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+;;; test-wttrin--replace-response-location.el --- Tests for wttrin--replace-response-location -*- lexical-binding: t; -*-
+
+;; Copyright (C) 2025-2026 Craig Jennings
+
+;;; Commentary:
+
+;; Unit tests for wttrin--replace-response-location function.
+;; Tests that the API's lowercase location prefix gets replaced with the
+;; user's original casing while preserving the weather data.
+
+;;; Code:
+
+(require 'ert)
+(require 'wttrin)
+(require 'testutil-wttrin)
+
+;;; Setup and Teardown
+
+(defun test-wttrin--replace-response-location-setup ()
+  "Setup for replace-response-location tests."
+  (testutil-wttrin-setup))
+
+(defun test-wttrin--replace-response-location-teardown ()
+  "Teardown for replace-response-location tests."
+  (testutil-wttrin-teardown))
+
+;;; Normal Cases
+
+(ert-deftest test-wttrin--replace-response-location-normal-replaces-lowercase ()
+  "API's lowercase location should be replaced with user's original casing."
+  (should (equal (wttrin--replace-response-location
+                  "new orleans, la: ☀️ +72°F Clear"
+                  "New Orleans, LA")
+                 "New Orleans, LA: ☀️ +72°F Clear")))
+
+(ert-deftest test-wttrin--replace-response-location-normal-preserves-weather-data ()
+  "Everything after the colon should be identical to the original response."
+  (let* ((response "berlin: 🌧 +48°F Light rain")
+         (result (wttrin--replace-response-location response "Berlin")))
+    ;; Weather data portion should be untouched
+    (should (string-suffix-p ": 🌧 +48°F Light rain" result))))
+
+;;; Boundary Cases
+
+(ert-deftest test-wttrin--replace-response-location-boundary-already-correct ()
+  "When API casing already matches user's, the string should be unchanged."
+  (should (equal (wttrin--replace-response-location
+                  "Paris: ☀️ +61°F Clear"
+                  "Paris")
+                 "Paris: ☀️ +61°F Clear")))
+
+(ert-deftest test-wttrin--replace-response-location-boundary-unicode ()
+  "Unicode location should be preserved correctly in the replacement."
+  (should (equal (wttrin--replace-response-location
+                  "são paulo: 🌤 +77°F Partly cloudy"
+                  "São Paulo")
+                 "São Paulo: 🌤 +77°F Partly cloudy")))
+
+(ert-deftest test-wttrin--replace-response-location-boundary-no-colon ()
+  "Response with no colon should be returned unchanged."
+  (should (equal (wttrin--replace-response-location
+                  "Unknown location"
+                  "Paris")
+                 "Unknown location")))
+
+;;; Integration: fetch populates cache with corrected casing
+
+(ert-deftest test-wttrin--mode-line-fetch-caches-user-location-casing ()
+  "After a successful fetch, the cached string should use the user's
+location casing, not the API's lowercase version."
+  (test-wttrin--replace-response-location-setup)
+  (unwind-protect
+      (let ((wttrin-favorite-location "New Orleans, LA")
+            (wttrin--mode-line-cache nil))
+        (testutil-wttrin-mock-http-response
+            "new orleans, la: ☀️ +72°F Clear"
+          (wttrin--mode-line-fetch-weather)
+          (should wttrin--mode-line-cache)
+          (let ((cached-string (cdr wttrin--mode-line-cache)))
+            (should (string-prefix-p "New Orleans, LA:" cached-string)))))
+    (test-wttrin--replace-response-location-teardown)))
+
+(provide 'test-wttrin--replace-response-location)
+;;; test-wttrin--replace-response-location.el ends here