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+;;; test-custom-line-paragraph-underscore-line.el --- Tests for cj/underscore-line -*- lexical-binding: t; -*-
+
+;;; Commentary:
+;; Tests for the cj/underscore-line function from custom-line-paragraph.el
+;;
+;; This function underlines the current line by inserting a row of characters below it.
+;; If the line is empty or contains only whitespace, it aborts with a message.
+;;
+;; The function uses (read-char) to get the underline character from the user.
+;; In tests, we mock this using cl-letf.
+
+;;; 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 expand-region package
+(provide 'expand-region)
+
+;; Now load the actual production module
+(require 'custom-line-paragraph)
+
+;;; Setup and Teardown
+
+(defun test-underscore-line-setup ()
+  "Setup for underscore-line tests."
+  (cj/create-test-base-dir))
+
+(defun test-underscore-line-teardown ()
+  "Teardown for underscore-line tests."
+  (cj/delete-test-base-dir))
+
+;;; Normal Cases
+
+(ert-deftest test-underscore-line-simple-text ()
+  "Should underline simple text line."
+  (test-underscore-line-setup)
+  (unwind-protect
+      (with-temp-buffer
+        (insert "Hello World")
+        (goto-char (point-min))
+        (cl-letf (((symbol-function 'read-char) (lambda (&rest _) ?-)))
+          (cj/underscore-line)
+          (should (string-match-p "Hello World\n-----------" (buffer-string)))))
+    (test-underscore-line-teardown)))
+
+(ert-deftest test-underscore-line-preserve-original ()
+  "Should preserve original line text."
+  (test-underscore-line-setup)
+  (unwind-protect
+      (with-temp-buffer
+        (insert "Original Text")
+        (goto-char (point-min))
+        (cl-letf (((symbol-function 'read-char) (lambda (&rest _) ?=)))
+          (cj/underscore-line)
+          (goto-char (point-min))
+          (should (looking-at "Original Text"))))
+    (test-underscore-line-teardown)))
+
+(ert-deftest test-underscore-line-use-specified-character ()
+  "Should use the character provided by user."
+  (test-underscore-line-setup)
+  (unwind-protect
+      (with-temp-buffer
+        (insert "Test")
+        (goto-char (point-min))
+        (cl-letf (((symbol-function 'read-char) (lambda (&rest _) ?*)))
+          (cj/underscore-line)
+          (should (string-match-p "\\*\\*\\*\\*" (buffer-string)))))
+    (test-underscore-line-teardown)))
+
+(ert-deftest test-underscore-line-match-width ()
+  "Should create underline matching line width."
+  (test-underscore-line-setup)
+  (unwind-protect
+      (with-temp-buffer
+        (insert "12345")
+        (goto-char (point-min))
+        (cl-letf (((symbol-function 'read-char) (lambda (&rest _) ?-)))
+          (cj/underscore-line)
+          ;; Should have exactly 5 dashes
+          (should (string-match-p "12345\n-----$" (buffer-string)))))
+    (test-underscore-line-teardown)))
+
+(ert-deftest test-underscore-line-insert-newline ()
+  "Should insert newline before underline."
+  (test-underscore-line-setup)
+  (unwind-protect
+      (with-temp-buffer
+        (insert "Line")
+        (goto-char (point-min))
+        (cl-letf (((symbol-function 'read-char) (lambda (&rest _) ?-)))
+          (cj/underscore-line)
+          (should (= 2 (count-lines (point-min) (point-max))))))
+    (test-underscore-line-teardown)))
+
+(ert-deftest test-underscore-line-cursor-preserved ()
+  "Should preserve cursor position."
+  (test-underscore-line-setup)
+  (unwind-protect
+      (with-temp-buffer
+        (insert "Some text here")
+        (goto-char (point-min))
+        (forward-char 5)  ; Position in middle
+        (let ((original-pos (point)))
+          (cl-letf (((symbol-function 'read-char) (lambda (&rest _) ?-)))
+            (cj/underscore-line)
+            (should (= (point) original-pos)))))
+    (test-underscore-line-teardown)))
+
+;;; Boundary Cases
+
+(ert-deftest test-underscore-line-empty-line-aborts ()
+  "Should abort on empty line."
+  (test-underscore-line-setup)
+  (unwind-protect
+      (with-temp-buffer
+        (insert "")
+        (cj/underscore-line)
+        ;; Buffer should remain empty
+        (should (string= "" (buffer-string))))
+    (test-underscore-line-teardown)))
+
+(ert-deftest test-underscore-line-whitespace-only-aborts ()
+  "Should abort on whitespace-only line."
+  (test-underscore-line-setup)
+  (unwind-protect
+      (with-temp-buffer
+        (insert "   \t  ")
+        (goto-char (point-min))
+        (let ((original (buffer-string)))
+          (cj/underscore-line)
+          ;; Buffer should be unchanged
+          (should (string= original (buffer-string)))))
+    (test-underscore-line-teardown)))
+
+(ert-deftest test-underscore-line-single-character ()
+  "Should underline single character line."
+  (test-underscore-line-setup)
+  (unwind-protect
+      (with-temp-buffer
+        (insert "X")
+        (goto-char (point-min))
+        (cl-letf (((symbol-function 'read-char) (lambda (&rest _) ?-)))
+          (cj/underscore-line)
+          (should (string= "X\n-" (buffer-string)))))
+    (test-underscore-line-teardown)))
+
+(ert-deftest test-underscore-line-very-long-line ()
+  "Should handle very long lines (5000+ chars)."
+  (test-underscore-line-setup)
+  (unwind-protect
+      (with-temp-buffer
+        (let ((long-line (make-string 5000 ?x)))
+          (insert long-line)
+          (goto-char (point-min))
+          (cl-letf (((symbol-function 'read-char) (lambda (&rest _) ?-)))
+            (cj/underscore-line)
+            ;; Should have 5000 dashes
+            (should (= 5000 (how-many "-" (point-min) (point-max)))))))
+    (test-underscore-line-teardown)))
+
+(ert-deftest test-underscore-line-with-tabs ()
+  "Should account for tab expansion in column width."
+  (test-underscore-line-setup)
+  (unwind-protect
+      (with-temp-buffer
+        (insert "a\tb")
+        (goto-char (point-min))
+        (cl-letf (((symbol-function 'read-char) (lambda (&rest _) ?-)))
+          (cj/underscore-line)
+          ;; Tab expands to column, underline should match visual width
+          (let ((underline-length (save-excursion
+                                    (goto-char (point-min))
+                                    (forward-line 1)
+                                    (- (line-end-position) (line-beginning-position)))))
+            (should (> underline-length 2)))))  ; More than just "a" and "b"
+    (test-underscore-line-teardown)))
+
+(ert-deftest test-underscore-line-leading-whitespace ()
+  "Should include leading whitespace in width calculation."
+  (test-underscore-line-setup)
+  (unwind-protect
+      (with-temp-buffer
+        (insert "  text")
+        (goto-char (point-min))
+        (cl-letf (((symbol-function 'read-char) (lambda (&rest _) ?-)))
+          (cj/underscore-line)
+          ;; Should have 6 dashes (2 spaces + 4 chars)
+          (should (string= "  text\n------" (buffer-string)))))
+    (test-underscore-line-teardown)))
+
+(ert-deftest test-underscore-line-trailing-whitespace ()
+  "Should include trailing whitespace in width calculation."
+  (test-underscore-line-setup)
+  (unwind-protect
+      (with-temp-buffer
+        (insert "text  ")
+        (goto-char (point-min))
+        (cl-letf (((symbol-function 'read-char) (lambda (&rest _) ?-)))
+          (cj/underscore-line)
+          ;; Should have 6 dashes (4 chars + 2 spaces)
+          (should (string= "text  \n------" (buffer-string)))))
+    (test-underscore-line-teardown)))
+
+(ert-deftest test-underscore-line-unicode-emoji ()
+  "Should handle Unicode and emoji characters."
+  (test-underscore-line-setup)
+  (unwind-protect
+      (with-temp-buffer
+        (insert "Hello 👋")
+        (goto-char (point-min))
+        (cl-letf (((symbol-function 'read-char) (lambda (&rest _) ?-)))
+          (cj/underscore-line)
+          ;; Should create underline
+          (should (string-match-p "Hello 👋\n-" (buffer-string)))))
+    (test-underscore-line-teardown)))
+
+(ert-deftest test-underscore-line-rtl-text ()
+  "Should handle RTL text."
+  (test-underscore-line-setup)
+  (unwind-protect
+      (with-temp-buffer
+        (insert "مرحبا")
+        (goto-char (point-min))
+        (cl-letf (((symbol-function 'read-char) (lambda (&rest _) ?-)))
+          (cj/underscore-line)
+          (should (string-match-p "مرحبا\n-" (buffer-string)))))
+    (test-underscore-line-teardown)))
+
+(ert-deftest test-underscore-line-combining-characters ()
+  "Should handle Unicode combining characters."
+  (test-underscore-line-setup)
+  (unwind-protect
+      (with-temp-buffer
+        (insert "cafe\u0301")  ; e with combining acute
+        (goto-char (point-min))
+        (cl-letf (((symbol-function 'read-char) (lambda (&rest _) ?-)))
+          (cj/underscore-line)
+          (should (string-match-p "cafe\u0301\n-" (buffer-string)))))
+    (test-underscore-line-teardown)))
+
+(ert-deftest test-underscore-line-at-buffer-start ()
+  "Should work on first line in buffer."
+  (test-underscore-line-setup)
+  (unwind-protect
+      (with-temp-buffer
+        (insert "First line\nSecond line")
+        (goto-char (point-min))
+        (cl-letf (((symbol-function 'read-char) (lambda (&rest _) ?=)))
+          (cj/underscore-line)
+          (should (string-match-p "First line\n==========" (buffer-string)))))
+    (test-underscore-line-teardown)))
+
+(ert-deftest test-underscore-line-at-buffer-end ()
+  "Should work on last line in buffer."
+  (test-underscore-line-setup)
+  (unwind-protect
+      (with-temp-buffer
+        (insert "First line\nLast line")
+        (goto-char (point-max))
+        (cl-letf (((symbol-function 'read-char) (lambda (&rest _) ?=)))
+          (cj/underscore-line)
+          (should (string-match-p "Last line\n=========$" (buffer-string)))))
+    (test-underscore-line-teardown)))
+
+(ert-deftest test-underscore-line-different-characters ()
+  "Should work with various underline characters."
+  (test-underscore-line-setup)
+  (unwind-protect
+      (with-temp-buffer
+        (insert "Test")
+        (goto-char (point-min))
+        (dolist (char '(?- ?= ?* ?# ?~ ?_))
+          (goto-char (point-min))
+          (delete-region (point-min) (point-max))
+          (insert "Test")
+          (goto-char (point-min))
+          (cl-letf (((symbol-function 'read-char) (lambda (&rest _) char)))
+            (cj/underscore-line)
+            (should (string-match-p (regexp-quote (make-string 4 char)) (buffer-string))))))
+    (test-underscore-line-teardown)))
+
+(ert-deftest test-underscore-line-special-characters ()
+  "Should work with special non-alphanumeric characters."
+  (test-underscore-line-setup)
+  (unwind-protect
+      (with-temp-buffer
+        (insert "Text")
+        (goto-char (point-min))
+        (cl-letf (((symbol-function 'read-char) (lambda (&rest _) ?@)))
+          (cj/underscore-line)
+          (should (string-match-p "@@@@" (buffer-string)))))
+    (test-underscore-line-teardown)))
+
+(ert-deftest test-underscore-line-cursor-in-middle ()
+  "Should work regardless of cursor position on line."
+  (test-underscore-line-setup)
+  (unwind-protect
+      (with-temp-buffer
+        (insert "Hello World")
+        (goto-char (point-min))
+        (forward-char 6)  ; Position after "Hello "
+        (cl-letf (((symbol-function 'read-char) (lambda (&rest _) ?-)))
+          (cj/underscore-line)
+          (should (string-match-p "Hello World\n-----------" (buffer-string)))))
+    (test-underscore-line-teardown)))
+
+(ert-deftest test-underscore-line-cursor-at-start ()
+  "Should work when cursor at line beginning."
+  (test-underscore-line-setup)
+  (unwind-protect
+      (with-temp-buffer
+        (insert "Text")
+        (goto-char (point-min))
+        (cl-letf (((symbol-function 'read-char) (lambda (&rest _) ?-)))
+          (cj/underscore-line)
+          (should (string-match-p "Text\n----" (buffer-string)))))
+    (test-underscore-line-teardown)))
+
+(ert-deftest test-underscore-line-cursor-at-end ()
+  "Should work when cursor at line end."
+  (test-underscore-line-setup)
+  (unwind-protect
+      (with-temp-buffer
+        (insert "Text")
+        (goto-char (point-max))
+        (cl-letf (((symbol-function 'read-char) (lambda (&rest _) ?-)))
+          (cj/underscore-line)
+          (should (string-match-p "Text\n----" (buffer-string)))))
+    (test-underscore-line-teardown)))
+
+;;; Error Cases
+
+(ert-deftest test-underscore-line-read-only-buffer ()
+  "Should error when attempting to modify read-only buffer."
+  (test-underscore-line-setup)
+  (unwind-protect
+      (with-temp-buffer
+        (insert "Read only text")
+        (goto-char (point-min))
+        (read-only-mode 1)
+        (cl-letf (((symbol-function 'read-char) (lambda (&rest _) ?-)))
+          (should-error (cj/underscore-line))))
+    (test-underscore-line-teardown)))
+
+(ert-deftest test-underscore-line-buffer-modified-flag ()
+  "Should set buffer modified flag."
+  (test-underscore-line-setup)
+  (unwind-protect
+      (with-temp-buffer
+        (insert "Text")
+        (set-buffer-modified-p nil)
+        (goto-char (point-min))
+        (cl-letf (((symbol-function 'read-char) (lambda (&rest _) ?-)))
+          (cj/underscore-line)
+          (should (buffer-modified-p))))
+    (test-underscore-line-teardown)))
+
+(ert-deftest test-underscore-line-undo-behavior ()
+  "Should support undo after underlining."
+  (test-underscore-line-setup)
+  (unwind-protect
+      (let* ((temp-file (expand-file-name "test-undo-underline.txt" cj/test-base-dir))
+             (original-content "Test line"))
+        ;; Create file with initial content
+        (with-temp-file temp-file
+          (insert original-content))
+        ;; Open file and test undo
+        (find-file temp-file)
+        (buffer-enable-undo)
+        ;; Establish undo history
+        (goto-char (point-min))
+        (insert " ")
+        (delete-char -1)
+        (undo-boundary)
+        (goto-char (point-min))
+        (let ((before-underline (buffer-string)))
+          (cl-letf (((symbol-function 'read-char) (lambda (&rest _) ?-)))
+            (cj/underscore-line))
+          (undo-boundary)
+          (let ((after-underline (buffer-string)))
+            (should-not (string= before-underline after-underline))
+            (undo)
+            (should (string= before-underline (buffer-string)))))
+        (kill-buffer (current-buffer)))
+    (test-underscore-line-teardown)))
+
+(provide 'test-custom-line-paragraph-underscore-line)
+;;; test-custom-line-paragraph-underscore-line.el ends here