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+;;; test-modeline-config-string-truncate-p.el --- Tests for cj/modeline-string-truncate-p -*- lexical-binding: t; -*-
+
+;;; Commentary:
+;; Tests for the cj/modeline-string-truncate-p function from modeline-config.el
+;;
+;; This function returns non-nil when ALL conditions are met:
+;; 1. STR is a string
+;; 2. STR is not empty
+;; 3. Window is narrow (< 100 chars via cj/modeline-window-narrow-p)
+;; 4. String length exceeds cj/modeline-string-truncate-length
+;; 5. Not in a single-window frame (one-window-p returns nil)
+;;
+;; We mock window functions to isolate the logic.
+
+;;; 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-window-state (narrow-p one-window-p &rest body)
+  "Execute BODY with mocked window state.
+NARROW-P controls `cj/modeline-window-narrow-p' return value.
+ONE-WINDOW-P controls `one-window-p' return value."
+  (declare (indent 2))
+  `(cl-letf (((symbol-function 'cj/modeline-window-narrow-p)
+              (lambda () ,narrow-p))
+             ((symbol-function 'one-window-p)
+              (lambda (&rest _args) ,one-window-p)))
+     ,@body))
+
+;;; Normal Cases - All conditions met (should return non-nil)
+
+(ert-deftest test-modeline-config-string-truncate-p-normal-long-string-narrow-multi-window ()
+  "Should return non-nil when string is long, window is narrow, and multiple windows."
+  (with-window-state t nil
+    (let ((cj/modeline-string-truncate-length 12))
+      (should (cj/modeline-string-truncate-p "a-very-long-filename.el")))))
+
+(ert-deftest test-modeline-config-string-truncate-p-normal-exactly-over-length ()
+  "Should return non-nil when string is one char over truncation length."
+  (with-window-state t nil
+    (let ((cj/modeline-string-truncate-length 12))
+      (should (cj/modeline-string-truncate-p "1234567890123")))))  ; 13 chars
+
+;;; Normal Cases - Conditions not met (should return nil)
+
+(ert-deftest test-modeline-config-string-truncate-p-normal-short-string ()
+  "Should return nil when string is shorter than truncation length."
+  (with-window-state t nil
+    (let ((cj/modeline-string-truncate-length 12))
+      (should-not (cj/modeline-string-truncate-p "init.el")))))
+
+(ert-deftest test-modeline-config-string-truncate-p-normal-wide-window ()
+  "Should return nil when window is wide (not narrow)."
+  (with-window-state nil nil
+    (let ((cj/modeline-string-truncate-length 12))
+      (should-not (cj/modeline-string-truncate-p "a-very-long-filename.el")))))
+
+(ert-deftest test-modeline-config-string-truncate-p-normal-single-window ()
+  "Should return nil when only one window is open."
+  (with-window-state t t
+    (let ((cj/modeline-string-truncate-length 12))
+      (should-not (cj/modeline-string-truncate-p "a-very-long-filename.el")))))
+
+;;; Boundary Cases
+
+(ert-deftest test-modeline-config-string-truncate-p-boundary-exact-length ()
+  "Should return nil when string is exactly at truncation length."
+  (with-window-state t nil
+    (let ((cj/modeline-string-truncate-length 12))
+      (should-not (cj/modeline-string-truncate-p "123456789012")))))  ; exactly 12
+
+(ert-deftest test-modeline-config-string-truncate-p-boundary-empty-string ()
+  "Should return nil for empty string."
+  (with-window-state t nil
+    (let ((cj/modeline-string-truncate-length 12))
+      (should-not (cj/modeline-string-truncate-p "")))))
+
+(ert-deftest test-modeline-config-string-truncate-p-boundary-single-char ()
+  "Should return nil for single character string."
+  (with-window-state t nil
+    (let ((cj/modeline-string-truncate-length 12))
+      (should-not (cj/modeline-string-truncate-p "x")))))
+
+(ert-deftest test-modeline-config-string-truncate-p-boundary-truncate-length-1 ()
+  "Should return non-nil for any string > 1 when truncation length is 1."
+  (with-window-state t nil
+    (let ((cj/modeline-string-truncate-length 1))
+      (should (cj/modeline-string-truncate-p "ab")))))
+
+(ert-deftest test-modeline-config-string-truncate-p-boundary-truncate-length-0 ()
+  "Should return non-nil for any non-empty string when truncation length is 0."
+  (with-window-state t nil
+    (let ((cj/modeline-string-truncate-length 0))
+      (should (cj/modeline-string-truncate-p "a")))))
+
+;;; Error Cases
+
+(ert-deftest test-modeline-config-string-truncate-p-error-nil-input ()
+  "Should return nil for nil input."
+  (with-window-state t nil
+    (let ((cj/modeline-string-truncate-length 12))
+      (should-not (cj/modeline-string-truncate-p nil)))))
+
+(ert-deftest test-modeline-config-string-truncate-p-error-number-input ()
+  "Should return nil for number input."
+  (with-window-state t nil
+    (let ((cj/modeline-string-truncate-length 12))
+      (should-not (cj/modeline-string-truncate-p 42)))))
+
+(ert-deftest test-modeline-config-string-truncate-p-error-symbol-input ()
+  "Should return nil for symbol input."
+  (with-window-state t nil
+    (let ((cj/modeline-string-truncate-length 12))
+      (should-not (cj/modeline-string-truncate-p 'some-symbol)))))
+
+(provide 'test-modeline-config-string-truncate-p)
+;;; test-modeline-config-string-truncate-p.el ends here