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<h1>Search Engine<a name="search-engine" href="#search-engine" class="heading-anchor">🔗</a></h1>
<p>The Search Engine powers the Search input in the note list and the Goto Anything dialog.</p>
<h2>Search algorithm<a name="search-algorithm" href="#search-algorithm" class="heading-anchor">🔗</a></h2>
<h3>Discretely using only the most critical parameter in sorting<a name="discretely-using-only-the-most-critical-parameter-in-sorting" href="#discretely-using-only-the-most-critical-parameter-in-sorting" class="heading-anchor">🔗</a></h3>
<p>Sorting occurs as the Search Engine processes results, after searching for and weighting these results.</p>
<p>Parameters include fuzziness, title matching, weight (based on BM25 and age), the completed status of to-dos, and the note's age.</p>
<p>The Search Engine uses only the first relevant parameter to determine the order, rather than a weighted average.<br>
In effect, this means search results with note title matches will appear above all results that only matched the note body,<br>
regardless of weight or other parameters.</p>
<h3>Determining weight as a sorting parameter<a name="determining-weight-as-a-sorting-parameter" href="#determining-weight-as-a-sorting-parameter" class="heading-anchor">🔗</a></h3>
<p>The Search Engine determines the weight parameter using both <a href="https://en.wikipedia.org/wiki/Okapi_BM25">BM25</a><br>
and the number of days since last user update.</p>
<h4>BM25<a name="bm25" href="#bm25" class="heading-anchor">🔗</a></h4>
<p>The Search Engine determines BM25 based on "term frequency-inverse document frequency."<br>
The "TF–IDF" value increases proportionally to the number of times a word appears in the document<br>
and is offset by the number of documents in the corpus that contain the word, which helps to adjust<br>
for the fact that some words appear more frequently in general.</p>
<p>BM25 returns weight zero for a search term that occurs in more than half the notes.<br>
So terms that are abundant in all notes to have zero relevance w.r.t. BM25.</p>
<h4>Days since last user update<a name="days-since-last-user-update" href="#days-since-last-user-update" class="heading-anchor">🔗</a></h4>
<p>Sorting increases the BM25 weight by the inverse number of days since the note was updated.<br>
Recent notes will, therefore, be weighted highly in the search results.<br>
This time-based weight decays logarithmically, becoming less of a factor than BM25 after months have passed.</p>
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