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<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1">
<title>Site search with Perl + CGI + SA</title>
<link rel="stylesheet" href="/assets/css/main.css">
<link rel="stylesheet" href="/assets/css/skeleton.css">
</head>
<body>
<div id="nav-container" class="container">
<ul id="navlist" class="left">
<li >
<a href="/" class="link-decor-none">hme</a>
</li>
<li class="active">
<a href="/log/" class="link-decor-none">log</a>
</li>
<li >
<a href="/projects/" class="link-decor-none">poc</a>
</li>
<li >
<a href="/about/" class="link-decor-none">abt</a>
</li>
<li>
<a href="/cgi-bin/find.cgi" class="link-decor-none">sws</a>
</li>
<li>
<a href="/feed.xml" class="link-decor-none">rss</a>
</li>
</ul>
</div>
<main>
<div class="container">
<div class="container-2">
<h2 class="center" id="title">SITE SEARCH WITH PERL + CGI + SA</h2>
<h5 class="center">02 JANUARY 2026</h5>
<br>
<div class="twocol justify"><p>Number of articles on the site is growing. Need a way to find them.</p>
<p>Search must match substrings, be case-insensitive, fast, and secure. Site must
continue to work without JavaScript, on text web browsers–so no client-side
search. Page rank not applicable.</p>
<p>Architecture. OpenBSD’s httpd speaks FastCGI. Perl and slowcgi are in the base
system. Search using a Perl script. Invoke it from httpd via slowcgi. Send the
results back over FastCGI.</p>
<p>Data structure. Suffix array. Build-time indexer generates three files:
corpus.bin–articles, sa.bin-suffix array, file_map.dat–article metadata
(title, path).</p>
<h2 id="indexer">Indexer</h2>
<p>Crawl the directory containing posts, extract HTML with regex
(no-regex-for-HTML doctrinaires getting a seizure right about now), lower-case
the text (for case-insensitive search), convert to raw octets, concatenate.</p>
<p>Sort the text lexicographically, and store their byte offsets in sa.bin:</p>
<div class="language-plaintext highlighter-rouge"><div class="highlight"><pre class="highlight"><code># Use a block that forces byte-level comparison
{
use bytes;
@sa = sort {
# First 64 bytes check (fast path)
(substr($corpus, $a, 64) cmp substr($corpus, $b, 64)) ||
# Full string fallback (required for correctness)
(substr($corpus, $a) cmp substr($corpus, $b))
} @sa;
}
</code></pre></div></div>
<p>This is the slowest part of the indexer running at ~ O(L⋅NlogN), N is the
suffix count, L the median length of suffixes. The fast path caps L at 64 bytes
for most cases, bringing complexity down to O(NlogN)–64-byte length chosen to
fit in cache lines.</p>
<div class="language-plaintext highlighter-rouge"><div class="highlight"><pre class="highlight"><code>CORPUS: a s c i \0 i m x
OFFSET: 0 1 2 3 4 5 6 7
SORTED SUFFIXES:
[4] \0imx
[0] asci\0imx
[2] ci\0imx
[3] i\0imx <-- "i" from "asci"
[5] imx <-- "i" from "imx"
[6] mx
[1] sci\0imx
[7] x
</code></pre></div></div>
<p>Sentinel (null byte) must be lexicographically smaller than every other
character in the alphabet (offset 3 and 5 handled correctly at the document
boundary).</p>
<p>Benchmarks.</p>
<p>Performed on a T490 running OpenBSD 7.8. CPU: Intel(R) Core(TM)
i7-10510U CPU @ 1.80GHz. AMD64. 15 GB memory.</p>
<p>All articles are 16 KB (2x my avg post size) documents generated with seed.sh.</p>
<p>Total Time: 0.1475 seconds<br />
Files Processed: 500 <br />
File Sizes:<br />
corpus.bin 33.59 KB<br />
sa.bin 134.34 KB<br />
file_map.dat 37.01 KB<br />
Total Index: 204.94 KB<br /></p>
<p>Total Time: 0.3101 seconds<br />
Files Processed: 1000<br />
File Sizes:<br />
corpus.bin 67.28 KB<br />
sa.bin 269.11 KB<br />
file_map.dat 74.12 KB<br />
Total Index: 410.51 KB<br /></p>
<p>Total Time: 10.9661 seconds<br />
Files Processed: 10000<br />
File Sizes:<br />
corpus.bin 682.51 KB<br />
sa.bin 2730.05 KB<br />
file_map.dat 750.88 KB<br />
Total Index: 4163.44 KB<br /></p>
<p>There are 11 posts today. Unlikely that it’d grow to 10K. Even if it did, the
index is very manageable.</p>
<h2 id="search">Search</h2>
<p>Need to perform a range query to find offsets of all matches. Suffix
array is sorted–use binary search.</p>
<div class="language-plaintext highlighter-rouge"><div class="highlight"><pre class="highlight"><code># left bound
while ($low <= $high) {
# Int overflow: Perl handles by promoting to float.
my $mid = int(($low + $high) / 2);
seek($fh_sa, $mid * 4, 0);
read($fh_sa, my $bin_off, 4);
my $off = unpack("L", $bin_off);
seek($fh_cp, $off, 0);
read($fh_cp, my $text, $query_len);
my $cmp = $text cmp $query;
if ($cmp >= 0) {
$first_hit = $mid if $cmp == 0;
$high = $mid - 1;
} else {
$low = $mid + 1;
}
}
# Continue with right bound...
</code></pre></div></div>
<p>Each offset is a 32-bit unsigned int in sa.bin. Use the word-aligned offsets to
seek/read exact chunks into memory. Since the FastCGI script is invoked per
query, small, frequent allocations are better than less frequent large
allocations (see bm_*.txt).</p>
<p>Once the range is found through binary search, the results are collected using
a simple for loop. Max result count is capped at 20.</p>
<p>Benchmarks. Search for ‘arduino’ (expect 0 matches). Left: SA-based index
search. Right: Naive search using regex.</p>
<p>500 articles:</p>
<table>
<tr>
<td style="padding: 0">Files Read:</td>
<td style="padding: 0; text-align: right;">3</td>
<td style="padding: 0; text-align: right;">500</td>
</tr>
<tr>
<td style="padding: 0">User CPU (s):</td>
<td style="padding: 0; text-align: right;">0.0100</td>
<td style="padding: 0; text-align: right;">0.0600</td>
</tr>
<tr>
<td style="padding: 0">System CPU (s):</td>
<td style="padding: 0; text-align: right;">0.0200</td>
<td style="padding: 0; text-align: right;">0.0200</td>
</tr>
<tr>
<td style="padding: 0">Total Time (s):</td>
<td style="padding: 0; text-align: right;">0.0012</td>
<td style="padding: 0; text-align: right;">0.0407</td>
</tr>
<tr>
<td style="padding: 0">Peak RAM (KB):</td>
<td style="padding: 0; text-align: right;">8828</td>
<td style="padding: 0; text-align: right;">9136</td>
</tr>
</table>
<p>1,000 articles:</p>
<table>
<tr>
<td style="padding: 0">Files Read:</td>
<td style="padding: 0; text-align: right;">3</td>
<td style="padding: 0; text-align: right;">1000</td>
</tr>
<tr>
<td style="padding: 0">User CPU (s):</td>
<td style="padding: 0; text-align: right;">0.0200</td>
<td style="padding: 0; text-align: right;">0.0500</td>
</tr>
<tr>
<td style="padding: 0">System CPU (s):</td>
<td style="padding: 0; text-align: right;">0.0100</td>
<td style="padding: 0; text-align: right;">0.0500</td>
</tr>
<tr>
<td style="padding: 0">Total Time (s):</td>
<td style="padding: 0; text-align: right;">0.0019</td>
<td style="padding: 0; text-align: right;">0.0795</td>
</tr>
<tr>
<td style="padding: 0">Peak RAM (KB):</td>
<td style="padding: 0; text-align: right;">8980</td>
<td style="padding: 0; text-align: right;">9460</td>
</tr>
</table>
<p>10,000 articles:</p>
<table>
<tr>
<td style="padding: 0">Files Read:</td>
<td style="padding: 0; text-align: right;">3</td>
<td style="padding: 0; text-align: right;">10000</td>
</tr>
<tr>
<td style="padding: 0">User CPU (s):</td>
<td style="padding: 0; text-align: right;">0.0300</td>
<td style="padding: 0; text-align: right;">0.4000</td>
</tr>
<tr>
<td style="padding: 0">System CPU (s):</td>
<td style="padding: 0; text-align: right;">0.0300</td>
<td style="padding: 0; text-align: right;">0.5400</td>
</tr>
<tr>
<td style="padding: 0">Total Time (s):</td>
<td style="padding: 0; text-align: right;">0.0161</td>
<td style="padding: 0; text-align: right;">0.9120</td>
</tr>
<tr>
<td style="padding: 0">Peak RAM (KB):</td>
<td style="padding: 0; text-align: right;">12504</td>
<td style="padding: 0; text-align: right;">12804</td>
</tr>
</table>
<p>Note: Total time < User + System CPU time is likely a measurement artifact–the
script finishes faster than the OS tick interval.</p>
<h2 id="security">Security</h2>
<p>Memory usage remains a concern. Parallel queries could exhaust memory. Limit
parallel searches using a semaphore implemented with lock files:</p>
<div class="language-plaintext highlighter-rouge"><div class="highlight"><pre class="highlight"><code>my $max_parallel = 50;
mkdir $lock_dir, 0777 unless -d $lock_dir;
my $active_count = 0;
my $now = time();
opendir(my $dh, $lock_dir);
while (my $file = readdir($dh)) {
next unless $file =~ /\.lock$/;
my $path = "$lock_dir/$file";
my $mtime = (stat($path))[9] || 0;
($now - $mtime > $lock_timeout) ? unlink($path) : $active_count++;
}
closedir($dh);
</code></pre></div></div>
<p>XSS attacks. Escape all HTML using HTML::Escape qw(escape_html).</p>
<p>ReDOS: Sanitize search text by tossing non-printable characters, limit length,
work within utf-8. Use <code class="language-plaintext highlighter-rouge">\Q...\E</code> in searches.</p>
<div class="language-plaintext highlighter-rouge"><div class="highlight"><pre class="highlight"><code>if (($ENV{QUERY_STRING} || '') =~ /^q=([^&]*)/) {
$search_text = $1;
$search_text =~ s/%([0-9A-Fa-f]{2})/chr(hex($1))/eg;
$search_text = decode_utf8($search_text // "");
$search_text =~ s/\P{Print}//g;
$search_text = substr($search_text, 0, 64);
$search_text =~ s/^\s+|\s+$//g;
}
</code></pre></div></div>
<p>Command injection: no exec()/system() calls. Non-privileged user.
Read-only directory.</p>
<p>Symlink attacks: paths are hard-coded. Script can’t be modified (554).</p>
<p>No API keys, passwords involved.</p>
<p>Last line of defence: chroot.</p>
<p>Can’t think of anything else.</p>
<p>Verdict: Fast SA index lookup. Mitigated primary attack vectors. No
dependencies. Works on every conceivable device.</p>
<p>Commit:
<a href="https://git.asciimx.com/www/commit/?h=term&id=9889acaac984817731ad9f9355f067f9acd02efd">9889aca</a>
| Benchmarks:
<a href="https://git.asciimx.com/site-search-bm/commit/?id=8a4da6809cf9368cd6a5dd7351181ea4256453f9">8a4da68</a></p>
</div>
<p class="post-author right">by W. D. Sadeep Madurange</p>
</div>
</div>
</main>
<div class="footer">
<div class="container">
<div class="twelve columns right container-2">
<p id="footer-text">© ASCIIMX - 2026</p>
</div>
</div>
</div>
</body>
</html>
|
