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+;;; test-modeline-config-string-cut-middle.el --- Tests for cj/modeline-string-cut-middle -*- lexical-binding: t; -*-
+
+;;; Commentary:
+;; Tests for the cj/modeline-string-cut-middle function from modeline-config.el
+;;
+;; This function truncates a string in the middle with "..." when conditions
+;; are met (narrow window, multi-window, string exceeds truncation length).
+;; Example: "my-very-long-name.el" → "my-ver...me.el"
+;;
+;; We mock cj/modeline-string-truncate-p to isolate the string manipulation
+;; logic from window state checks.
+
+;;; Code:
+
+(require 'ert)
+
+;; Add modules directory to load path
+(add-to-list 'load-path (expand-file-name "modules" user-emacs-directory))
+
+;; Stub dependencies before loading the module
+(unless (boundp 'cj/buffer-status-colors)
+  (defvar cj/buffer-status-colors
+    '((unmodified . "#FFFFFF")
+      (modified   . "#00FF00")
+      (read-only  . "#FF0000")
+      (overwrite  . "#FFD700"))))
+
+(require 'modeline-config)
+
+;;; Test Helpers
+
+(defmacro with-truncation-enabled (&rest body)
+  "Execute BODY with truncation forced on.
+Mocks `cj/modeline-string-truncate-p' to always return t."
+  `(cl-letf (((symbol-function 'cj/modeline-string-truncate-p)
+              (lambda (_str) t)))
+     ,@body))
+
+(defmacro with-truncation-disabled (&rest body)
+  "Execute BODY with truncation forced off.
+Mocks `cj/modeline-string-truncate-p' to always return nil."
+  `(cl-letf (((symbol-function 'cj/modeline-string-truncate-p)
+              (lambda (_str) nil)))
+     ,@body))
+
+;;; Normal Cases
+
+(ert-deftest test-modeline-config-string-cut-middle-normal-truncates-long-string ()
+  "Should truncate a long string in the middle with '...'."
+  (with-truncation-enabled
+    (let* ((cj/modeline-string-truncate-length 12)
+           (result (cj/modeline-string-cut-middle "my-very-long-name.el")))
+      (should (string-match-p "\\.\\.\\." result))
+      (should (< (length result) (length "my-very-long-name.el"))))))
+
+(ert-deftest test-modeline-config-string-cut-middle-normal-preserves-start-and-end ()
+  "Should keep the beginning and end of the string."
+  (with-truncation-enabled
+    (let* ((cj/modeline-string-truncate-length 12)
+           (result (cj/modeline-string-cut-middle "my-very-long-name.el")))
+      (should (string-prefix-p "my-ver" result))
+      (should (string-suffix-p "me.el" result)))))
+
+(ert-deftest test-modeline-config-string-cut-middle-normal-result-length ()
+  "Truncated result should be truncate-length + 3 (for '...')."
+  (with-truncation-enabled
+    (let* ((cj/modeline-string-truncate-length 12)
+           (result (cj/modeline-string-cut-middle "abcdefghijklmnopqrstuvwxyz")))
+      ;; half=6, so 6 chars + "..." + 6 chars = 15
+      (should (= (length result) 15)))))
+
+(ert-deftest test-modeline-config-string-cut-middle-normal-no-truncation-when-disabled ()
+  "Should return string unchanged when truncation is disabled."
+  (with-truncation-disabled
+    (let ((result (cj/modeline-string-cut-middle "my-very-long-name.el")))
+      (should (string= result "my-very-long-name.el")))))
+
+(ert-deftest test-modeline-config-string-cut-middle-normal-short-string-unchanged ()
+  "Short string should pass through unchanged when truncation is disabled."
+  (with-truncation-disabled
+    (let ((result (cj/modeline-string-cut-middle "init.el")))
+      (should (string= result "init.el")))))
+
+;;; Boundary Cases
+
+(ert-deftest test-modeline-config-string-cut-middle-boundary-empty-string ()
+  "Empty string should pass through unchanged."
+  (with-truncation-disabled
+    (let ((result (cj/modeline-string-cut-middle "")))
+      (should (string= result "")))))
+
+(ert-deftest test-modeline-config-string-cut-middle-boundary-single-char ()
+  "Single character string should pass through unchanged."
+  (with-truncation-disabled
+    (let ((result (cj/modeline-string-cut-middle "x")))
+      (should (string= result "x")))))
+
+(ert-deftest test-modeline-config-string-cut-middle-boundary-odd-truncate-length ()
+  "Odd truncation length should floor the half correctly."
+  (with-truncation-enabled
+    (let* ((cj/modeline-string-truncate-length 11)
+           (result (cj/modeline-string-cut-middle "abcdefghijklmnopqrstuvwxyz")))
+      ;; half = (floor 11 2) = 5, so 5 + "..." + 5 = 13
+      (should (= (length result) 13))
+      (should (string= (substring result 0 5) "abcde"))
+      (should (string= (substring result -5) "vwxyz")))))
+
+(ert-deftest test-modeline-config-string-cut-middle-boundary-truncate-length-2 ()
+  "Minimum practical truncation length of 2."
+  (with-truncation-enabled
+    (let* ((cj/modeline-string-truncate-length 2)
+           (result (cj/modeline-string-cut-middle "abcdefghij")))
+      ;; half = (floor 2 2) = 1, so 1 + "..." + 1 = 5
+      (should (= (length result) 5))
+      (should (string= result "a...j")))))
+
+(ert-deftest test-modeline-config-string-cut-middle-boundary-unicode-filename ()
+  "Should handle unicode characters in filename."
+  (with-truncation-enabled
+    (let* ((cj/modeline-string-truncate-length 10)
+           (result (cj/modeline-string-cut-middle "überlangerDateiname.el")))
+      (should (string-match-p "\\.\\.\\." result))
+      (should (< (length result) (length "überlangerDateiname.el"))))))
+
+(ert-deftest test-modeline-config-string-cut-middle-boundary-dots-in-filename ()
+  "Should handle filenames with dots (common in elisp)."
+  (with-truncation-enabled
+    (let* ((cj/modeline-string-truncate-length 12)
+           (result (cj/modeline-string-cut-middle "test-custom-whitespace-collapse.el")))
+      (should (string-match-p "\\.\\.\\." result)))))
+
+;;; Error Cases
+
+(ert-deftest test-modeline-config-string-cut-middle-error-nil-input ()
+  "Nil input should pass through (truncate-p returns nil for non-strings)."
+  (with-truncation-disabled
+    (should (null (cj/modeline-string-cut-middle nil)))))
+
+(ert-deftest test-modeline-config-string-cut-middle-error-number-input ()
+  "Number input should pass through (truncate-p returns nil for non-strings)."
+  (with-truncation-disabled
+    (should (= (cj/modeline-string-cut-middle 42) 42))))
+
+(provide 'test-modeline-config-string-cut-middle)
+;;; test-modeline-config-string-cut-middle.el ends here