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+;;; test-prog-lsp--add-file-watch-ignored-extras.el --- Tests for cj/lsp--add-file-watch-ignored-extras -*- lexical-binding: t; -*-
+
+;;; Commentary:
+;; Tests for cj/lsp--add-file-watch-ignored-extras in prog-lsp.el.
+;; The function adds project-agnostic build/cache directory patterns to
+;; `lsp-file-watch-ignored-directories' without replacing lsp-mode's
+;; defaults.  Patterns are sourced from `cj/lsp-file-watch-ignored-extras'.
+
+;;; Code:
+
+(require 'ert)
+(require 'cl-lib)
+
+;; Declare lsp-mode's defcustom as a special variable so `let' binds it
+;; dynamically.  Real definition is lsp-mode's; loaded only when use-package
+;; activates lsp-mode.  In the test environment, this stub provides the value
+;; cell `add-to-list' needs.
+(defvar lsp-file-watch-ignored-directories nil)
+
+(require 'prog-lsp)
+
+;;; Normal Cases
+
+(ert-deftest test-prog-lsp--add-file-watch-ignored-extras-normal-adds-all-patterns ()
+  "Normal: every entry from `cj/lsp-file-watch-ignored-extras' lands in the list."
+  (let ((lsp-file-watch-ignored-directories nil))
+    (cj/lsp--add-file-watch-ignored-extras)
+    (should (= (length lsp-file-watch-ignored-directories)
+               (length cj/lsp-file-watch-ignored-extras)))
+    (dolist (pattern cj/lsp-file-watch-ignored-extras)
+      (should (member pattern lsp-file-watch-ignored-directories)))))
+
+(ert-deftest test-prog-lsp--add-file-watch-ignored-extras-normal-extends-not-replaces ()
+  "Normal: pre-existing entries (lsp-mode defaults) are preserved."
+  (let ((lsp-file-watch-ignored-directories
+         '("[/\\\\]\\.git\\'" "[/\\\\]\\.svn\\'" "[/\\\\]\\.idea\\'")))
+    (cj/lsp--add-file-watch-ignored-extras)
+    (should (member "[/\\\\]\\.git\\'" lsp-file-watch-ignored-directories))
+    (should (member "[/\\\\]\\.svn\\'" lsp-file-watch-ignored-directories))
+    (should (member "[/\\\\]\\.idea\\'" lsp-file-watch-ignored-directories))
+    (dolist (pattern cj/lsp-file-watch-ignored-extras)
+      (should (member pattern lsp-file-watch-ignored-directories)))))
+
+(ert-deftest test-prog-lsp--add-file-watch-ignored-extras-normal-key-patterns-present ()
+  "Normal: specific expected directory names appear in the constant."
+  (dolist (name '("node_modules" "target" "__pycache__" ".venv" "venv"
+                  "dist" "coverage" "test-results" "playwright-report"
+                  ".terraform" ".ruff_cache" ".pytest_cache" ".mypy_cache"))
+    (should (cl-some (lambda (p) (string-match-p (regexp-quote name) p))
+                     cj/lsp-file-watch-ignored-extras))))
+
+;;; Boundary Cases
+
+(ert-deftest test-prog-lsp--add-file-watch-ignored-extras-boundary-idempotent ()
+  "Boundary: calling twice doesn't duplicate entries."
+  (let ((lsp-file-watch-ignored-directories nil))
+    (cj/lsp--add-file-watch-ignored-extras)
+    (cj/lsp--add-file-watch-ignored-extras)
+    (should (= (length lsp-file-watch-ignored-directories)
+               (length cj/lsp-file-watch-ignored-extras)))))
+
+(ert-deftest test-prog-lsp--add-file-watch-ignored-extras-boundary-all-patterns-non-empty ()
+  "Boundary: every pattern is a non-empty string."
+  (dolist (pattern cj/lsp-file-watch-ignored-extras)
+    (should (stringp pattern))
+    (should (not (string-empty-p pattern)))))
+
+(ert-deftest test-prog-lsp--add-file-watch-ignored-extras-boundary-all-patterns-valid-regex ()
+  "Boundary: every pattern compiles as a valid Emacs regex."
+  (dolist (pattern cj/lsp-file-watch-ignored-extras)
+    (condition-case err
+        ;; string-match-p compiles the regex; invalid syntax raises invalid-regexp.
+        (string-match-p pattern "/some/sample/path")
+      (invalid-regexp
+       (ert-fail (format "Invalid regex %S: %s"
+                         pattern (error-message-string err)))))))
+
+;;; Error Cases
+
+(ert-deftest test-prog-lsp--add-file-watch-ignored-extras-error-non-list-target ()
+  "Error: non-list target value triggers `add-to-list' wrong-type-argument."
+  (let ((lsp-file-watch-ignored-directories "not-a-list"))
+    (should-error (cj/lsp--add-file-watch-ignored-extras)
+                  :type 'wrong-type-argument)))
+
+(provide 'test-prog-lsp--add-file-watch-ignored-extras)
+;;; test-prog-lsp--add-file-watch-ignored-extras.el ends here