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+;;; test-wttrin--mode-line-update-display.el --- Tests for mode-line display update -*- lexical-binding: t; -*-
+
+;; Copyright (C) 2025 Craig Jennings
+
+;;; Commentary:
+;; Unit tests for wttrin--mode-line-update-display and
+;; wttrin--mode-line-valid-response-p.
+
+;;; Code:
+
+(require 'ert)
+(require 'wttrin)
+(require 'testutil-wttrin)
+
+;;; Setup and Teardown
+
+(defun test-wttrin--mode-line-update-display-setup ()
+  "Setup for mode-line update display tests."
+  (testutil-wttrin-setup)
+  (setq wttrin-mode-line-string nil)
+  (setq wttrin--mode-line-tooltip-data nil))
+
+(defun test-wttrin--mode-line-update-display-teardown ()
+  "Teardown for mode-line update display tests."
+  (testutil-wttrin-teardown)
+  (setq wttrin-mode-line-string nil)
+  (setq wttrin--mode-line-tooltip-data nil))
+
+;;; --------------------------------------------------------------------------
+;;; wttrin--mode-line-valid-response-p
+;;; --------------------------------------------------------------------------
+
+;;; Normal Cases
+
+(ert-deftest test-wttrin--mode-line-valid-response-p-normal-standard-response ()
+  "Valid response with location, emoji, temp, and conditions."
+  (should (wttrin--mode-line-valid-response-p "Paris: ☀️ +61°F Clear")))
+
+(ert-deftest test-wttrin--mode-line-valid-response-p-normal-minimal-with-colon ()
+  "Minimal valid response containing a colon."
+  (should (wttrin--mode-line-valid-response-p "X: Y")))
+
+;;; Boundary Cases
+
+(ert-deftest test-wttrin--mode-line-valid-response-p-boundary-colon-only ()
+  "Response that is just a colon is technically valid (has expected delimiter)."
+  (should (wttrin--mode-line-valid-response-p ":")))
+
+;;; Error Cases
+
+(ert-deftest test-wttrin--mode-line-valid-response-p-error-nil ()
+  "Nil input is invalid."
+  (should-not (wttrin--mode-line-valid-response-p nil)))
+
+(ert-deftest test-wttrin--mode-line-valid-response-p-error-empty-string ()
+  "Empty string is invalid."
+  (should-not (wttrin--mode-line-valid-response-p "")))
+
+(ert-deftest test-wttrin--mode-line-valid-response-p-error-no-colon ()
+  "String without colon is invalid (doesn't match expected format)."
+  (should-not (wttrin--mode-line-valid-response-p "no colon here")))
+
+(ert-deftest test-wttrin--mode-line-valid-response-p-error-not-a-string ()
+  "Non-string input is invalid."
+  (should-not (wttrin--mode-line-valid-response-p 42)))
+
+;;; --------------------------------------------------------------------------
+;;; wttrin--mode-line-update-display
+;;; --------------------------------------------------------------------------
+
+;;; Normal Cases
+
+(ert-deftest test-wttrin--mode-line-update-display-normal-sets-mode-line-string ()
+  "Display update sets wttrin-mode-line-string to non-nil."
+  (test-wttrin--mode-line-update-display-setup)
+  (unwind-protect
+      (progn
+        (wttrin--mode-line-update-display "Paris: ☀️ +61°F Clear")
+        (should wttrin-mode-line-string))
+    (test-wttrin--mode-line-update-display-teardown)))
+
+(ert-deftest test-wttrin--mode-line-update-display-normal-stores-tooltip-data ()
+  "Display update stores weather string as tooltip data."
+  (test-wttrin--mode-line-update-display-setup)
+  (unwind-protect
+      (progn
+        (wttrin--mode-line-update-display "Paris: ☀️ +61°F Clear")
+        (should (equal wttrin--mode-line-tooltip-data "Paris: ☀️ +61°F Clear")))
+    (test-wttrin--mode-line-update-display-teardown)))
+
+(ert-deftest test-wttrin--mode-line-update-display-normal-extracts-emoji ()
+  "Display update extracts emoji character into mode-line string."
+  (test-wttrin--mode-line-update-display-setup)
+  (unwind-protect
+      (let ((wttrin-mode-line-emoji-font nil))
+        (wttrin--mode-line-update-display "Paris: X +61°F Clear")
+        ;; Mode-line string should contain the extracted character
+        (should (string-match-p "X" (substring-no-properties wttrin-mode-line-string))))
+    (test-wttrin--mode-line-update-display-teardown)))
+
+(ert-deftest test-wttrin--mode-line-update-display-normal-has-help-echo ()
+  "Display update sets help-echo property for tooltip."
+  (test-wttrin--mode-line-update-display-setup)
+  (unwind-protect
+      (progn
+        (wttrin--mode-line-update-display "Paris: ☀️ +61°F Clear")
+        (should (get-text-property 0 'help-echo wttrin-mode-line-string)))
+    (test-wttrin--mode-line-update-display-teardown)))
+
+(ert-deftest test-wttrin--mode-line-update-display-normal-has-local-map ()
+  "Display update sets local-map property for mouse interaction."
+  (test-wttrin--mode-line-update-display-setup)
+  (unwind-protect
+      (progn
+        (wttrin--mode-line-update-display "Paris: ☀️ +61°F Clear")
+        (should (eq (get-text-property 0 'local-map wttrin-mode-line-string)
+                    wttrin--mode-line-map)))
+    (test-wttrin--mode-line-update-display-teardown)))
+
+;;; Boundary Cases
+
+(ert-deftest test-wttrin--mode-line-update-display-boundary-no-emoji-match-uses-fallback ()
+  "When emoji regex doesn't match, fallback character '?' is used."
+  (test-wttrin--mode-line-update-display-setup)
+  (unwind-protect
+      (let ((wttrin-mode-line-emoji-font nil))
+        (wttrin--mode-line-update-display "no colon here")
+        (should (string-match-p "\\?" (substring-no-properties wttrin-mode-line-string))))
+    (test-wttrin--mode-line-update-display-teardown)))
+
+;;; Tooltip Lambda Tests
+
+(ert-deftest test-wttrin--mode-line-update-display-normal-tooltip-returns-weather-data ()
+  "Tooltip lambda returns weather data when available."
+  (test-wttrin--mode-line-update-display-setup)
+  (unwind-protect
+      (progn
+        (wttrin--mode-line-update-display "Paris: ☀️ +61°F Clear")
+        (let ((tooltip-fn (get-text-property 0 'help-echo wttrin-mode-line-string)))
+          (should (equal (funcall tooltip-fn nil nil nil) "Paris: ☀️ +61°F Clear"))))
+    (test-wttrin--mode-line-update-display-teardown)))
+
+(ert-deftest test-wttrin--mode-line-update-display-boundary-tooltip-empty-string-uses-fallback ()
+  "Tooltip lambda falls back when tooltip data is empty string."
+  (test-wttrin--mode-line-update-display-setup)
+  (unwind-protect
+      (let ((wttrin-favorite-location "Paris"))
+        (wttrin--mode-line-update-display "Paris: ☀️ +61°F Clear")
+        ;; Simulate empty tooltip data (as would happen with bad response)
+        (setq wttrin--mode-line-tooltip-data "")
+        (let ((tooltip-fn (get-text-property 0 'help-echo wttrin-mode-line-string)))
+          (should (string-match-p "Weather for Paris" (funcall tooltip-fn nil nil nil)))))
+    (test-wttrin--mode-line-update-display-teardown)))
+
+(ert-deftest test-wttrin--mode-line-update-display-boundary-tooltip-nil-uses-fallback ()
+  "Tooltip lambda falls back when tooltip data is nil."
+  (test-wttrin--mode-line-update-display-setup)
+  (unwind-protect
+      (let ((wttrin-favorite-location "Paris"))
+        (wttrin--mode-line-update-display "Paris: ☀️ +61°F Clear")
+        (setq wttrin--mode-line-tooltip-data nil)
+        (let ((tooltip-fn (get-text-property 0 'help-echo wttrin-mode-line-string)))
+          (should (string-match-p "Weather for Paris" (funcall tooltip-fn nil nil nil)))))
+    (test-wttrin--mode-line-update-display-teardown)))
+
+;;; --------------------------------------------------------------------------
+;;; wttrin--mode-line-fetch-weather (validation integration)
+;;; --------------------------------------------------------------------------
+
+(ert-deftest test-wttrin--mode-line-fetch-weather-error-empty-response-keeps-previous ()
+  "Empty API response does not overwrite previous valid display."
+  (test-wttrin--mode-line-update-display-setup)
+  (unwind-protect
+      (let ((wttrin-favorite-location "Paris"))
+        ;; Set up a valid prior state
+        (wttrin--mode-line-update-display "Paris: ☀️ +61°F Clear")
+        (let ((previous-string wttrin-mode-line-string)
+              (previous-tooltip wttrin--mode-line-tooltip-data))
+          ;; Simulate fetch returning empty response
+          (testutil-wttrin-mock-http-response ""
+            (wttrin--mode-line-fetch-weather)
+            ;; Previous state should be preserved
+            (should (equal wttrin-mode-line-string previous-string))
+            (should (equal wttrin--mode-line-tooltip-data previous-tooltip)))))
+    (test-wttrin--mode-line-update-display-teardown)))
+
+(ert-deftest test-wttrin--mode-line-fetch-weather-error-no-colon-response-keeps-previous ()
+  "Malformed API response without colon does not overwrite previous valid display."
+  (test-wttrin--mode-line-update-display-setup)
+  (unwind-protect
+      (let ((wttrin-favorite-location "Paris"))
+        (wttrin--mode-line-update-display "Paris: ☀️ +61°F Clear")
+        (let ((previous-string wttrin-mode-line-string)
+              (previous-tooltip wttrin--mode-line-tooltip-data))
+          (testutil-wttrin-mock-http-response "Unknown location"
+            (wttrin--mode-line-fetch-weather)
+            (should (equal wttrin-mode-line-string previous-string))
+            (should (equal wttrin--mode-line-tooltip-data previous-tooltip)))))
+    (test-wttrin--mode-line-update-display-teardown)))
+
+(ert-deftest test-wttrin--mode-line-fetch-weather-error-nil-location-does-not-fetch ()
+  "No fetch is attempted when wttrin-favorite-location is nil."
+  (test-wttrin--mode-line-update-display-setup)
+  (unwind-protect
+      (let ((wttrin-favorite-location nil)
+            (fetch-called nil))
+        (cl-letf (((symbol-function 'wttrin--fetch-url)
+                   (lambda (_url _callback) (setq fetch-called t))))
+          (wttrin--mode-line-fetch-weather)
+          (should-not fetch-called)))
+    (test-wttrin--mode-line-update-display-teardown)))
+
+(ert-deftest test-wttrin--mode-line-fetch-weather-normal-valid-response-updates-display ()
+  "Valid API response updates the mode-line display."
+  (test-wttrin--mode-line-update-display-setup)
+  (unwind-protect
+      (let ((wttrin-favorite-location "Paris"))
+        (testutil-wttrin-mock-http-response "Paris: ☀️ +61°F Clear"
+          (wttrin--mode-line-fetch-weather)
+          (should wttrin-mode-line-string)
+          (should (equal wttrin--mode-line-tooltip-data "Paris: ☀️ +61°F Clear"))))
+    (test-wttrin--mode-line-update-display-teardown)))
+
+(provide 'test-wttrin--mode-line-update-display)
+;;; test-wttrin--mode-line-update-display.el ends here