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docs/development/testing.mdx

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+# Testing
+
+## Complement
+
+Have a look at [Complement's repository][complement] for an explanation of what
+it is.
+
+To test against Complement, with Nix (or [Lix](https://lix.systems) and
+[direnv installed and set up][direnv] (run `direnv allow` after setting up the hook), you can:
+
+* Run `./bin/complement "$COMPLEMENT_SRC"` to build a Complement image, run
+the tests, and output the logs and results to the specified paths. This will also output the OCI image
+at `result`
+* Run `nix build .#complement` from the root of the repository to just build a
+Complement OCI image outputted to `result` (it's a `.tar.gz` file)
+* Or download the latest Complement OCI image from the CI workflow artifacts
+output from the commit/revision you want to test (e.g. from main)
+[here][ci-workflows]
+
+If you want to use your own prebuilt OCI image (such as from our CI) without needing
+Nix installed, put the image at `complement_oci_image.tar.gz` in the root of the repo
+and run the script.
+
+If you're on macOS and need to build an image, run `nix build .#linux-complement`.
+
+We have a Complement fork as some tests have needed to be fixed. This can be found
+at [continuwuation/complement](https://forgejo.ellis.link/continuwuation/complement)
+
+[ci-workflows]:
+https://forgejo.ellis.link/continuwuation/continuwuity/actions/?workflow=ci.yml&actor=0&status=1
+[complement]: https://github.com/matrix-org/complement
+[direnv]: https://direnv.net/docs/hook.html