose our list as-is, to
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make sense of the digits. Ignore the last row for a second, and
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only consider the first 3 rows:</p>
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<div class="code">
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</div>
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</div>
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<pre><code>123
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45
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+
6</code></pre>
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<div class="row">
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<p>Upon transposing this, we get:</p>
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<div class="code">
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</div>
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</div>
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<pre><code>1
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24
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+
356</code></pre>
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<div class="row">
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<p>This is exactly what we need!</p>
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<div class="code">
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</div>
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</div>
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<div class="row">
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+
<p>Let us start with a similar base, using <code>lines</code> to
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break things up, and <code>rot</code> to move the line of
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operators to the start. Our <code>align</code> function can now be
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defined as below.</p>
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<div class="code">
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<div class="sourceCode" id="cb7"><pre
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class="sourceCode haskell"><code class="sourceCode haskell"><span id="cb7-1"><a href="#cb7-1" aria-hidden="true" tabindex="-1"></a>p2 <span class="ot">=</span> <span class="fu">sum</span> <span class="op">.</span> <span class="fu">map</span> solve <span class="op">.</span> align <span class="op">.</span> rot <span class="op">.</span> <span class="fu">lines</span></span>
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<span id="cb7-2"><a href="#cb7-2" aria-hidden="true" tabindex="-1"></a> <span class="kw">where</span></span>
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<span id="cb7-3"><a href="#cb7-3" aria-hidden="true" tabindex="-1"></a> align <span class="ot">=</span></span>
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<span id="cb7-4"><a href="#cb7-4" aria-hidden="true" tabindex="-1"></a> <span class="fu">zipWith</span> (<span class="op">:</span>)</span>
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<span id="cb7-5"><a href="#cb7-5" aria-hidden="true" tabindex="-1"></a> <span class="op"><$></span> (<span class="fu">words</span> <span class="op">.</span> <span class="fu">head</span>)</span>
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<span id="cb7-6"><a href="#cb7-6" aria-hidden="true" tabindex="-1"></a> <span class="op"><*></span> (splitWhen (<span class="fu">all</span> <span class="fu">isSpace</span>) <span class="op">.</span> transpose <span class="op">.</span> <span class="fu">tail</span>)</span></code></pre></div>
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</div>
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</div>
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<div class="row">
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<p>We first start with <code>zipWith (:)</code> to join the
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+
operation <code>"+"</code> with the rest of the numbers to
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produce:</p>
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<div class="code">
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+
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+
</div>
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</div>
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<pre><code>["+", "1", "2", ...]</code></pre>
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<div class="row">
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<p>Remember that this is the format <code>solve</code>
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+
accepts!.</p>
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<div class="code">
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</div>
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</div>
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<div class="row">
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<p>We are using <code><*></code> to perform “sequential
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application”. The first part …</p>
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<div class="code">
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</div>
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</div>
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<pre><code>words . head</code></pre>
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<div class="row">
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<p>… is used to split up the operator line into a list of
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operators.</p>
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<div class="code">
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</div>
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</div>
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<div class="row">
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<p>The second part …</p>
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<div class="code">
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</div>
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</div>
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<pre><code>splitWhen (all isSpace) . transpose . tail</code></pre>
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<div class="row">
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<p>…is used to transpose numbers! Note what happens when we
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transpose the lines containing numbers, we get:</p>
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<div class="code">
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+
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</div>
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</div>
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<pre><code>["1 ","24 ","356"," ","369","248","8 ", ...]</code></pre>
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+
<div class="row">
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<p>There is an element containing all spaces (which was present
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between each column!). This is what we use to separate each
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column.</p>
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<div class="code">
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</div>
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</div>
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<div class="row">
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<p>Finally, a main function to wrap it all up:</p>
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<div class="code">
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<div class="sourceCode" id="cb12"><pre
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+
class="sourceCode haskell"><code class="sourceCode haskell"><span id="cb12-1"><a href="#cb12-1" aria-hidden="true" tabindex="-1"></a>main <span class="ot">=</span> <span class="kw">do</span></span>
256
+
<span id="cb12-2"><a href="#cb12-2" aria-hidden="true" tabindex="-1"></a> n <span class="ot"><-</span> <span class="fu">getContents</span></span>
257
+
<span id="cb12-3"><a href="#cb12-3" aria-hidden="true" tabindex="-1"></a> <span class="fu">print</span> <span class="op">$</span> p1 n</span>
258
+
<span id="cb12-4"><a href="#cb12-4" aria-hidden="true" tabindex="-1"></a> <span class="fu">print</span> <span class="op">$</span> p2 n</span></code></pre></div>
259
+
</div>
260
+
</div>
261
+
</body>
262
+
</html>
+1
out/index.html
+1
out/index.html
···
56
56
<li><a href="2.1-day-2-1.html#day-2-1" id="toc-day-2-1">Day 2</a></li>
57
57
<li><a href="2.2-day-4.html#day-4" id="toc-day-4">Day 4</a></li>
58
58
<li><a href="2.3-day-5.html#day-5" id="toc-day-5">Day 5</a></li>
59
+
<li><a href="2.4-day-6.html#day-6" id="toc-day-6">Day 6</a></li>
59
60
</ul></li>
60
61
</ul>
61
62
</nav>
+1
-1
out/sitemap.json
+1
-1
out/sitemap.json
···
1
-
{"section":{"id":"","level":"0","number":null,"path":"index.html","title":""},"subsections":[{"section":{"id":"section","level":"1","number":"1","path":"1-section.html#section","title":"2016"},"subsections":[{"section":{"id":"day-1","level":"2","number":"1.1","path":"1.1-day-1.html#day-1","title":"Day 1"},"subsections":[{"section":{"id":"part-1","level":"3","number":"1.1.1","path":"1.1-day-1.html#part-1","title":"Part 1"},"subsections":[]},{"section":{"id":"part-2","level":"3","number":"1.1.2","path":"1.1-day-1.html#part-2","title":"Part 2"},"subsections":[]}]},{"section":{"id":"day-2","level":"2","number":"1.2","path":"1.2-day-2.html#day-2","title":"Day 2"},"subsections":[{"section":{"id":"part-1-1","level":"3","number":"1.2.1","path":"1.2-day-2.html#part-1-1","title":"Part 1"},"subsections":[]}]}]},{"section":{"id":"section-1","level":"1","number":"2","path":"2-section-1.html#section-1","title":"2025"},"subsections":[{"section":{"id":"day-2-1","level":"2","number":"2.1","path":"2.1-day-2-1.html#day-2-1","title":"Day 2"},"subsections":[{"section":{"id":"part-1-2","level":"3","number":"2.1.1","path":"2.1-day-2-1.html#part-1-2","title":"Part 1"},"subsections":[]},{"section":{"id":"part-2-1","level":"3","number":"2.1.2","path":"2.1-day-2-1.html#part-2-1","title":"Part 2"},"subsections":[]}]},{"section":{"id":"day-4","level":"2","number":"2.2","path":"2.2-day-4.html#day-4","title":"Day 4"},"subsections":[{"section":{"id":"part-1-3","level":"3","number":"2.2.1","path":"2.2-day-4.html#part-1-3","title":"Part 1"},"subsections":[]},{"section":{"id":"part-2-2","level":"3","number":"2.2.2","path":"2.2-day-4.html#part-2-2","title":"Part 2"},"subsections":[]}]},{"section":{"id":"day-5","level":"2","number":"2.3","path":"2.3-day-5.html#day-5","title":"Day 5"},"subsections":[{"section":{"id":"part-1-4","level":"3","number":"2.3.1","path":"2.3-day-5.html#part-1-4","title":"Part 1"},"subsections":[]},{"section":{"id":"part-2-3","level":"3","number":"2.3.2","path":"2.3-day-5.html#part-2-3","title":"Part 2"},"subsections":[]}]}]}]}
1
+
{"section":{"id":"","level":"0","number":null,"path":"index.html","title":""},"subsections":[{"section":{"id":"section","level":"1","number":"1","path":"1-section.html#section","title":"2016"},"subsections":[{"section":{"id":"day-1","level":"2","number":"1.1","path":"1.1-day-1.html#day-1","title":"Day 1"},"subsections":[{"section":{"id":"part-1","level":"3","number":"1.1.1","path":"1.1-day-1.html#part-1","title":"Part 1"},"subsections":[]},{"section":{"id":"part-2","level":"3","number":"1.1.2","path":"1.1-day-1.html#part-2","title":"Part 2"},"subsections":[]}]},{"section":{"id":"day-2","level":"2","number":"1.2","path":"1.2-day-2.html#day-2","title":"Day 2"},"subsections":[{"section":{"id":"part-1-1","level":"3","number":"1.2.1","path":"1.2-day-2.html#part-1-1","title":"Part 1"},"subsections":[]}]}]},{"section":{"id":"section-1","level":"1","number":"2","path":"2-section-1.html#section-1","title":"2025"},"subsections":[{"section":{"id":"day-2-1","level":"2","number":"2.1","path":"2.1-day-2-1.html#day-2-1","title":"Day 2"},"subsections":[{"section":{"id":"part-1-2","level":"3","number":"2.1.1","path":"2.1-day-2-1.html#part-1-2","title":"Part 1"},"subsections":[]},{"section":{"id":"part-2-1","level":"3","number":"2.1.2","path":"2.1-day-2-1.html#part-2-1","title":"Part 2"},"subsections":[]}]},{"section":{"id":"day-4","level":"2","number":"2.2","path":"2.2-day-4.html#day-4","title":"Day 4"},"subsections":[{"section":{"id":"part-1-3","level":"3","number":"2.2.1","path":"2.2-day-4.html#part-1-3","title":"Part 1"},"subsections":[]},{"section":{"id":"part-2-2","level":"3","number":"2.2.2","path":"2.2-day-4.html#part-2-2","title":"Part 2"},"subsections":[]}]},{"section":{"id":"day-5","level":"2","number":"2.3","path":"2.3-day-5.html#day-5","title":"Day 5"},"subsections":[{"section":{"id":"part-1-4","level":"3","number":"2.3.1","path":"2.3-day-5.html#part-1-4","title":"Part 1"},"subsections":[]},{"section":{"id":"part-2-3","level":"3","number":"2.3.2","path":"2.3-day-5.html#part-2-3","title":"Part 2"},"subsections":[]}]},{"section":{"id":"day-6","level":"2","number":"2.4","path":"2.4-day-6.html#day-6","title":"Day 6"},"subsections":[{"section":{"id":"part-1-5","level":"3","number":"2.4.1","path":"2.4-day-6.html#part-1-5","title":"Part 1"},"subsections":[]},{"section":{"id":"part-2-4","level":"3","number":"2.4.2","path":"2.4-day-6.html#part-2-4","title":"Part 2"},"subsections":[]}]}]}]}
+44
-1
out/style.css
+44
-1
out/style.css
···
1
+
:root {
2
+
--bg-color: #ffffff;
3
+
--text-color: #1a1a1a;
4
+
--border-color: #e0e0e0;
5
+
--link-color: #0066cc;
6
+
--link-hover: #004499;
7
+
--link-visited: #551a8b;
8
+
--syntax-datatype: #8f4e8b;
9
+
--syntax-comment: #6a737d;
10
+
--syntax-keyword: #1a1a1a;
11
+
}
12
+
13
+
@media (prefers-color-scheme: dark) {
14
+
:root {
15
+
--bg-color: #000000;
16
+
--text-color: #e8e8e8;
17
+
--border-color: #404040;
18
+
--link-color: #6eb5ff;
19
+
--link-hover: #9fcdff;
20
+
--link-visited: #d4a5ff;
21
+
--syntax-datatype: #c792ea;
22
+
--syntax-comment: #b0b0b0;
23
+
--syntax-keyword: #e8e8e8;
24
+
}
25
+
}
26
+
1
27
body {
2
28
max-width: 1400px;
3
29
margin-left: auto;
···
5
31
box-sizing: border-box;
6
32
padding: 0 5vw;
7
33
font-family: "Libre Baskerville", serif;
34
+
background-color: var(--bg-color);
35
+
color: var(--text-color);
8
36
}
9
37
10
38
.sitenav {
···
35
63
overflow: auto;
36
64
}
37
65
66
+
a {
67
+
color: var(--link-color);
68
+
}
69
+
70
+
a:hover {
71
+
color: var(--link-hover);
72
+
}
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+
74
+
a:visited {
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+
color: var(--link-visited);
76
+
}
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+
78
+
code span.dt { color: var(--syntax-datatype); }
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+
code span.co { color: var(--syntax-comment); font-style: italic; }
80
+
code span.kw { color: var(--syntax-keyword); font-weight: bold; }
81
+
38
82
@media (max-width: 768px) {
39
83
body {
40
84
max-width: 100%;
···
54
98
padding: 0;
55
99
}
56
100
}
57
-
+94
src/2025/06.lhs
+94
src/2025/06.lhs
···
1
+
## Day 6
2
+
3
+
This day is largely string munging, which is quite boring, but also quite easy to do with Haskell!
4
+
5
+
### Part 1
6
+
7
+
This part needs us to consider each operation on a column. `Data.List.transpose` fortunately makes this very simple. If we are able to massage our input into the form `["+", "1", "2", "3"]`, we can use this simple `solve` function to produce the result. The only motivation to use this format is the ease of pattern-matching over the start of the list.
8
+
9
+
```haskell
10
+
solve ("+" : rest) = sum . map read $ rest
11
+
solve ("*" : rest) = product . map read $ rest
12
+
```
13
+
14
+
Thus, to solve the first part, we can break up our input using `lines`. `rot` is a simple function that rotates a list forwards to move the last element to the first. By doing `rot . lines` upon our input, we have a list of the form `["+ *", "1 2 3", "4 5 6"]`. Next, we define an `align` function to line up the colums. This method simply breaks each item in the list by spaces, and then transposes them, so we end up with `[["+", "1", "2", "3"], ["*", "4", "5", "6"]]`:
15
+
16
+
```haskell-top
17
+
import Data.Char (isSpace)
18
+
import Data.List (transpose)
19
+
import Data.List.Split (splitWhen)
20
+
```
21
+
22
+
```haskell
23
+
rot = liftA2 (:) last init
24
+
25
+
p1 = sum . map solve . align . rot . lines
26
+
where
27
+
align = transpose . map words
28
+
```
29
+
30
+
### Part 2
31
+
32
+
We are now tasked with lining up the digits in a columnar fashion to for a number:
33
+
34
+
123 ...
35
+
45 ...
36
+
6 ...
37
+
* ...
38
+
39
+
Here, the numbers to operate on are 356, 24 and 1. Unfortunately, we can no longer employ `words` as it breaks on all whitespace. We must now transpose our list as-is, to make sense of the digits. Ignore the last row for a second, and only consider the first 3 rows:
40
+
41
+
123
42
+
45
43
+
6
44
+
45
+
Upon transposing this, we get:
46
+
47
+
1
48
+
24
49
+
356
50
+
51
+
This is exactly what we need!
52
+
53
+
Let us start with a similar base, using `lines` to break things up, and `rot` to move the line of operators to the start. Our `align` function can now be defined as below.
54
+
55
+
```haskell
56
+
p2 = sum . map solve . align . rot . lines
57
+
where
58
+
align =
59
+
zipWith (:)
60
+
<$> (words . head)
61
+
<*> (splitWhen (all isSpace) . transpose . tail)
62
+
```
63
+
64
+
We first start with `zipWith (:)` to join the operation `"+"` with the rest of the numbers to produce:
65
+
66
+
["+", "1", "2", ...]
67
+
68
+
Remember that this is the format `solve` accepts!
69
+
70
+
We are using `<*>` to perform "sequential application". The first part ...
71
+
72
+
words . head
73
+
74
+
... is used to split up the operator line into a list of operators.
75
+
76
+
The second part ...
77
+
78
+
splitWhen (all isSpace) . transpose . tail
79
+
80
+
...is used to transpose numbers! Note what happens when we transpose the lines containing numbers, we get:
81
+
82
+
["1 ","24 ","356"," ","369","248","8 ", ...]
83
+
84
+
There is an element containing all spaces (which was present between each column), `Data.List.Split.splitWhen` is employed to detect and chunk our list into runs of valid numbers.
85
+
86
+
87
+
Finally, a main function to wrap it all up:
88
+
89
+
```haskell
90
+
main = do
91
+
n <- getContents
92
+
print $ p1 n
93
+
print $ p2 n
94
+
```