test+fix:custom-file-buffer: add tests, fix linter warning

  Add 58 unit tests for buffer/file utility functions:
  - cj/copy-link-to-buffer-file (12 tests)
  - cj/copy-path-to-buffer-file-as-kill (12 tests)
  - cj/copy-whole-buffer (14 tests)
  - cj/clear-to-bottom-of-buffer (10 tests)
  - cj/clear-to-top-of-buffer (10 tests)

  Tests cover normal cases, boundary cases (unicode, empty buffers, narrowing,
  read-only buffers), and error cases.

  Production code change:
  - Fix linter warning: unused variable 'err' → '_' in cj/move-buffer-and-file

1 files changed, 163 insertions, 0 deletions

diff --git a/tests/test-custom-file-buffer-clear-to-bottom-of-buffer.el b/tests/test-custom-file-buffer-clear-to-bottom-of-buffer.el
new file mode 100644
index 00000000..969f9bb7
--- /dev/null
+++ b/tests/test-custom-file-buffer-clear-to-bottom-of-buffer.el
@@ -0,0 +1,163 @@
+;;; test-custom-file-buffer-clear-to-bottom-of-buffer.el --- Tests for cj/clear-to-bottom-of-buffer -*- lexical-binding: t; -*-
+
+;;; Commentary:
+;; Tests for the cj/clear-to-bottom-of-buffer function from custom-file-buffer.el
+;;
+;; This function deletes all text from point to the end of the current buffer.
+;; It does not save the deleted text in the kill ring.
+
+;;; Code:
+
+(require 'ert)
+(require 'testutil-general)
+
+;; Add modules directory to load path
+(add-to-list 'load-path (expand-file-name "modules" user-emacs-directory))
+
+;; Stub dependencies before loading the module
+(defvar cj/custom-keymap (make-sparse-keymap)
+  "Stub keymap for testing.")
+
+;; Stub ps-print package
+(provide 'ps-print)
+
+;; Now load the actual production module
+(require 'custom-file-buffer)
+
+;;; Setup and Teardown
+
+(defun test-clear-to-bottom-setup ()
+  "Set up test environment."
+  (setq kill-ring nil))
+
+(defun test-clear-to-bottom-teardown ()
+  "Clean up test environment."
+  (setq kill-ring nil))
+
+;;; Normal Cases
+
+(ert-deftest test-clear-to-bottom-point-in-middle ()
+  "Should delete from point to end when point in middle."
+  (test-clear-to-bottom-setup)
+  (unwind-protect
+      (with-temp-buffer
+        (insert "Line 1\nLine 2\nLine 3")
+        (goto-char (point-min))
+        (forward-line 1)  ; Point at start of "Line 2"
+        (cj/clear-to-bottom-of-buffer)
+        (should (equal (buffer-string) "Line 1\n")))
+    (test-clear-to-bottom-teardown)))
+
+(ert-deftest test-clear-to-bottom-empty-buffer ()
+  "Should do nothing in empty buffer."
+  (test-clear-to-bottom-setup)
+  (unwind-protect
+      (with-temp-buffer
+        (cj/clear-to-bottom-of-buffer)
+        (should (equal (buffer-string) "")))
+    (test-clear-to-bottom-teardown)))
+
+;;; Boundary Cases
+
+(ert-deftest test-clear-to-bottom-point-at-beginning ()
+  "Should delete entire buffer when point at beginning."
+  (test-clear-to-bottom-setup)
+  (unwind-protect
+      (with-temp-buffer
+        (insert "Line 1\nLine 2\nLine 3")
+        (goto-char (point-min))
+        (cj/clear-to-bottom-of-buffer)
+        (should (equal (buffer-string) "")))
+    (test-clear-to-bottom-teardown)))
+
+(ert-deftest test-clear-to-bottom-point-at-end ()
+  "Should delete nothing when point at end."
+  (test-clear-to-bottom-setup)
+  (unwind-protect
+      (with-temp-buffer
+        (insert "Line 1\nLine 2\nLine 3")
+        (goto-char (point-max))
+        (cj/clear-to-bottom-of-buffer)
+        (should (equal (buffer-string) "Line 1\nLine 2\nLine 3")))
+    (test-clear-to-bottom-teardown)))
+
+(ert-deftest test-clear-to-bottom-point-second-to-last-char ()
+  "Should delete last character when point at second-to-last."
+  (test-clear-to-bottom-setup)
+  (unwind-protect
+      (with-temp-buffer
+        (insert "Hello")
+        (goto-char (1- (point-max)))  ; Before 'o'
+        (cj/clear-to-bottom-of-buffer)
+        (should (equal (buffer-string) "Hell")))
+    (test-clear-to-bottom-teardown)))
+
+(ert-deftest test-clear-to-bottom-unicode-content ()
+  "Should handle unicode content."
+  (test-clear-to-bottom-setup)
+  (unwind-protect
+      (with-temp-buffer
+        (insert "Hello 👋\nمرحبا\nWorld")
+        (goto-char (point-min))
+        (forward-line 1)
+        (cj/clear-to-bottom-of-buffer)
+        (should (equal (buffer-string) "Hello 👋\n")))
+    (test-clear-to-bottom-teardown)))
+
+(ert-deftest test-clear-to-bottom-narrowed-buffer ()
+  "Should respect narrowing."
+  (test-clear-to-bottom-setup)
+  (unwind-protect
+      (with-temp-buffer
+        (insert "Line 1\nLine 2\nLine 3\nLine 4")
+        (goto-char (point-min))
+        (forward-line 1)
+        (let ((start (point)))
+          (forward-line 2)
+          (narrow-to-region start (point))
+          (goto-char (point-min))
+          (forward-line 1)  ; Point at "Line 3"
+          (cj/clear-to-bottom-of-buffer)
+          (should (equal (buffer-string) "Line 2\n"))))
+    (test-clear-to-bottom-teardown)))
+
+(ert-deftest test-clear-to-bottom-multiple-windows ()
+  "Should update all windows showing buffer."
+  (test-clear-to-bottom-setup)
+  (unwind-protect
+      (with-temp-buffer
+        (insert "Line 1\nLine 2\nLine 3")
+        (goto-char (point-min))
+        (forward-line 1)
+        (cj/clear-to-bottom-of-buffer)
+        ;; Just verify content changed
+        (should (equal (buffer-string) "Line 1\n")))
+    (test-clear-to-bottom-teardown)))
+
+(ert-deftest test-clear-to-bottom-does-not-affect-kill-ring ()
+  "Should not add deleted text to kill ring."
+  (test-clear-to-bottom-setup)
+  (unwind-protect
+      (with-temp-buffer
+        (insert "Line 1\nLine 2\nLine 3")
+        (goto-char (point-min))
+        (setq kill-ring nil)
+        (cj/clear-to-bottom-of-buffer)
+        (should (null kill-ring)))
+    (test-clear-to-bottom-teardown)))
+
+;;; Error Cases
+
+(ert-deftest test-clear-to-bottom-read-only-buffer ()
+  "Should signal error in read-only buffer."
+  (test-clear-to-bottom-setup)
+  (unwind-protect
+      (with-temp-buffer
+        (insert "Read-only content")
+        (read-only-mode 1)
+        (goto-char (point-min))
+        (should-error (cj/clear-to-bottom-of-buffer)))
+    (test-clear-to-bottom-teardown)))
+
+(provide 'test-custom-file-buffer-clear-to-bottom-of-buffer)
+;;; test-custom-file-buffer-clear-to-bottom-of-buffer.el ends here