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+;;; test-wttrin--resolve-favorite-location.el --- Tests for wttrin--resolve-favorite-location -*- lexical-binding: t; -*-
+
+;; Copyright (C) 2026 Craig Jennings
+
+;;; Commentary:
+
+;; Unit tests for `wttrin--resolve-favorite-location' and its helpers.
+;; The resolver maps the three modes of `wttrin-favorite-location'
+;; (nil / string / t) onto a returned location string, kicking off an
+;; async geolocation lookup the first time t is seen.
+
+;;; Code:
+
+(require 'ert)
+(require 'cl-lib)
+(require 'wttrin)
+(require 'testutil-wttrin)
+
+;; Load wttrin-geolocation up front so cl-letf mocks of
+;; `wttrin-geolocation-detect' aren't undone by the resolver's lazy
+;; (require 'wttrin-geolocation) — which would otherwise re-evaluate
+;; the defun and overwrite the mocked symbol-function.
+(require 'wttrin-geolocation
+         (expand-file-name "wttrin-geolocation.el"
+                           (file-name-directory (locate-library "wttrin"))))
+
+;;; Helpers
+
+(defmacro test-wttrin--resolve-with-geolocation-mock (returned-location &rest body)
+  "Run BODY with `wttrin-geolocation-detect' mocked to call back with
+RETURNED-LOCATION synchronously.  Resets resolver caches before BODY."
+  (declare (indent 1))
+  `(let ((wttrin--resolved-favorite-location nil)
+         (wttrin--favorite-location-pending nil))
+     (cl-letf (((symbol-function 'wttrin-geolocation-detect)
+                (lambda (callback) (funcall callback ,returned-location))))
+       ,@body)))
+
+;;; Normal Cases
+
+(ert-deftest test-wttrin--resolve-favorite-location-normal-nil-returns-nil ()
+  "Disabled mode (nil) resolves to nil."
+  (let ((wttrin-favorite-location nil)
+        (wttrin--resolved-favorite-location nil)
+        (wttrin--favorite-location-pending nil))
+    (should-not (wttrin--resolve-favorite-location))))
+
+(ert-deftest test-wttrin--resolve-favorite-location-normal-string-returns-itself ()
+  "Explicit string resolves to itself."
+  (let ((wttrin-favorite-location "Berkeley, CA")
+        (wttrin--resolved-favorite-location nil)
+        (wttrin--favorite-location-pending nil))
+    (should (string= "Berkeley, CA" (wttrin--resolve-favorite-location)))))
+
+(ert-deftest test-wttrin--resolve-favorite-location-normal-t-with-cache-returns-cached ()
+  "When t and cache populated, resolver returns cached value without re-fetching."
+  (let ((wttrin-favorite-location t)
+        (wttrin--resolved-favorite-location "Cached, Place")
+        (wttrin--favorite-location-pending nil)
+        (detect-calls 0))
+    (cl-letf (((symbol-function 'wttrin-geolocation-detect)
+               (lambda (_callback) (cl-incf detect-calls))))
+      (should (string= "Cached, Place" (wttrin--resolve-favorite-location)))
+      (should (= 0 detect-calls)))))
+
+(ert-deftest test-wttrin--resolve-favorite-location-normal-t-detection-success-caches-result ()
+  "t-mode detect callback populates the resolver cache."
+  (let ((wttrin-favorite-location t))
+    (test-wttrin--resolve-with-geolocation-mock "Detected, City"
+      (wttrin--resolve-favorite-location)
+      (should (string= "Detected, City" wttrin--resolved-favorite-location))
+      (should-not wttrin--favorite-location-pending))))
+
+(ert-deftest test-wttrin--resolve-favorite-location-normal-second-call-returns-cached-from-first ()
+  "After detection completes, the second call returns the cached string."
+  (let ((wttrin-favorite-location t))
+    (test-wttrin--resolve-with-geolocation-mock "Detected, City"
+      (wttrin--resolve-favorite-location)
+      (should (string= "Detected, City" (wttrin--resolve-favorite-location))))))
+
+;;; Boundary Cases
+
+(ert-deftest test-wttrin--resolve-favorite-location-boundary-t-without-cache-returns-nil ()
+  "First call with t and empty cache returns nil while detection runs."
+  (let ((wttrin-favorite-location t)
+        (wttrin--resolved-favorite-location nil)
+        (wttrin--favorite-location-pending nil))
+    (cl-letf (((symbol-function 'wttrin-geolocation-detect)
+               (lambda (_callback)
+                 ;; Simulate an async lookup that has NOT called back yet.
+                 (setq wttrin--favorite-location-pending t))))
+      (should-not (wttrin--resolve-favorite-location)))))
+
+(ert-deftest test-wttrin--resolve-favorite-location-boundary-pending-suppresses-duplicate-detect ()
+  "When a lookup is already pending, the resolver does not start another."
+  (let ((wttrin-favorite-location t)
+        (wttrin--resolved-favorite-location nil)
+        (wttrin--favorite-location-pending t)
+        (detect-calls 0))
+    (cl-letf (((symbol-function 'wttrin-geolocation-detect)
+               (lambda (_callback) (cl-incf detect-calls))))
+      (wttrin--resolve-favorite-location)
+      (should (= 0 detect-calls)))))
+
+;;; Error Cases
+
+(ert-deftest test-wttrin--resolve-favorite-location-error-detection-failure-leaves-cache-empty ()
+  "When the detect callback returns nil, the cache stays empty and pending clears."
+  (let ((wttrin-favorite-location t))
+    (test-wttrin--resolve-with-geolocation-mock nil
+      (wttrin--resolve-favorite-location)
+      (should-not wttrin--resolved-favorite-location)
+      (should-not wttrin--favorite-location-pending))))
+
+(ert-deftest test-wttrin--resolve-favorite-location-error-detection-failure-allows-retry ()
+  "After a failed detection, a subsequent resolve call kicks off a new lookup."
+  (let ((wttrin-favorite-location t)
+        (wttrin--resolved-favorite-location nil)
+        (wttrin--favorite-location-pending nil)
+        (detect-calls 0))
+    (cl-letf (((symbol-function 'wttrin-geolocation-detect)
+               (lambda (callback)
+                 (cl-incf detect-calls)
+                 (funcall callback nil))))
+      (wttrin--resolve-favorite-location)
+      (wttrin--resolve-favorite-location)
+      (should (= 2 detect-calls)))))
+
+;;; Display Name Helper
+
+(ert-deftest test-wttrin--favorite-location-display-name-resolved-string-returns-string ()
+  "Display name returns the resolved string when available."
+  (let ((wttrin-favorite-location "Tokyo, JP")
+        (wttrin--resolved-favorite-location nil)
+        (wttrin--favorite-location-pending nil))
+    (should (string= "Tokyo, JP" (wttrin--favorite-location-display-name)))))
+
+(ert-deftest test-wttrin--favorite-location-display-name-t-pending-returns-current-location-label ()
+  "Display name returns \"current location\" when t-mode lookup is pending."
+  (let ((wttrin-favorite-location t)
+        (wttrin--resolved-favorite-location nil)
+        (wttrin--favorite-location-pending t))
+    (should (string= "current location"
+                     (wttrin--favorite-location-display-name)))))
+
+(ert-deftest test-wttrin--favorite-location-display-name-nil-returns-nil ()
+  "Display name returns nil when feature is disabled."
+  (let ((wttrin-favorite-location nil)
+        (wttrin--resolved-favorite-location nil)
+        (wttrin--favorite-location-pending nil))
+    (should-not (wttrin--favorite-location-display-name))))
+
+(provide 'test-wttrin--resolve-favorite-location)
+;;; test-wttrin--resolve-favorite-location.el ends here