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+;;; test-wttrin--buffer-cache-refresh.el --- Tests for wttrin--buffer-cache-refresh -*- lexical-binding: t; -*-
+
+;; Copyright (C) 2025 Craig Jennings
+
+;;; Commentary:
+
+;; Unit tests for wttrin--buffer-cache-refresh function.
+;; Tests the proactive background refresh that keeps buffer cache fresh.
+
+;;; Code:
+
+(require 'ert)
+(require 'wttrin)
+(require 'testutil-wttrin)
+
+;;; Setup and Teardown
+
+(defun test-wttrin--buffer-cache-refresh-setup ()
+  "Setup for buffer-cache-refresh tests."
+  (testutil-wttrin-setup))
+
+(defun test-wttrin--buffer-cache-refresh-teardown ()
+  "Teardown for buffer-cache-refresh tests."
+  (testutil-wttrin-teardown))
+
+;;; Normal Cases
+
+(ert-deftest test-wttrin--buffer-cache-refresh-normal-success-updates-cache ()
+  "Successful fetch should store fresh data in the buffer cache."
+  (test-wttrin--buffer-cache-refresh-setup)
+  (unwind-protect
+      (let ((wttrin-favorite-location "Paris"))
+        (testutil-wttrin-mock-http-response "Fresh weather data for Paris"
+          (wttrin--buffer-cache-refresh)
+          ;; Cache should now have an entry for Paris
+          (let* ((cache-key (wttrin--make-cache-key "Paris"))
+                 (cached (gethash cache-key wttrin--cache)))
+            (should cached)
+            (should (equal (cdr cached) "Fresh weather data for Paris")))))
+    (test-wttrin--buffer-cache-refresh-teardown)))
+
+(ert-deftest test-wttrin--buffer-cache-refresh-normal-uses-favorite-location ()
+  "Refresh should fetch weather for the configured favorite location."
+  (test-wttrin--buffer-cache-refresh-setup)
+  (unwind-protect
+      (let ((wttrin-favorite-location "Tokyo, JP")
+            (fetched-query nil))
+        (cl-letf (((symbol-function 'wttrin-fetch-raw-string)
+                   (lambda (query callback)
+                     (setq fetched-query query)
+                     (funcall callback "some data"))))
+          (wttrin--buffer-cache-refresh)
+          (should (equal fetched-query "Tokyo, JP"))))
+    (test-wttrin--buffer-cache-refresh-teardown)))
+
+(ert-deftest test-wttrin--buffer-cache-refresh-normal-cache-key-respects-unit-system ()
+  "Cache entry should use the correct key based on unit system settings."
+  (test-wttrin--buffer-cache-refresh-setup)
+  (unwind-protect
+      (let ((wttrin-favorite-location "Paris")
+            (wttrin-unit-system "m"))
+        (testutil-wttrin-mock-http-response "metric data"
+          (wttrin--buffer-cache-refresh)
+          ;; Cache key should include unit system
+          (let* ((expected-key (wttrin--make-cache-key "Paris"))
+                 (cached (gethash expected-key wttrin--cache)))
+            (should cached)
+            (should (equal (cdr cached) "metric data")))))
+    (test-wttrin--buffer-cache-refresh-teardown)))
+
+;;; Boundary Cases
+
+(ert-deftest test-wttrin--buffer-cache-refresh-boundary-nil-location-is-noop ()
+  "When favorite-location is nil, no fetch should be attempted."
+  (test-wttrin--buffer-cache-refresh-setup)
+  (unwind-protect
+      (let ((wttrin-favorite-location nil)
+            (fetch-called nil))
+        (cl-letf (((symbol-function 'wttrin-fetch-raw-string)
+                   (lambda (_query _callback) (setq fetch-called t))))
+          (wttrin--buffer-cache-refresh)
+          (should-not fetch-called)
+          (should (= 0 (testutil-wttrin-cache-size)))))
+    (test-wttrin--buffer-cache-refresh-teardown)))
+
+(ert-deftest test-wttrin--buffer-cache-refresh-boundary-overwrites-stale-entry ()
+  "A refresh should replace any existing stale cache entry for the same location."
+  (test-wttrin--buffer-cache-refresh-setup)
+  (unwind-protect
+      (let ((wttrin-favorite-location "Paris"))
+        ;; Seed cache with old data
+        (testutil-wttrin-add-to-cache "Paris" "old stale data" 9999)
+        (testutil-wttrin-mock-http-response "fresh new data"
+          (wttrin--buffer-cache-refresh)
+          (let* ((cache-key (wttrin--make-cache-key "Paris"))
+                 (cached (gethash cache-key wttrin--cache)))
+            (should (equal (cdr cached) "fresh new data")))))
+    (test-wttrin--buffer-cache-refresh-teardown)))
+
+;;; Error Cases
+
+(ert-deftest test-wttrin--buffer-cache-refresh-error-fetch-failure-preserves-cache ()
+  "Failed fetch should not overwrite existing cache entry."
+  (test-wttrin--buffer-cache-refresh-setup)
+  (unwind-protect
+      (let ((wttrin-favorite-location "Paris"))
+        ;; Seed cache with existing data
+        (testutil-wttrin-add-to-cache "Paris" "existing good data" 300)
+        (cl-letf (((symbol-function 'wttrin-fetch-raw-string)
+                   (lambda (_query callback) (funcall callback nil))))
+          (wttrin--buffer-cache-refresh)
+          ;; Existing cache should be untouched
+          (let* ((cache-key (wttrin--make-cache-key "Paris"))
+                 (cached (gethash cache-key wttrin--cache)))
+            (should cached)
+            (should (equal (cdr cached) "existing good data")))))
+    (test-wttrin--buffer-cache-refresh-teardown)))
+
+(provide 'test-wttrin--buffer-cache-refresh)
+;;; test-wttrin--buffer-cache-refresh.el ends here