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+;;; test-custom-case-downcase-dwim.el --- Tests for cj/downcase-dwim -*- lexical-binding: t; -*-
+
+;;; Commentary:
+;; Tests for the cj/downcase-dwim function from custom-case.el.
+;;
+;; Mirror of upcase-dwim: if a region is active, downcase the region;
+;; otherwise downcase the symbol at point. Signals user-error if no symbol.
+;; Tests focus on the dispatch logic, not on Emacs's downcase-region.
+
+;;; Code:
+
+(require 'ert)
+
+(unless (boundp 'cj/custom-keymap)
+  (defvar cj/custom-keymap (make-sparse-keymap)))
+
+(add-to-list 'load-path (expand-file-name "modules" user-emacs-directory))
+(require 'custom-case)
+
+;;; Normal Cases
+
+(ert-deftest test-custom-case-downcase-dwim-normal-region-downcased ()
+  "Active region should be downcased."
+  (with-temp-buffer
+    (insert "HELLO WORLD")
+    (set-mark (point-min))
+    (goto-char (point-max))
+    (activate-mark)
+    (cj/downcase-dwim)
+    (should (equal (buffer-string) "hello world"))))
+
+(ert-deftest test-custom-case-downcase-dwim-normal-partial-region ()
+  "Only the selected region should be downcased."
+  (with-temp-buffer
+    (insert "HELLO WORLD")
+    (set-mark 1)
+    (goto-char 6)
+    (activate-mark)
+    (cj/downcase-dwim)
+    (should (equal (buffer-string) "hello WORLD"))))
+
+(ert-deftest test-custom-case-downcase-dwim-normal-symbol-at-point ()
+  "Without region, symbol at point should be downcased."
+  (with-temp-buffer
+    (insert "HELLO WORLD")
+    (goto-char 1)
+    (cj/downcase-dwim)
+    (should (equal (buffer-string) "hello WORLD"))))
+
+(ert-deftest test-custom-case-downcase-dwim-normal-symbol-mid-word ()
+  "Point in the middle of a symbol should downcase the whole symbol."
+  (with-temp-buffer
+    (insert "HELLO WORLD")
+    (goto-char 3)
+    (cj/downcase-dwim)
+    (should (equal (buffer-string) "hello WORLD"))))
+
+(ert-deftest test-custom-case-downcase-dwim-normal-already-lowercase ()
+  "Already lowercase text should remain unchanged."
+  (with-temp-buffer
+    (insert "hello WORLD")
+    (goto-char 1)
+    (cj/downcase-dwim)
+    (should (equal (buffer-string) "hello WORLD"))))
+
+;;; Boundary Cases
+
+(ert-deftest test-custom-case-downcase-dwim-boundary-single-char-symbol ()
+  "Single character symbol should be downcased."
+  (with-temp-buffer
+    (insert "A")
+    (goto-char 1)
+    (cj/downcase-dwim)
+    (should (equal (buffer-string) "a"))))
+
+;;; Error Cases
+
+(ert-deftest test-custom-case-downcase-dwim-error-no-symbol-signals-error ()
+  "With no region and no symbol at point, should signal user-error."
+  (with-temp-buffer
+    (insert "   ")
+    (goto-char 2)
+    (should-error (cj/downcase-dwim) :type 'user-error)))
+
+(provide 'test-custom-case-downcase-dwim)
+;;; test-custom-case-downcase-dwim.el ends here