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+;;; test-custom-misc-format-region.el --- Tests for cj/--format-region -*- lexical-binding: t; -*-
+
+;;; Commentary:
+;; Tests for the cj/--format-region function from custom-misc.el
+;;
+;; This function reformats text by applying three operations:
+;; 1. untabify - converts tabs to spaces
+;; 2. indent-region - reindents according to major mode
+;; 3. delete-trailing-whitespace - removes trailing whitespace
+;;
+;; Note: indent-region behavior is major-mode dependent. We test in
+;; emacs-lisp-mode and fundamental-mode for predictable results.
+;;
+;; We test the NON-INTERACTIVE implementation (cj/--format-region)
+;; 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-misc)
+
+;;; Test Helpers
+
+(defun test-format-region (input-text &optional mode)
+  "Test cj/--format-region on INPUT-TEXT.
+MODE is the major mode to use (defaults to fundamental-mode).
+Returns the buffer string after operation."
+  (with-temp-buffer
+    (funcall (or mode #'fundamental-mode))
+    (insert input-text)
+    (cj/--format-region (point-min) (point-max))
+    (buffer-string)))
+
+;;; Normal Cases - Tab Conversion
+
+(ert-deftest test-format-region-converts-tabs ()
+  "Should convert tabs to spaces."
+  (let ((result (test-format-region "hello\tworld")))
+    (should-not (string-match-p "\t" result))
+    (should (string-match-p " " result))))
+
+(ert-deftest test-format-region-multiple-tabs ()
+  "Should convert multiple tabs."
+  (let ((result (test-format-region "\t\thello\t\tworld\t\t")))
+    (should-not (string-match-p "\t" result))))
+
+;;; Normal Cases - Trailing Whitespace
+
+(ert-deftest test-format-region-removes-trailing-spaces ()
+  "Should remove trailing spaces."
+  (let ((result (test-format-region "hello world   ")))
+    (should (string= result "hello world"))))
+
+(ert-deftest test-format-region-removes-trailing-tabs ()
+  "Should remove trailing tabs."
+  (let ((result (test-format-region "hello world\t\t")))
+    (should (string= result "hello world"))))
+
+(ert-deftest test-format-region-removes-trailing-mixed ()
+  "Should remove trailing mixed whitespace."
+  (let ((result (test-format-region "hello world \t \t ")))
+    (should (string= result "hello world"))))
+
+(ert-deftest test-format-region-multiline-trailing ()
+  "Should remove trailing whitespace from multiple lines."
+  (let ((result (test-format-region "line1   \nline2\t\t\nline3 \t ")))
+    (should (string= result "line1\nline2\nline3"))))
+
+;;; Normal Cases - Combined Operations
+
+(ert-deftest test-format-region-tabs-and-trailing ()
+  "Should handle both tabs and trailing whitespace."
+  (let ((result (test-format-region "\thello\tworld\t\t")))
+    (should-not (string-match-p "\t" result))
+    ;; Should not end with whitespace
+    (should-not (string-match-p "[ \t]+$" result))))
+
+(ert-deftest test-format-region-preserves-interior-spaces ()
+  "Should preserve interior spaces while fixing edges."
+  (let ((result (test-format-region "\thello   world\t")))
+    (should (string-match-p "hello   world" result))
+    (should-not (string-match-p "\t" result))))
+
+;;; Normal Cases - Indentation (Mode-Specific)
+
+(ert-deftest test-format-region-elisp-indentation ()
+  "Should reindent Elisp code."
+  (let* ((input "(defun foo ()\n(+ 1 2))")
+         (result (test-format-region input #'emacs-lisp-mode))
+         (lines (split-string result "\n")))
+    ;; The inner form should be indented - second line should start with 2 spaces
+    (should (= 2 (length lines)))
+    (should (string-prefix-p "(defun foo ()" (car lines)))
+    (should (string-prefix-p "  " (cadr lines)))))
+
+;;; Boundary Cases
+
+(ert-deftest test-format-region-empty-string ()
+  "Should handle empty string."
+  (let ((result (test-format-region "")))
+    (should (string= result ""))))
+
+(ert-deftest test-format-region-no-issues ()
+  "Should handle text with no formatting issues (no-op)."
+  (let ((result (test-format-region "hello world")))
+    (should (string= result "hello world"))))
+
+(ert-deftest test-format-region-only-whitespace ()
+  "Should handle text with only whitespace."
+  (let ((result (test-format-region "\t   \t   ")))
+    ;; Should become empty or just spaces, no tabs
+    (should-not (string-match-p "\t" result))))
+
+(ert-deftest test-format-region-single-line ()
+  "Should handle single line."
+  (let ((result (test-format-region "\thello\t")))
+    (should-not (string-match-p "\t" result))))
+
+(ert-deftest test-format-region-very-long-text ()
+  "Should handle very long text."
+  (let* ((long-text (mapconcat (lambda (_) "\thello\t") (make-list 100 nil) "\n"))
+         (result (test-format-region long-text)))
+    (should-not (string-match-p "\t" result))))
+
+(ert-deftest test-format-region-newlines-preserved ()
+  "Should preserve newlines while fixing formatting."
+  (let ((result (test-format-region "line1\t \nline2\t \nline3\t ")))
+    (should (= 2 (cl-count ?\n result)))))
+
+;;; Error Cases
+
+(ert-deftest test-format-region-start-greater-than-end ()
+  "Should error when start > end."
+  (should-error
+   (with-temp-buffer
+     (insert "hello world")
+     (cj/--format-region (point-max) (point-min)))
+   :type 'error))
+
+(ert-deftest test-format-region-empty-region ()
+  "Should handle empty region (start == end) without error."
+  (with-temp-buffer
+    (insert "hello world")
+    (let ((pos (/ (+ (point-min) (point-max)) 2)))
+      (cj/--format-region pos pos)
+      ;; Should complete without error
+      (should (string= (buffer-string) "hello world")))))
+
+(provide 'test-custom-misc-format-region)
+;;; test-custom-misc-format-region.el ends here