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+;;; test-dev-fkeys--f6-source-stem.el --- Tests for cj/--f6-source-stem -*- lexical-binding: t -*-
+
+;;; Commentary:
+;; Tests for the stem extractor. Given any source-or-test filename, returns
+;; the source module name with directory, extension, and any test-pattern
+;; prefix/suffix stripped. Used to map source files to their tests and
+;; vice versa.
+
+;;; Code:
+
+(require 'ert)
+(add-to-list 'load-path (expand-file-name "modules" user-emacs-directory))
+(require 'dev-fkeys)
+
+;;; Normal Cases — Elisp
+
+(ert-deftest test-dev-fkeys-f6-source-stem-elisp-source ()
+  "Normal: a regular elisp source file's stem is its basename."
+  (should (string= (cj/--f6-source-stem "modules/foo.el") "foo")))
+
+(ert-deftest test-dev-fkeys-f6-source-stem-elisp-test ()
+  "Normal: an elisp test file's stem strips the `test-' prefix."
+  (should (string= (cj/--f6-source-stem "tests/test-foo.el") "foo")))
+
+(ert-deftest test-dev-fkeys-f6-source-stem-elisp-test-double-dash ()
+  "Normal: an elisp per-helper test file `test-<module>--<helper>.el' yields
+the module name (everything from `--' onward is dropped)."
+  (should (string= (cj/--f6-source-stem "tests/test-dev-fkeys--detect-project-type.el")
+                   "dev-fkeys")))
+
+;;; Normal Cases — Python
+
+(ert-deftest test-dev-fkeys-f6-source-stem-python-source ()
+  "Normal: a regular Python source file's stem is its basename."
+  (should (string= (cj/--f6-source-stem "pkg/foo.py") "foo")))
+
+(ert-deftest test-dev-fkeys-f6-source-stem-python-test-prefix ()
+  "Normal: a `test_<name>.py' file's stem strips the `test_' prefix."
+  (should (string= (cj/--f6-source-stem "tests/test_foo.py") "foo")))
+
+(ert-deftest test-dev-fkeys-f6-source-stem-python-test-suffix ()
+  "Normal: a `<name>_test.py' file's stem strips the `_test' suffix."
+  (should (string= (cj/--f6-source-stem "pkg/foo_test.py") "foo")))
+
+;;; Normal Cases — Go
+
+(ert-deftest test-dev-fkeys-f6-source-stem-go-source ()
+  "Normal: a regular Go source file's stem is its basename."
+  (should (string= (cj/--f6-source-stem "pkg/foo.go") "foo")))
+
+(ert-deftest test-dev-fkeys-f6-source-stem-go-test ()
+  "Normal: a `<name>_test.go' file's stem strips the `_test' suffix."
+  (should (string= (cj/--f6-source-stem "pkg/foo_test.go") "foo")))
+
+;;; Boundary Cases
+
+(ert-deftest test-dev-fkeys-f6-source-stem-elisp-multi-segment-name ()
+  "Boundary: hyphenated module names round-trip cleanly."
+  (should (string= (cj/--f6-source-stem "modules/calendar-sync.el") "calendar-sync"))
+  (should (string= (cj/--f6-source-stem "tests/test-calendar-sync.el") "calendar-sync")))
+
+(ert-deftest test-dev-fkeys-f6-source-stem-elisp-test-with-dashes-after-double-dash ()
+  "Boundary: a per-helper test file with hyphens in both module and helper:
+`test-foo-bar--baz-qux.el' → `foo-bar' (stops at first `--')."
+  (should (string= (cj/--f6-source-stem "tests/test-foo-bar--baz-qux.el")
+                   "foo-bar")))
+
+(ert-deftest test-dev-fkeys-f6-source-stem-unknown-language-falls-back-to-basename ()
+  "Boundary: an unsupported extension just returns the basename without extension."
+  (should (string= (cj/--f6-source-stem "foo.rs") "foo"))
+  (should (string= (cj/--f6-source-stem "foo.txt") "foo")))
+
+;;; Error Cases
+
+(ert-deftest test-dev-fkeys-f6-source-stem-nil-returns-nil ()
+  "Error: nil filename returns nil without erroring."
+  (should (null (cj/--f6-source-stem nil))))
+
+(ert-deftest test-dev-fkeys-f6-source-stem-empty-returns-nil ()
+  "Error: empty string returns nil."
+  (should (null (cj/--f6-source-stem ""))))
+
+(provide 'test-dev-fkeys--f6-source-stem)
+;;; test-dev-fkeys--f6-source-stem.el ends here