docs: add design notes

README.md

@@ -740,9 +740,9 @@ return {
 ## 🙏 PRs are welcome
 
 Improvement suggestion PRs to this repo are very much welcome, and I encourage
-you to begin in the
-[discussions](https://github.com/fredrikaverpil/neotest-golang/discussions) in
-case the change is not trivial.
+you to begin by reading the below paragraph on the adapter design and engage in
+the [discussions](https://github.com/fredrikaverpil/neotest-golang/discussions)
+in case the change is not trivial.
 
 You can run tests, formatting and linting locally with `make all`. Install
 dependencies with `make install`. Have a look at the [Makefile](Makefile) for
@@ -769,3 +769,78 @@ query and play around. You can paste in queries from
 [`query.lua`](https://github.com/fredrikaverpil/neotest-golang/blob/main/lua/neotest-golang/query.lua)
 in the editor, to see how the query behaves and highlights parts of your Go test
 file.
+
+## General design of the adapter
+
+### Treesitter queries detect tests
+
+Neotest leverages treesitter AST-parsing of source code to detect tests. This
+adapter supplies queries so to figure out what is considered a test.
+
+From the result of these queries, a Neotest "position" tree is built (can be
+visualized through the "Neotest summary"). Each position in the tree represents
+either a `dir`, `file` or `test` type. Neotest also has a notion of a
+`namespace` position type, but this is ignored by default by this adapter (but
+leveraged to supply testify support).
+
+### Generating valid `go test` commands
+
+The `dir`, `file` and `test` tree position types cannot be directly translated
+over to Go so to produce a valid `go test` command. Go primarily cares about a
+Go package's import path, test name regexp filters and the current working
+directory.
+
+For example, these are all valid `go test` command:
+
+```bash
+# run all tests, recursing sub-packages, in the current working directory.
+go test ./...
+
+# run all tests in a given package 'x', by specifying the full import path
+go test github.com/fredrikaverpil/neotest-golang/x
+
+# run all tests in a given package 'x', recursing sub-packages
+go test github.com/fredrikaverpil/neotest-golang/x/...
+
+# run _some_ tests in a given package, based on a regexp filter
+go test github.com/fredrikaverpil/neotest-golang -run "^TestFoo$|^TestBar$"
+```
+
+> [!NOTE]
+>
+> All the above commands must be run somewhere beneath the location of the
+> `go.mod` file specifying the _module_ name, which in this example is
+> `github.com/fredrikaverpil/neotest-golang`.
+
+I figured out that by executing `go list -json ./...` in the `go.mod` root
+location, the output provides valuable information about test files/folders and
+their corresponding Go package's import path. This data is key to being able to
+take the Neotest/treesitter position type and generate a valid `go test` command
+for it. In essence, this approach is what makes neotest-golang so robust.
+
+### Output processing
+
+Neotest captures the stdout from the test execution command and writes it to
+disk as a temporary file. The adapter is responsible for reading the file(s) and
+reporting back status and output to the Neotest tree (and specifically the
+position in the tree which was executed). It is therefore crucial for outputting
+structured data, which in this case is done with `go test -json`.
+
+One challenge here is that Go build errors are not part of the strucutured JSON
+output (although captured in the stdout) and needs to be looked for in other
+ways.
+
+Another challenge is to properly populate statuses and errors into the
+corresponding Neotest tree position. This becomes increasingly difficult when
+you consider running tests in a recursive manner (e.g. `go test -json ./...`).
+
+Errors are recorded and populated, per position type, along with its
+corresponding buffer's line number. Neotest can then show the errors inline as
+diagnostics.
+
+I've taken an approach with this adapter where I record test outcome for each
+Neotest position type and populate it onto each of them, when applicable.
+
+On some systems and terminals, there are great issues with the `go test` output.
+I've therefore made it possible to make the adapter rely on output saved
+directly to disk without going through stdout, by leveraging `gotestsum`.