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+# `docsim`
+
+A simple, fast command-line tool for scoring the similarity of text documents.
+
+``` console
+$ docsim --query some-file.txt --show-scores ~/documents/notes
+0.000  completely-dissimilar-file.txt
+0.152  somewhat-similar-file.md
+0.469  pretty-similar-file.md
+0.872  very-similar-file.org
+1.000  potentially-identical-file.txt
+```
+
+Given a query document and a collection of potential matches, `docsim` ranks
+each document in the collection by its textual similarity to the query.
+
+## Examples
+
+Check the [`man` page] for the definitive documentation, but these should get
+you started.
+
+[`man` page]: ./man/docsim.1
+
+Search for similar files in a given directory:
+
+``` console
+$ docsim --query some-file.txt ~/documents/notes
+[...]
+```
+
+If no paths are provided `docsim` will search the current working directory.
+
+``` console
+$ docsim --query some-file.txt
+[...]
+```
+
+Without a provided `--query` document `docsim` takes input from STDIN:
+
+``` console
+$ echo "Here's a query to search for." | docsim ~/documents/notes
+[...]
+```
+
+Only show the top 3 matches, with the best at the top:
+
+``` console
+$ docsim --query some-file.txt --limit 3 --best-first ~/documents/notes
+potentially-identical-file.txt
+very-similar-file.org
+pretty-similar-file.md
+```
+
+Find Go files similar to `main.go` in the current directory. Don't use stemming
+or stoplists, since these aren't English documents.
+
+``` console
+$ docsim --query main.go --no-stemming --no-stoplist **/*.go
+[...]
+```
+
+Notice that because `docsim` uses an English stoplist and an English stemming
+algorithm, you'll almost certainly want to use the `--no-stoplist` and
+`--no-stemming` flags if your documents are written in another language
+(including source code).
+
+## Installation
+
+If you've got a Go toolchain handy, you can either:
+
+``` console
+$ git clone git@github.com:hrs/docsim.git
+$ cd docsim
+$ sudo make install
+```
+
+Or just:
+
+``` console
+$ go install github.com/hrs/docsim/docsim@latest
+```
+
+Note that using `go install` that doesn't include the [`man` page][], which you
+can optionally install manually by copying into e.g.
+`/usr/local/share/man/man1`.
+
+[`man` page]: ./man/docsim.1
+
+## Running tests
+
+``` console
+$ make test
+```
+
+## How it works
+
+`docsim` uses [TF-IDF][] weighting and [cosine similarity][] to numerically
+score the textual similarity between the query and every other document.
+
+[TF-IDF]: https://en.wikipedia.org/wiki/Tf%E2%80%93idf
+[cosine similarity]: https://en.wikipedia.org/wiki/Vector_space_model#Applications
+
+"Textual similarity" roughly means "uses the same words." Each document is
+parsed into a big bag of words, which are passed through a common English
+[stoplist][], [stemmed][] (so "spins," "spinner," and "spinning" might all
+reduce down to just "spin"), and assigned weights based on how often they appear
+in the document and how rare they are in the corpus as a whole.
+
+[stoplist]: https://en.wikipedia.org/wiki/Stop_word
+[stemmed]: https://en.wikipedia.org/wiki/Stemming
+
+We can think of each of these documents as a [vector in term space][], where
+each word is a dimension with its weight as a magnitude. Two documents are
+"similar," then, inasmuch as they point in the same direction, so we define
+similarity by the size of the angle between them.
+
+[vector in term space]: https://en.wikipedia.org/wiki/Vector_space_model
+
+## Contributing
+
+`docsim` is still in a nascent state, so I'm happy just writing the code myself
+for now, but please feel free to [report any issues][] you encounter!
+
+[report any issues]: https://github.com/hrs/docsim/issues