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+Contributing to `bfs`
+=====================
+
+License
+-------
+
+`bfs` is licensed under the [Zero-Clause BSD License](https://opensource.org/licenses/0BSD), a maximally permissive license.
+Contributions must use the same license.
+
+Individual files contain the following tag instead of the full license text:
+
+    SPDX-License-Identifier: 0BSD
+
+This enables machine processing of license information based on the SPDX License Identifiers that are available here: https://spdx.org/licenses/
+
+
+Implementation
+--------------
+
+`bfs` is written in [C](https://en.wikipedia.org/wiki/C_(programming_language)), specifically [C17](https://en.wikipedia.org/wiki/C17_(C_standard_revision)).
+You can get a feel for the coding style by skimming the source code.
+[`main.c`](/src/main.c) contains an overview of the rest of source files.
+A quick summary:
+
+- Tabs for indentation, spaces for alignment.
+- Most types and functions should be namespaced with `bfs_`.
+  Exceptions are made for things that could be generally useful outside of `bfs`.
+- Error handling follows the C standard library conventions: return a nonzero `int` or a `NULL` pointer, with the error code in `errno`.
+  All failure cases should be handled, including `malloc()` failures.
+- `goto` is not considered harmful for cleaning up in error paths.
+
+
+Tests
+-----
+
+`bfs` includes an extensive test suite.
+See the [build documentation](BUILDING.md#testing) for details on running the tests.
+
+Test cases are grouped by the standard or `find` implementation that supports the tested feature(s):
+
+| Group                           | Description                           |
+|---------------------------------|---------------------------------------|
+| [`tests/posix`](/tests/posix)   | POSIX compatibility tests             |
+| [`tests/bsd`](/tests/bsd)       | BSD `find` features                   |
+| [`tests/gnu`](/tests/gnu)       | GNU `find` features                   |
+| [`tests/common`](/tests/common) | Features common to BSD and GNU `find` |
+| [`tests/bfs`](/tests/bfs)       | `bfs`-specific tests                  |
+
+Both new features and bug fixes should have associated tests.
+To add a test, create a new `*.sh` file in the appropriate group.
+Snapshot tests use the `bfs_diff` function to automatically compare the generated and expected outputs.
+For example,
+
+```bash
+# posix/something.sh
+bfs_diff basic -name something
+```
+
+`basic` is one of the directory trees generated for test cases; others include `links`, `loops`, `deep`, and `rainbow`.
+
+Run `./tests/tests.sh posix/something --update` to generate the reference snapshot (and don't forget to `git add` it).