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+;;; test-custom-whitespace-delete-blank-lines.el --- Tests for cj/--delete-blank-lines -*- lexical-binding: t; -*-
+
+;;; Commentary:
+;; Tests for the cj/--delete-blank-lines function from custom-whitespace.el
+;;
+;; This function deletes blank lines from text, where blank lines are defined
+;; as lines containing only whitespace (spaces, tabs) or nothing at all.
+;; Uses the regexp ^[[:space:]]*$ to match blank lines.
+;;
+;; We test the NON-INTERACTIVE implementation (cj/--delete-blank-lines)
+;; to avoid mocking user prompts. 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-whitespace)
+
+;;; Test Helpers
+
+(defun test-delete-blank-lines (input-text)
+  "Test cj/--delete-blank-lines on INPUT-TEXT.
+Returns the buffer string after operation."
+  (with-temp-buffer
+    (insert input-text)
+    (cj/--delete-blank-lines (point-min) (point-max))
+    (buffer-string)))
+
+;;; Normal Cases
+
+(ert-deftest test-delete-blank-lines-single-blank ()
+  "Should delete single blank line between text."
+  (let ((result (test-delete-blank-lines "line1\n\nline2")))
+    (should (string= result "line1\nline2"))))
+
+(ert-deftest test-delete-blank-lines-multiple-consecutive ()
+  "Should delete multiple consecutive blank lines."
+  (let ((result (test-delete-blank-lines "line1\n\n\n\nline2")))
+    (should (string= result "line1\nline2"))))
+
+(ert-deftest test-delete-blank-lines-spaces-only ()
+  "Should delete lines with spaces only."
+  (let ((result (test-delete-blank-lines "line1\n   \nline2")))
+    (should (string= result "line1\nline2"))))
+
+(ert-deftest test-delete-blank-lines-tabs-only ()
+  "Should delete lines with tabs only."
+  (let ((result (test-delete-blank-lines "line1\n\t\t\nline2")))
+    (should (string= result "line1\nline2"))))
+
+(ert-deftest test-delete-blank-lines-mixed-whitespace ()
+  "Should delete lines with mixed whitespace."
+  (let ((result (test-delete-blank-lines "line1\n \t \t \nline2")))
+    (should (string= result "line1\nline2"))))
+
+(ert-deftest test-delete-blank-lines-no-blank-lines ()
+  "Should handle text with no blank lines (no-op)."
+  (let ((result (test-delete-blank-lines "line1\nline2\nline3")))
+    (should (string= result "line1\nline2\nline3"))))
+
+(ert-deftest test-delete-blank-lines-at-start ()
+  "Should delete blank lines at start of region."
+  (let ((result (test-delete-blank-lines "\n\nline1\nline2")))
+    (should (string= result "line1\nline2"))))
+
+(ert-deftest test-delete-blank-lines-at-end ()
+  "Should delete blank lines at end of region."
+  (let ((result (test-delete-blank-lines "line1\nline2\n\n")))
+    (should (string= result "line1\nline2\n"))))
+
+(ert-deftest test-delete-blank-lines-scattered ()
+  "Should delete blank lines scattered throughout text."
+  (let ((result (test-delete-blank-lines "line1\n\nline2\n   \nline3\n\t\nline4")))
+    (should (string= result "line1\nline2\nline3\nline4"))))
+
+;;; Boundary Cases
+
+(ert-deftest test-delete-blank-lines-empty-string ()
+  "Should handle empty string."
+  (let ((result (test-delete-blank-lines "")))
+    (should (string= result ""))))
+
+(ert-deftest test-delete-blank-lines-only-blank-lines ()
+  "Should delete all lines if only blank lines exist."
+  (let ((result (test-delete-blank-lines "\n\n\n")))
+    (should (string= result ""))))
+
+(ert-deftest test-delete-blank-lines-only-whitespace ()
+  "Should delete lines containing only whitespace."
+  (let ((result (test-delete-blank-lines "   \n\t\t\n \t ")))
+    (should (string= result ""))))
+
+(ert-deftest test-delete-blank-lines-single-line-content ()
+  "Should handle single line with content (no-op)."
+  (let ((result (test-delete-blank-lines "hello world")))
+    (should (string= result "hello world"))))
+
+(ert-deftest test-delete-blank-lines-single-blank-line ()
+  "Should delete single blank line."
+  (let ((result (test-delete-blank-lines "\n")))
+    (should (string= result ""))))
+
+(ert-deftest test-delete-blank-lines-very-long-region ()
+  "Should handle very long region with many blank lines."
+  (let* ((lines (make-list 100 "content"))
+         (input (mapconcat #'identity lines "\n\n"))
+         (expected (mapconcat #'identity lines "\n"))
+         (result (test-delete-blank-lines input)))
+    (should (string= result expected))))
+
+(ert-deftest test-delete-blank-lines-preserve-content-lines ()
+  "Should preserve lines with any non-whitespace content."
+  (let ((result (test-delete-blank-lines "x\n\ny\n   \nz")))
+    (should (string= result "x\ny\nz"))))
+
+;;; Error Cases
+
+(ert-deftest test-delete-blank-lines-start-greater-than-end ()
+  "Should error when start > end."
+  (should-error
+   (with-temp-buffer
+     (insert "line1\n\nline2")
+     (cj/--delete-blank-lines (point-max) (point-min)))
+   :type 'error))
+
+(ert-deftest test-delete-blank-lines-empty-region ()
+  "Should handle empty region (start == end) without error."
+  (with-temp-buffer
+    (insert "line1\n\nline2")
+    (let ((pos (/ (+ (point-min) (point-max)) 2)))
+      (cj/--delete-blank-lines pos pos)
+      ;; Should complete without error
+      (should (string-match-p "line1" (buffer-string))))))
+
+(provide 'test-custom-whitespace-delete-blank-lines)
+;;; test-custom-whitespace-delete-blank-lines.el ends here