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+;;; test-custom-ordering-reverse-lines.el --- Tests for cj/--reverse-lines -*- lexical-binding: t; -*-
+
+;;; Commentary:
+;; Tests for the cj/--reverse-lines function from custom-ordering.el
+;;
+;; This function reverses the order of lines in a region.
+;; The first line becomes last, last becomes first, etc.
+;;
+;; Examples:
+;; Input:  "line1\nline2\nline3"
+;; Output: "line3\nline2\nline1"
+;;
+;; We test the NON-INTERACTIVE implementation (cj/--reverse-lines) to avoid
+;; mocking region selection. This follows our testing best practice of
+;; separating business logic from UI interaction.
+
+;;; 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.")
+
+;; Now load the actual production module
+(require 'custom-ordering)
+
+;;; Test Helpers
+
+(defun test-reverse-lines (input-text)
+  "Test cj/--reverse-lines on INPUT-TEXT.
+Returns the transformed string."
+  (with-temp-buffer
+    (insert input-text)
+    (cj/--reverse-lines (point-min) (point-max))))
+
+;;; Normal Cases
+
+(ert-deftest test-reverse-lines-three-lines ()
+  "Should reverse three lines."
+  (let ((result (test-reverse-lines "line1\nline2\nline3")))
+    (should (string= result "line3\nline2\nline1"))))
+
+(ert-deftest test-reverse-lines-two-lines ()
+  "Should reverse two lines."
+  (let ((result (test-reverse-lines "first\nsecond")))
+    (should (string= result "second\nfirst"))))
+
+(ert-deftest test-reverse-lines-many-lines ()
+  "Should reverse many lines."
+  (let ((result (test-reverse-lines "a\nb\nc\nd\ne")))
+    (should (string= result "e\nd\nc\nb\na"))))
+
+(ert-deftest test-reverse-lines-with-content ()
+  "Should reverse lines with actual content."
+  (let ((result (test-reverse-lines "apple banana\ncherry date\negg fig")))
+    (should (string= result "egg fig\ncherry date\napple banana"))))
+
+(ert-deftest test-reverse-lines-bidirectional ()
+  "Should reverse back and forth correctly."
+  (let* ((original "line1\nline2\nline3")
+         (reversed (test-reverse-lines original))
+         (back (test-reverse-lines reversed)))
+    (should (string= reversed "line3\nline2\nline1"))
+    (should (string= back original))))
+
+;;; Boundary Cases
+
+(ert-deftest test-reverse-lines-empty-string ()
+  "Should handle empty string."
+  (let ((result (test-reverse-lines "")))
+    (should (string= result ""))))
+
+(ert-deftest test-reverse-lines-single-line ()
+  "Should handle single line (no change)."
+  (let ((result (test-reverse-lines "single line")))
+    (should (string= result "single line"))))
+
+(ert-deftest test-reverse-lines-empty-lines ()
+  "Should reverse including empty lines."
+  (let ((result (test-reverse-lines "a\n\nb")))
+    (should (string= result "b\n\na"))))
+
+(ert-deftest test-reverse-lines-trailing-newline ()
+  "Should handle trailing newline."
+  (let ((result (test-reverse-lines "line1\nline2\n")))
+    (should (string= result "\nline2\nline1"))))
+
+(ert-deftest test-reverse-lines-only-newlines ()
+  "Should reverse lines that are only newlines."
+  (let ((result (test-reverse-lines "\n\n\n")))
+    (should (string= result "\n\n\n"))))
+
+(ert-deftest test-reverse-lines-numbers ()
+  "Should reverse numbered lines."
+  (let ((result (test-reverse-lines "1\n2\n3\n4\n5")))
+    (should (string= result "5\n4\n3\n2\n1"))))
+
+(ert-deftest test-reverse-lines-very-long ()
+  "Should reverse very long list."
+  (let* ((lines (mapcar #'number-to-string (number-sequence 1 100)))
+         (input (mapconcat #'identity lines "\n"))
+         (result (test-reverse-lines input))
+         (result-lines (split-string result "\n")))
+    (should (= 100 (length result-lines)))
+    (should (string= "100" (car result-lines)))
+    (should (string= "1" (car (last result-lines))))))
+
+;;; Error Cases
+
+(ert-deftest test-reverse-lines-start-greater-than-end ()
+  "Should error when start > end."
+  (should-error
+   (with-temp-buffer
+     (insert "line1\nline2")
+     (cj/--reverse-lines (point-max) (point-min)))
+   :type 'error))
+
+(ert-deftest test-reverse-lines-empty-region ()
+  "Should handle empty region (start == end)."
+  (with-temp-buffer
+    (insert "line1\nline2")
+    (let ((pos (/ (+ (point-min) (point-max)) 2)))
+      (should (string= "" (cj/--reverse-lines pos pos))))))
+
+(provide 'test-custom-ordering-reverse-lines)
+;;; test-custom-ordering-reverse-lines.el ends here