feat: Initial clean release of CHM

readme/dev/spec/search_sorting.md

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