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+;;; test-custom-text-enclose-append.el --- Tests for cj/--append-to-lines -*- lexical-binding: t; -*-
+
+;;; Commentary:
+;; Tests for the cj/--append-to-lines function from custom-text-enclose.el
+;;
+;; This function appends a suffix string to the end of each line in text.
+;; It preserves the structure of lines and handles trailing newlines correctly.
+;;
+;; Examples:
+;; Input:  "line1\nline2", suffix: ";"
+;; Output: "line1;\nline2;"
+;;
+;; Input:  "single", suffix: "!"
+;; Output: "single!"
+;;
+;; We test the NON-INTERACTIVE implementation (cj/--append-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-text-enclose)
+
+;;; Test Helpers
+
+(defun test-append-to-lines (text suffix)
+  "Test cj/--append-to-lines on TEXT with SUFFIX.
+Returns the transformed string."
+  (cj/--append-to-lines text suffix))
+
+;;; Normal Cases - Single Line
+
+(ert-deftest test-append-single-line ()
+  "Should append to single line."
+  (let ((result (test-append-to-lines "hello" ";")))
+    (should (string= result "hello;"))))
+
+(ert-deftest test-append-single-line-semicolon ()
+  "Should append semicolon to single line."
+  (let ((result (test-append-to-lines "var x = 5" ";")))
+    (should (string= result "var x = 5;"))))
+
+(ert-deftest test-append-single-line-exclamation ()
+  "Should append exclamation mark to single line."
+  (let ((result (test-append-to-lines "Hello world" "!")))
+    (should (string= result "Hello world!"))))
+
+;;; Normal Cases - Multiple Lines
+
+(ert-deftest test-append-two-lines ()
+  "Should append to two lines."
+  (let ((result (test-append-to-lines "line1\nline2" ";")))
+    (should (string= result "line1;\nline2;"))))
+
+(ert-deftest test-append-three-lines ()
+  "Should append to three lines."
+  (let ((result (test-append-to-lines "a\nb\nc" ".")))
+    (should (string= result "a.\nb.\nc."))))
+
+(ert-deftest test-append-many-lines ()
+  "Should append to many lines."
+  (let* ((lines (make-list 10 "line"))
+         (input (mapconcat #'identity lines "\n"))
+         (result (test-append-to-lines input ";"))
+         (result-lines (split-string result "\n")))
+    (should (= 10 (length result-lines)))
+    (should (cl-every (lambda (line) (string-suffix-p ";" line)) result-lines))))
+
+;;; Normal Cases - Various Suffixes
+
+(ert-deftest test-append-comma ()
+  "Should append comma to lines."
+  (let ((result (test-append-to-lines "apple\nbanana" ",")))
+    (should (string= result "apple,\nbanana,"))))
+
+(ert-deftest test-append-multi-char ()
+  "Should append multi-character suffix."
+  (let ((result (test-append-to-lines "line" " // comment")))
+    (should (string= result "line // comment"))))
+
+(ert-deftest test-append-pipe ()
+  "Should append pipe character."
+  (let ((result (test-append-to-lines "col1\ncol2" " |")))
+    (should (string= result "col1 |\ncol2 |"))))
+
+(ert-deftest test-append-empty-suffix ()
+  "Should handle empty suffix."
+  (let ((result (test-append-to-lines "line1\nline2" "")))
+    (should (string= result "line1\nline2"))))
+
+;;; Boundary Cases - Trailing Newlines
+
+(ert-deftest test-append-with-trailing-newline ()
+  "Should preserve trailing newline."
+  (let ((result (test-append-to-lines "line1\nline2\n" ";")))
+    (should (string= result "line1;\nline2;\n"))))
+
+(ert-deftest test-append-no-trailing-newline ()
+  "Should work without trailing newline."
+  (let ((result (test-append-to-lines "line1\nline2" ";")))
+    (should (string= result "line1;\nline2;"))))
+
+(ert-deftest test-append-single-line-with-newline ()
+  "Should preserve trailing newline on single line."
+  (let ((result (test-append-to-lines "line\n" ";")))
+    (should (string= result "line;\n"))))
+
+;;; Boundary Cases - Empty Lines
+
+(ert-deftest test-append-empty-line-between ()
+  "Should append to empty line between other lines."
+  (let ((result (test-append-to-lines "line1\n\nline3" ";")))
+    (should (string= result "line1;\n;\nline3;"))))
+
+(ert-deftest test-append-only-empty-lines ()
+  "Should append to only empty lines."
+  (let ((result (test-append-to-lines "\n\n" ";")))
+    (should (string= result ";\n;\n"))))
+
+(ert-deftest test-append-empty-first-line ()
+  "Should append to empty first line."
+  (let ((result (test-append-to-lines "\nline2\nline3" ";")))
+    (should (string= result ";\nline2;\nline3;"))))
+
+;;; Boundary Cases - Whitespace
+
+(ert-deftest test-append-preserves-leading-whitespace ()
+  "Should preserve leading whitespace."
+  (let ((result (test-append-to-lines "  line1\n    line2" ";")))
+    (should (string= result "  line1;\n    line2;"))))
+
+(ert-deftest test-append-preserves-trailing-whitespace ()
+  "Should preserve trailing whitespace on line."
+  (let ((result (test-append-to-lines "line1  \nline2  " ";")))
+    (should (string= result "line1  ;\nline2  ;"))))
+
+(ert-deftest test-append-whitespace-only-line ()
+  "Should append to whitespace-only line."
+  (let ((result (test-append-to-lines "line1\n   \nline3" ";")))
+    (should (string= result "line1;\n   ;\nline3;"))))
+
+;;; Boundary Cases - Special Cases
+
+(ert-deftest test-append-empty-string ()
+  "Should handle empty string."
+  (let ((result (test-append-to-lines "" ";")))
+    (should (string= result ";"))))
+
+(ert-deftest test-append-very-long-line ()
+  "Should append to very long line."
+  (let* ((long-line (make-string 1000 ?a))
+         (result (test-append-to-lines long-line ";")))
+    (should (string-suffix-p ";" result))
+    (should (= (length result) 1001))))
+
+(ert-deftest test-append-with-existing-suffix ()
+  "Should append even if line already has the suffix."
+  (let ((result (test-append-to-lines "line;" ";")))
+    (should (string= result "line;;"))))
+
+;;; Edge Cases - Special Characters in Suffix
+
+(ert-deftest test-append-newline-suffix ()
+  "Should append newline as suffix."
+  (let ((result (test-append-to-lines "line1\nline2" "\n")))
+    (should (string= result "line1\n\nline2\n"))))
+
+(ert-deftest test-append-tab-suffix ()
+  "Should append tab as suffix."
+  (let ((result (test-append-to-lines "col1\ncol2" "\t")))
+    (should (string= result "col1\t\ncol2\t"))))
+
+(ert-deftest test-append-quote-suffix ()
+  "Should append quote as suffix."
+  (let ((result (test-append-to-lines "value1\nvalue2" "\"")))
+    (should (string= result "value1\"\nvalue2\""))))
+
+(provide 'test-custom-text-enclose-append)
+;;; test-custom-text-enclose-append.el ends here