test: Add unit tests for `join-line-or-region` and `join-paragraph`

Add comprehensive test cases for the `cj/join-line-or-region` and
`cj/join-paragraph` functions within the custom-line-paragraph
module. These tests cover normal, boundary, and error cases to
ensure the proper functionality of line and paragraph joining
features. They address various scenarios including line breaks,
whitespace, Unicode content, and cursor positioning, enhancing the
robustness of existing functionality.

2 files changed, 787 insertions, 0 deletions

diff --git a/tests/test-custom-line-paragraph-join-line-or-region.el b/tests/test-custom-line-paragraph-join-line-or-region.el
new file mode 100644
index 00000000..674d4618
--- /dev/null
+++ b/tests/test-custom-line-paragraph-join-line-or-region.el
@@ -0,0 +1,427 @@
+;;; test-custom-line-paragraph-join-line-or-region.el --- Tests for cj/join-line-or-region -*- lexical-binding: t; -*-
+
+;;; Commentary:
+;; Tests for the cj/join-line-or-region function from custom-line-paragraph.el
+;;
+;; IMPORTANT NOTE ON REGION ACTIVATION IN BATCH MODE:
+;; When testing functions that use (use-region-p) in batch mode, you must
+;; explicitly activate the region. Unlike interactive Emacs, batch mode does
+;; not automatically activate regions when you set mark and point.
+;;
+;; To properly test region-based behavior in batch mode:
+;; 1. Enable transient-mark-mode: (transient-mark-mode 1)
+;; 2. Set mark and point as needed
+;; 3. Explicitly activate the mark: (activate-mark)
+;;
+;; Without these steps, (use-region-p) will return nil even when mark and
+;; point are set, causing the function to take the no-region code path.
+;; This is a common pitfall that junior developers may miss when writing
+;; ERT tests for region-aware commands.
+
+;;; 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.")
+
+;; Stub expand-region package
+(provide 'expand-region)
+
+;; Now load the actual production module
+(require 'custom-line-paragraph)
+
+;;; Setup and Teardown
+
+(defun test-join-line-or-region-setup ()
+  "Setup for join-line-or-region tests."
+  (cj/create-test-base-dir))
+
+(defun test-join-line-or-region-teardown ()
+  "Teardown for join-line-or-region tests."
+  (cj/delete-test-base-dir))
+
+;;; Normal Cases
+
+(ert-deftest test-join-line-or-region-no-region-joins-with-previous-line ()
+  "Without region, should join current line with previous line."
+  (test-join-line-or-region-setup)
+  (unwind-protect
+      (with-temp-buffer
+        (insert "line one\nline two")
+        (goto-char (point-max))
+        (cj/join-line-or-region)
+        (should (string-match-p "line one line two" (buffer-string))))
+    (test-join-line-or-region-teardown)))
+
+(ert-deftest test-join-line-or-region-no-region-adds-newline-after-join ()
+  "Without region, should add newline after joining."
+  (test-join-line-or-region-setup)
+  (unwind-protect
+      (with-temp-buffer
+        (insert "line one\nline two")
+        (goto-char (point-max))
+        (cj/join-line-or-region)
+        (should (string-suffix-p "\n" (buffer-string))))
+    (test-join-line-or-region-teardown)))
+
+(ert-deftest test-join-line-or-region-with-region-joins-all-lines ()
+  "With region, should join all lines in region."
+  (test-join-line-or-region-setup)
+  (unwind-protect
+      (with-temp-buffer
+        (insert "line one\nline two\nline three")
+        (goto-char (point-min))
+        (set-mark (point))
+        (goto-char (point-max))
+        (activate-mark)
+        (cj/join-line-or-region)
+        (should (string-match-p "line one line two line three" (buffer-string))))
+    (test-join-line-or-region-teardown)))
+
+(ert-deftest test-join-line-or-region-with-region-adds-newline-at-end ()
+  "With region, should add newline at end."
+  (test-join-line-or-region-setup)
+  (unwind-protect
+      (with-temp-buffer
+        (insert "line one\nline two\nline three")
+        (goto-char (point-min))
+        (set-mark (point))
+        (goto-char (point-max))
+        (activate-mark)
+        (cj/join-line-or-region)
+        (should (string-suffix-p "\n" (buffer-string))))
+    (test-join-line-or-region-teardown)))
+
+(ert-deftest test-join-line-or-region-preserves-text-content ()
+  "Should preserve all text content when joining."
+  (test-join-line-or-region-setup)
+  (unwind-protect
+      (with-temp-buffer
+        (insert "hello\nworld")
+        (goto-char (point-min))
+        (set-mark (point))
+        (goto-char (point-max))
+        (activate-mark)
+        (cj/join-line-or-region)
+        (should (string-match-p "hello.*world" (buffer-string))))
+    (test-join-line-or-region-teardown)))
+
+(ert-deftest test-join-line-or-region-removes-line-breaks-between-words ()
+  "Should remove line breaks and add spaces between words."
+  (test-join-line-or-region-setup)
+  (unwind-protect
+      (with-temp-buffer
+        (insert "hello\nworld")
+        (goto-char (point-max))
+        (cj/join-line-or-region)
+        (should (string-match-p "hello world" (buffer-string))))
+    (test-join-line-or-region-teardown)))
+
+(ert-deftest test-join-line-or-region-multiple-lines-in-region ()
+  "Should handle multiple lines in region correctly."
+  (test-join-line-or-region-setup)
+  (unwind-protect
+      (with-temp-buffer
+        (insert "one\ntwo\nthree\nfour\nfive")
+        (goto-char (point-min))
+        (forward-line 1)
+        (set-mark (point))
+        (forward-line 3)
+        (activate-mark)
+        (cj/join-line-or-region)
+        (should (string-match-p "two three four" (buffer-string))))
+    (test-join-line-or-region-teardown)))
+
+;;; Boundary Cases
+
+(ert-deftest test-join-line-or-region-on-first-line-no-region-does-nothing-except-newline ()
+  "On first line without region, should only add newline."
+  (test-join-line-or-region-setup)
+  (unwind-protect
+      (with-temp-buffer
+        (insert "only line")
+        (goto-char (point-min))
+        (cj/join-line-or-region)
+        (should (string= "only line\n" (buffer-string))))
+    (test-join-line-or-region-teardown)))
+
+(ert-deftest test-join-line-or-region-empty-lines-in-region ()
+  "Should handle empty lines in region."
+  (test-join-line-or-region-setup)
+  (unwind-protect
+      (with-temp-buffer
+        (insert "line one\n\nline three")
+        (goto-char (point-min))
+        (set-mark (point))
+        (goto-char (point-max))
+        (activate-mark)
+        (cj/join-line-or-region)
+        (should (string-match-p "line one.*line three" (buffer-string))))
+    (test-join-line-or-region-teardown)))
+
+(ert-deftest test-join-line-or-region-single-line-region ()
+  "Should handle single-line region."
+  (test-join-line-or-region-setup)
+  (unwind-protect
+      (with-temp-buffer
+        (insert "only line")
+        (goto-char (point-min))
+        (set-mark (point))
+        (goto-char (point-max))
+        (activate-mark)
+        (cj/join-line-or-region)
+        (should (string= "only line\n" (buffer-string))))
+    (test-join-line-or-region-teardown)))
+
+(ert-deftest test-join-line-or-region-region-with-only-whitespace-lines ()
+  "Should handle region with only whitespace lines."
+  (test-join-line-or-region-setup)
+  (unwind-protect
+      (with-temp-buffer
+        (insert "   \n   \n   ")
+        (goto-char (point-min))
+        (set-mark (point))
+        (goto-char (point-max))
+        (activate-mark)
+        (cj/join-line-or-region)
+        (should (stringp (buffer-string))))
+    (test-join-line-or-region-teardown)))
+
+(ert-deftest test-join-line-or-region-lines-with-leading-whitespace ()
+  "Should handle lines with leading whitespace."
+  (test-join-line-or-region-setup)
+  (unwind-protect
+      (with-temp-buffer
+        (insert "line one\n  line two")
+        (goto-char (point-max))
+        (cj/join-line-or-region)
+        (should (string-match-p "line one.*line two" (buffer-string))))
+    (test-join-line-or-region-teardown)))
+
+(ert-deftest test-join-line-or-region-lines-with-trailing-whitespace ()
+  "Should handle lines with trailing whitespace."
+  (test-join-line-or-region-setup)
+  (unwind-protect
+      (with-temp-buffer
+        (insert "line one  \nline two")
+        (goto-char (point-max))
+        (cj/join-line-or-region)
+        (should (string-match-p "line one.*line two" (buffer-string))))
+    (test-join-line-or-region-teardown)))
+
+(ert-deftest test-join-line-or-region-lines-with-tabs ()
+  "Should handle lines with tab characters."
+  (test-join-line-or-region-setup)
+  (unwind-protect
+      (with-temp-buffer
+        (insert "line\tone\nline\ttwo")
+        (goto-char (point-max))
+        (cj/join-line-or-region)
+        (should (string-match-p "line.*one.*line.*two" (buffer-string))))
+    (test-join-line-or-region-teardown)))
+
+(ert-deftest test-join-line-or-region-lines-with-mixed-whitespace ()
+  "Should handle lines with mixed whitespace."
+  (test-join-line-or-region-setup)
+  (unwind-protect
+      (with-temp-buffer
+        (insert "  line \t one  \n\t line  two\t")
+        (goto-char (point-max))
+        (cj/join-line-or-region)
+        (should (string-match-p "line.*one.*line.*two" (buffer-string))))
+    (test-join-line-or-region-teardown)))
+
+(ert-deftest test-join-line-or-region-very-long-lines ()
+  "Should handle very long lines."
+  (test-join-line-or-region-setup)
+  (unwind-protect
+      (with-temp-buffer
+        (let ((long-line (make-string 5000 ?x)))
+          (insert long-line "\n" long-line)
+          (goto-char (point-max))
+          (cj/join-line-or-region)
+          (should (= (length (buffer-string)) (+ (* 2 5000) 1 1)))))
+    (test-join-line-or-region-teardown)))
+
+(ert-deftest test-join-line-or-region-unicode-characters ()
+  "Should handle unicode characters."
+  (test-join-line-or-region-setup)
+  (unwind-protect
+      (with-temp-buffer
+        (insert "café\nnaïve")
+        (goto-char (point-max))
+        (cj/join-line-or-region)
+        (should (string-match-p "café.*naïve" (buffer-string))))
+    (test-join-line-or-region-teardown)))
+
+(ert-deftest test-join-line-or-region-emoji-content ()
+  "Should handle emoji content."
+  (test-join-line-or-region-setup)
+  (unwind-protect
+      (with-temp-buffer
+        (insert "hello 👋\nworld 🌍")
+        (goto-char (point-max))
+        (cj/join-line-or-region)
+        (should (string-match-p "hello 👋.*world 🌍" (buffer-string))))
+    (test-join-line-or-region-teardown)))
+
+(ert-deftest test-join-line-or-region-rtl-text ()
+  "Should handle RTL text."
+  (test-join-line-or-region-setup)
+  (unwind-protect
+      (with-temp-buffer
+        (insert "مرحبا\nعالم")
+        (goto-char (point-max))
+        (cj/join-line-or-region)
+        (should (string-match-p "مرحبا.*عالم" (buffer-string))))
+    (test-join-line-or-region-teardown)))
+
+(ert-deftest test-join-line-or-region-region-at-buffer-start ()
+  "Should handle region starting at buffer beginning."
+  (test-join-line-or-region-setup)
+  (unwind-protect
+      (with-temp-buffer
+        (insert "line one\nline two\nline three")
+        (goto-char (point-min))
+        (set-mark (point))
+        (forward-line 2)
+        (activate-mark)
+        (cj/join-line-or-region)
+        (should (string-match-p "line one line two" (buffer-string))))
+    (test-join-line-or-region-teardown)))
+
+(ert-deftest test-join-line-or-region-region-at-buffer-end ()
+  "Should handle region ending at buffer end."
+  (test-join-line-or-region-setup)
+  (unwind-protect
+      (with-temp-buffer
+        (insert "line one\nline two\nline three")
+        (goto-char (point-min))
+        (forward-line 1)
+        (set-mark (point))
+        (goto-char (point-max))
+        (activate-mark)
+        (cj/join-line-or-region)
+        (should (string-match-p "line two line three" (buffer-string))))
+    (test-join-line-or-region-teardown)))
+
+(ert-deftest test-join-line-or-region-entire-buffer-as-region ()
+  "Should handle entire buffer selected as region."
+  (test-join-line-or-region-setup)
+  (unwind-protect
+      (with-temp-buffer
+        (insert "one\ntwo\nthree")
+        (transient-mark-mode 1)
+        (mark-whole-buffer)
+        (activate-mark)
+        (cj/join-line-or-region)
+        (should (string-match-p "one two three" (buffer-string))))
+    (test-join-line-or-region-teardown)))
+
+(ert-deftest test-join-line-or-region-single-character-lines ()
+  "Should handle single character lines."
+  (test-join-line-or-region-setup)
+  (unwind-protect
+      (with-temp-buffer
+        (insert "a\nb\nc")
+        (goto-char (point-min))
+        (set-mark (point))
+        (goto-char (point-max))
+        (activate-mark)
+        (cj/join-line-or-region)
+        (should (string-match-p "a b c" (buffer-string))))
+    (test-join-line-or-region-teardown)))
+
+(ert-deftest test-join-line-or-region-cursor-position-after-no-region ()
+  "Cursor should be at end after joining without region."
+  (test-join-line-or-region-setup)
+  (unwind-protect
+      (with-temp-buffer
+        (insert "line one\nline two")
+        (goto-char (point-max))
+        (cj/join-line-or-region)
+        (should (= (point) (point-max))))
+    (test-join-line-or-region-teardown)))
+
+(ert-deftest test-join-line-or-region-cursor-position-after-region ()
+  "Cursor should be at region end marker after joining region."
+  (test-join-line-or-region-setup)
+  (unwind-protect
+      (with-temp-buffer
+        (insert "line one\nline two\nline three")
+        (goto-char (point-min))
+        (set-mark (point))
+        (goto-char (point-max))
+        (let ((end-pos (point)))
+          (activate-mark)
+          (cj/join-line-or-region)
+          ;; Point should be near the original end position
+          (should (>= (point) (- end-pos 10)))))
+    (test-join-line-or-region-teardown)))
+
+(ert-deftest test-join-line-or-region-marker-validity-after-operation ()
+  "Marker should remain valid after operation."
+  (test-join-line-or-region-setup)
+  (unwind-protect
+      (with-temp-buffer
+        (insert "line one\nline two\nline three")
+        (let ((marker (set-marker (make-marker) (point-min))))
+          (goto-char (point-min))
+          (set-mark (point))
+          (goto-char (point-max))
+          (activate-mark)
+          (cj/join-line-or-region)
+          (should (marker-position marker))))
+    (test-join-line-or-region-teardown)))
+
+;;; Error Cases
+
+(ert-deftest test-join-line-or-region-empty-buffer-no-region ()
+  "Should handle empty buffer gracefully without region."
+  (test-join-line-or-region-setup)
+  (unwind-protect
+      (with-temp-buffer
+        (cj/join-line-or-region)
+        (should (string= "\n" (buffer-string))))
+    (test-join-line-or-region-teardown)))
+
+(ert-deftest test-join-line-or-region-empty-buffer-with-region ()
+  "Should handle empty buffer gracefully with region."
+  (test-join-line-or-region-setup)
+  (unwind-protect
+      (with-temp-buffer
+        (mark-whole-buffer)
+        (cj/join-line-or-region)
+        (should (string= "\n" (buffer-string))))
+    (test-join-line-or-region-teardown)))
+
+(ert-deftest test-join-line-or-region-single-line-buffer-no-region ()
+  "Should handle single line buffer without region."
+  (test-join-line-or-region-setup)
+  (unwind-protect
+      (with-temp-buffer
+        (insert "only line")
+        (goto-char (point-max))
+        (cj/join-line-or-region)
+        (should (string= "only line\n" (buffer-string))))
+    (test-join-line-or-region-teardown)))
+
+(ert-deftest test-join-line-or-region-read-only-buffer-should-error ()
+  "Should error when attempting to modify read-only buffer."
+  (test-join-line-or-region-setup)
+  (unwind-protect
+      (with-temp-buffer
+        (insert "line one\nline two")
+        (goto-char (point-max))
+        (read-only-mode 1)
+        (should-error (cj/join-line-or-region)))
+    (test-join-line-or-region-teardown)))
+
+(provide 'test-custom-line-paragraph-join-line-or-region)
+;;; test-custom-line-paragraph-join-line-or-region.el ends here
diff --git a/tests/test-custom-line-paragraph-join-paragraph.el b/tests/test-custom-line-paragraph-join-paragraph.el
new file mode 100644
index 00000000..a84adc6c
--- /dev/null
+++ b/tests/test-custom-line-paragraph-join-paragraph.el
@@ -0,0 +1,360 @@
+;;; test-custom-line-paragraph-join-paragraph.el --- Tests for cj/join-paragraph -*- lexical-binding: t; -*-
+
+;;; Commentary:
+;; Tests for the cj/join-paragraph function from custom-line-paragraph.el
+;;
+;; IMPORTANT NOTE ON REGION ACTIVATION IN BATCH MODE:
+;; When testing functions that use (use-region-p) in batch mode, you must
+;; explicitly activate the region. Unlike interactive Emacs, batch mode does
+;; not automatically activate regions when you set mark and point.
+;;
+;; To properly test region-based behavior in batch mode:
+;; 1. Enable transient-mark-mode: (transient-mark-mode 1)
+;; 2. Set mark and point as needed
+;; 3. Explicitly activate the mark: (activate-mark)
+;;
+;; Without these steps, (use-region-p) will return nil even when mark and
+;; point are set, causing the function to take the no-region code path.
+;; This is a common pitfall that junior developers may miss when writing
+;; ERT tests for region-aware commands.
+;;
+;; The cj/join-paragraph function uses er/mark-paragraph which sets a region,
+;; so we need to ensure transient-mark-mode is enabled in our tests.
+
+;;; Code:
+
+;; Add tests directory to load path for testutil-general
+(add-to-list 'load-path (expand-file-name "tests" user-emacs-directory))
+
+(require 'ert)
+(require 'testutil-general)
+
+;; Initialize package system to load expand-region
+(require 'package)
+(setq package-user-dir (expand-file-name "elpa" user-emacs-directory))
+(package-initialize)
+
+;; Add modules directory to load path
+(add-to-list 'load-path (expand-file-name "modules" user-emacs-directory))
+
+;; Add expand-region to load path explicitly
+(add-to-list 'load-path (expand-file-name "elpa/expand-region-1.0.0" user-emacs-directory))
+
+;; Stub dependencies before loading the module
+(defvar cj/custom-keymap (make-sparse-keymap)
+  "Stub keymap for testing.")
+
+;; Load expand-region for real (needed by cj/join-paragraph)
+(require 'expand-region)
+(require 'the-org-mode-expansions)
+
+;; Now load the actual production module
+(require 'custom-line-paragraph)
+
+;; -------------------------------- Test Fixtures ------------------------------
+
+(defun test-join-paragraph-setup ()
+  "Set up test environment."
+  (cj/create-test-base-dir))
+
+(defun test-join-paragraph-teardown ()
+  "Clean up test environment."
+  (cj/delete-test-base-dir))
+
+;; ---------------------------- Normal Cases -----------------------------------
+
+(ert-deftest test-join-paragraph-simple-multiline-cursor-at-start ()
+  "Join a simple 3-line paragraph with cursor at start."
+  (test-join-paragraph-setup)
+  (unwind-protect
+      (with-temp-buffer
+        (transient-mark-mode 1)
+        (insert "line one\nline two\nline three")
+        (goto-char (point-min))
+        (cj/join-paragraph)
+        (should (string= (buffer-substring-no-properties (point-min) (point-max))
+                        "line one line two line three\n")))
+    (test-join-paragraph-teardown)))
+
+(ert-deftest test-join-paragraph-simple-multiline-cursor-in-middle ()
+  "Join a simple 3-line paragraph with cursor in middle line."
+  (test-join-paragraph-setup)
+  (unwind-protect
+      (with-temp-buffer
+        (transient-mark-mode 1)
+        (insert "line one\nline two\nline three")
+        (goto-char (point-min))
+        (forward-line 1)
+        (cj/join-paragraph)
+        (should (string= (buffer-substring-no-properties (point-min) (point-max))
+                        "line one line two line three\n")))
+    (test-join-paragraph-teardown)))
+
+(ert-deftest test-join-paragraph-simple-multiline-cursor-at-end ()
+  "Join a simple 3-line paragraph with cursor at end."
+  (test-join-paragraph-setup)
+  (unwind-protect
+      (with-temp-buffer
+        (transient-mark-mode 1)
+        (insert "line one\nline two\nline three")
+        (goto-char (point-max))
+        (cj/join-paragraph)
+        (should (string= (buffer-substring-no-properties (point-min) (point-max))
+                        "line one line two line three\n")))
+    (test-join-paragraph-teardown)))
+
+(ert-deftest test-join-paragraph-surrounded-by-other-paragraphs ()
+  "Join only the current paragraph when surrounded by others."
+  (test-join-paragraph-setup)
+  (unwind-protect
+      (with-temp-buffer
+        (transient-mark-mode 1)
+        (insert "para one line one\npara one line two\n\n")
+        (insert "para two line one\npara two line two\n\n")
+        (insert "para three line one\npara three line two")
+        ;; Position in middle paragraph
+        (goto-char (point-min))
+        (forward-line 3)
+        (cj/join-paragraph)
+        (should (string-match-p "para one line one\npara one line two"
+                               (buffer-string)))
+        (should (string-match-p "para two line one para two line two"
+                               (buffer-string)))
+        (should (string-match-p "para three line one\npara three line two"
+                               (buffer-string))))
+    (test-join-paragraph-teardown)))
+
+(ert-deftest test-join-paragraph-with-leading-whitespace ()
+  "Join paragraph with indented lines, preserving appropriate spacing."
+  (test-join-paragraph-setup)
+  (unwind-protect
+      (with-temp-buffer
+        (transient-mark-mode 1)
+        (insert "  indented line one\n  indented line two\n  indented line three")
+        (goto-char (point-min))
+        (cj/join-paragraph)
+        (should (string= (buffer-substring-no-properties (point-min) (point-max))
+                        "  indented line one indented line two indented line three\n")))
+    (test-join-paragraph-teardown)))
+
+(ert-deftest test-join-paragraph-cursor-position-after ()
+  "Verify cursor moves forward one line after joining."
+  (test-join-paragraph-setup)
+  (unwind-protect
+      (with-temp-buffer
+        (transient-mark-mode 1)
+        (insert "line one\nline two\nline three\n")
+        (goto-char (point-min))
+        (let ((initial-line (line-number-at-pos)))
+          (cj/join-paragraph)
+          (should (= (line-number-at-pos) (1+ initial-line)))))
+    (test-join-paragraph-teardown)))
+
+(ert-deftest test-join-paragraph-multiple-paragraphs-first ()
+  "Join only first paragraph when three paragraphs exist."
+  (test-join-paragraph-setup)
+  (unwind-protect
+      (with-temp-buffer
+        (transient-mark-mode 1)
+        (insert "first one\nfirst two\n\nsecond one\nsecond two\n\nthird one\nthird two")
+        (goto-char (point-min))
+        (cj/join-paragraph)
+        (should (string-match-p "^first one first two\n" (buffer-string)))
+        (should (string-match-p "second one\nsecond two" (buffer-string)))
+        (should (string-match-p "third one\nthird two" (buffer-string))))
+    (test-join-paragraph-teardown)))
+
+(ert-deftest test-join-paragraph-multiple-paragraphs-middle ()
+  "Join only middle paragraph when three paragraphs exist."
+  (test-join-paragraph-setup)
+  (unwind-protect
+      (with-temp-buffer
+        (transient-mark-mode 1)
+        (insert "first one\nfirst two\n\nsecond one\nsecond two\n\nthird one\nthird two")
+        (goto-char (point-min))
+        (forward-line 3)
+        (cj/join-paragraph)
+        (should (string-match-p "first one\nfirst two" (buffer-string)))
+        (should (string-match-p "second one second two" (buffer-string)))
+        (should (string-match-p "third one\nthird two" (buffer-string))))
+    (test-join-paragraph-teardown)))
+
+(ert-deftest test-join-paragraph-multiple-paragraphs-last ()
+  "Join only last paragraph when three paragraphs exist."
+  (test-join-paragraph-setup)
+  (unwind-protect
+      (with-temp-buffer
+        (transient-mark-mode 1)
+        (insert "first one\nfirst two\n\nsecond one\nsecond two\n\nthird one\nthird two")
+        (goto-char (point-max))
+        (cj/join-paragraph)
+        (should (string-match-p "first one\nfirst two" (buffer-string)))
+        (should (string-match-p "second one\nsecond two" (buffer-string)))
+        (should (string-match-p "third one third two" (buffer-string))))
+    (test-join-paragraph-teardown)))
+
+;; ---------------------------- Boundary Cases ---------------------------------
+
+(ert-deftest test-join-paragraph-single-line-paragraph ()
+  "Handle paragraph with only one line gracefully."
+  (test-join-paragraph-setup)
+  (unwind-protect
+      (with-temp-buffer
+        (transient-mark-mode 1)
+        (insert "single line paragraph")
+        (goto-char (point-min))
+        (cj/join-paragraph)
+        ;; Should still work, even if nothing to join
+        (should (string= (buffer-substring-no-properties (point-min) (point-max))
+                        "single line paragraph\n")))
+    (test-join-paragraph-teardown)))
+
+(ert-deftest test-join-paragraph-at-buffer-start ()
+  "Join paragraph at very beginning of buffer."
+  (test-join-paragraph-setup)
+  (unwind-protect
+      (with-temp-buffer
+        (transient-mark-mode 1)
+        (insert "first line\nsecond line\nthird line\n\nother paragraph")
+        (goto-char (point-min))
+        (cj/join-paragraph)
+        (should (string-match-p "^first line second line third line\n" (buffer-string))))
+    (test-join-paragraph-teardown)))
+
+(ert-deftest test-join-paragraph-at-buffer-end ()
+  "Join paragraph at very end of buffer."
+  (test-join-paragraph-setup)
+  (unwind-protect
+      (with-temp-buffer
+        (transient-mark-mode 1)
+        (insert "other paragraph\n\nfirst line\nsecond line\nthird line")
+        (goto-char (point-max))
+        (cj/join-paragraph)
+        (should (string-match-p "first line second line third line\n$" (buffer-string))))
+    (test-join-paragraph-teardown)))
+
+(ert-deftest test-join-paragraph-very-long ()
+  "Join paragraph with many lines (20+ lines)."
+  (test-join-paragraph-setup)
+  (unwind-protect
+      (with-temp-buffer
+        (transient-mark-mode 1)
+        (dotimes (i 25)
+          (insert (format "line %d\n" (1+ i))))
+        (goto-char (point-min))
+        (cj/join-paragraph)
+        ;; Should have all 25 "line X" strings joined with spaces
+        (should (string-match-p "line 1 line 2 line 3.*line 24 line 25" (buffer-string)))
+        ;; Should not have multiple newlines in sequence
+        (should-not (string-match-p "\n.*\n.*\n" (buffer-string))))
+    (test-join-paragraph-teardown)))
+
+(ert-deftest test-join-paragraph-with-blank-lines-within ()
+  "Test behavior when expand-region might see internal structure."
+  (test-join-paragraph-setup)
+  (unwind-protect
+      (with-temp-buffer
+        (transient-mark-mode 1)
+        ;; This tests how er/mark-paragraph handles the content
+        (insert "line one\n\nline two\n\nother para")
+        (goto-char (point-min))
+        (cj/join-paragraph)
+        ;; er/mark-paragraph should mark just the first line in this case
+        (should (string-match-p "^line one\n" (buffer-string))))
+    (test-join-paragraph-teardown)))
+
+(ert-deftest test-join-paragraph-cursor-on-blank-line ()
+  "Handle cursor positioned on blank line between paragraphs."
+  (test-join-paragraph-setup)
+  (unwind-protect
+      (with-temp-buffer
+        (transient-mark-mode 1)
+        (insert "para one\npara one line two\n\npara two\npara two line two")
+        (goto-char (point-min))
+        (forward-line 2)  ;; Position on blank line
+        (cj/join-paragraph)
+        ;; Behavior depends on how er/mark-paragraph handles blank lines
+        ;; At minimum, should not error
+        (should (bufferp (current-buffer))))
+    (test-join-paragraph-teardown)))
+
+(ert-deftest test-join-paragraph-no-trailing-newline ()
+  "Handle paragraph at end of buffer with no trailing newline."
+  (test-join-paragraph-setup)
+  (unwind-protect
+      (with-temp-buffer
+        (transient-mark-mode 1)
+        (insert "line one\nline two\nline three")
+        (goto-char (point-min))
+        (cj/join-paragraph)
+        (should (string= (buffer-substring-no-properties (point-min) (point-max))
+                        "line one line two line three\n")))
+    (test-join-paragraph-teardown)))
+
+(ert-deftest test-join-paragraph-only-whitespace-lines ()
+  "Handle paragraph where lines contain only spaces/tabs."
+  (test-join-paragraph-setup)
+  (unwind-protect
+      (with-temp-buffer
+        (transient-mark-mode 1)
+        (insert "   \n\t\t\n  \t  ")
+        (goto-char (point-min))
+        (cj/join-paragraph)
+        ;; Should handle without error
+        (should (bufferp (current-buffer))))
+    (test-join-paragraph-teardown)))
+
+(ert-deftest test-join-paragraph-unicode-content ()
+  "Handle paragraph with emoji and special Unicode characters."
+  (test-join-paragraph-setup)
+  (unwind-protect
+      (with-temp-buffer
+        (transient-mark-mode 1)
+        (insert "Hello 👋 world\nこんにちは 世界\n🎉 celebration 🎊")
+        (goto-char (point-min))
+        (cj/join-paragraph)
+        (should (string= (buffer-substring-no-properties (point-min) (point-max))
+                        "Hello 👋 world こんにちは 世界 🎉 celebration 🎊\n")))
+    (test-join-paragraph-teardown)))
+
+;; ---------------------------- Error Cases ------------------------------------
+
+(ert-deftest test-join-paragraph-empty-buffer ()
+  "Handle empty buffer without error."
+  (test-join-paragraph-setup)
+  (unwind-protect
+      (with-temp-buffer
+        (transient-mark-mode 1)
+        ;; Empty buffer - should handle gracefully without error
+        (cj/join-paragraph)
+        (should (bufferp (current-buffer))))
+    (test-join-paragraph-teardown)))
+
+(ert-deftest test-join-paragraph-buffer-only-whitespace ()
+  "Handle buffer containing only whitespace."
+  (test-join-paragraph-setup)
+  (unwind-protect
+      (with-temp-buffer
+        (transient-mark-mode 1)
+        (insert "   \n\n\t\t\n  ")
+        (goto-char (point-min))
+        (cj/join-paragraph)
+        ;; Should handle without error
+        (should (bufferp (current-buffer))))
+    (test-join-paragraph-teardown)))
+
+(ert-deftest test-join-paragraph-buffer-single-character ()
+  "Handle buffer with minimal content."
+  (test-join-paragraph-setup)
+  (unwind-protect
+      (with-temp-buffer
+        (transient-mark-mode 1)
+        (insert "x")
+        (goto-char (point-min))
+        (cj/join-paragraph)
+        (should (string= (buffer-substring-no-properties (point-min) (point-max))
+                        "x\n")))
+    (test-join-paragraph-teardown)))
+
+(provide 'test-custom-line-paragraph-join-paragraph)
+;;; test-custom-line-paragraph-join-paragraph.el ends here