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+;;; test-custom-ordering-comma-to-lines.el --- Tests for cj/--comma-separated-text-to-lines -*- lexical-binding: t; -*-
+
+;;; Commentary:
+;; Tests for the cj/--comma-separated-text-to-lines function from custom-ordering.el
+;;
+;; This function converts comma-separated text to separate lines.
+;; It replaces commas with newlines and removes trailing whitespace from each line.
+;;
+;; Examples:
+;; Input:  "apple, banana, cherry"
+;; Output: "apple\nbanana\ncherry"
+;;
+;; Input:  "one,two,three"
+;; Output: "one\ntwo\nthree"
+;;
+;; We test the NON-INTERACTIVE implementation (cj/--comma-separated-text-to-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-comma-to-lines (input-text)
+  "Test cj/--comma-separated-text-to-lines on INPUT-TEXT.
+Returns the transformed string."
+  (with-temp-buffer
+    (insert input-text)
+    (cj/--comma-separated-text-to-lines (point-min) (point-max))))
+
+;;; Normal Cases - Simple Comma-Separated
+
+(ert-deftest test-comma-to-lines-simple ()
+  "Should convert simple comma-separated text to lines."
+  (let ((result (test-comma-to-lines "apple, banana, cherry")))
+    (should (string= result "apple\n banana\n cherry"))))
+
+(ert-deftest test-comma-to-lines-no-spaces ()
+  "Should convert comma-separated text without spaces."
+  (let ((result (test-comma-to-lines "one,two,three")))
+    (should (string= result "one\ntwo\nthree"))))
+
+(ert-deftest test-comma-to-lines-two-elements ()
+  "Should convert two comma-separated elements."
+  (let ((result (test-comma-to-lines "hello,world")))
+    (should (string= result "hello\nworld"))))
+
+(ert-deftest test-comma-to-lines-with-varied-spacing ()
+  "Should preserve leading spaces after commas."
+  (let ((result (test-comma-to-lines "alpha,  beta,   gamma")))
+    (should (string= result "alpha\n  beta\n   gamma"))))
+
+;;; Normal Cases - Trailing Whitespace
+
+(ert-deftest test-comma-to-lines-trailing-spaces ()
+  "Should remove trailing spaces but preserve leading spaces."
+  (let ((result (test-comma-to-lines "apple  ,  banana  ,  cherry  ")))
+    (should (string= result "apple\n  banana\n  cherry"))))
+
+(ert-deftest test-comma-to-lines-trailing-tabs ()
+  "Should remove trailing tabs after conversion."
+  (let ((result (test-comma-to-lines "apple\t,banana\t,cherry\t")))
+    (should (string= result "apple\nbanana\ncherry"))))
+
+;;; Boundary Cases
+
+(ert-deftest test-comma-to-lines-empty-string ()
+  "Should handle empty string."
+  (let ((result (test-comma-to-lines "")))
+    (should (string= result ""))))
+
+(ert-deftest test-comma-to-lines-single-element ()
+  "Should handle single element with no comma."
+  (let ((result (test-comma-to-lines "hello")))
+    (should (string= result "hello"))))
+
+(ert-deftest test-comma-to-lines-single-element-with-trailing-comma ()
+  "Should handle single element with trailing comma."
+  (let ((result (test-comma-to-lines "hello,")))
+    (should (string= result "hello\n"))))
+
+(ert-deftest test-comma-to-lines-leading-comma ()
+  "Should handle leading comma."
+  (let ((result (test-comma-to-lines ",apple,banana")))
+    (should (string= result "\napple\nbanana"))))
+
+(ert-deftest test-comma-to-lines-consecutive-commas ()
+  "Should handle consecutive commas."
+  (let ((result (test-comma-to-lines "apple,,banana")))
+    (should (string= result "apple\n\nbanana"))))
+
+(ert-deftest test-comma-to-lines-many-consecutive-commas ()
+  "Should handle many consecutive commas."
+  (let ((result (test-comma-to-lines "apple,,,banana")))
+    (should (string= result "apple\n\n\nbanana"))))
+
+(ert-deftest test-comma-to-lines-only-commas ()
+  "Should handle string with only commas (trailing blank lines removed)."
+  (let ((result (test-comma-to-lines ",,,")))
+    ;; delete-trailing-whitespace removes trailing blank lines
+    (should (string= result "\n"))))
+
+;;; Normal Cases - With Spaces Around Elements
+
+(ert-deftest test-comma-to-lines-leading-spaces ()
+  "Should preserve leading spaces within elements."
+  (let ((result (test-comma-to-lines " apple,  banana,   cherry")))
+    (should (string= result " apple\n  banana\n   cherry"))))
+
+(ert-deftest test-comma-to-lines-mixed-content ()
+  "Should handle mixed alphanumeric content."
+  (let ((result (test-comma-to-lines "item1,item2,item3")))
+    (should (string= result "item1\nitem2\nitem3"))))
+
+(ert-deftest test-comma-to-lines-with-numbers ()
+  "Should handle numbers."
+  (let ((result (test-comma-to-lines "1,2,3,4,5")))
+    (should (string= result "1\n2\n3\n4\n5"))))
+
+(ert-deftest test-comma-to-lines-very-long-list ()
+  "Should handle very long list."
+  (let* ((elements (mapcar #'number-to-string (number-sequence 1 100)))
+         (input (mapconcat #'identity elements ","))
+         (result (test-comma-to-lines input))
+         (lines (split-string result "\n")))
+    (should (= 100 (length lines)))))
+
+;;; Error Cases
+
+(ert-deftest test-comma-to-lines-start-greater-than-end ()
+  "Should error when start > end."
+  (should-error
+   (with-temp-buffer
+     (insert "a,b,c")
+     (cj/--comma-separated-text-to-lines (point-max) (point-min)))
+   :type 'error))
+
+(ert-deftest test-comma-to-lines-empty-region ()
+  "Should handle empty region (start == end)."
+  (with-temp-buffer
+    (insert "a,b,c")
+    (let ((pos (/ (+ (point-min) (point-max)) 2)))
+      (should (string= "" (cj/--comma-separated-text-to-lines pos pos))))))
+
+(provide 'test-custom-ordering-comma-to-lines)
+;;; test-custom-ordering-comma-to-lines.el ends here