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+;;; test-wttrin--format-staleness-header.el --- Tests for staleness header -*- lexical-binding: t; -*-
+
+;; Copyright (C) 2025 Craig Jennings
+
+;;; Commentary:
+
+;; Unit tests for wttrin--format-staleness-header function.
+;; Tests that buffer staleness information is formatted correctly.
+
+;;; Code:
+
+(require 'ert)
+(require 'wttrin)
+(require 'testutil-wttrin)
+
+;;; Setup and Teardown
+
+(defun test-wttrin--format-staleness-header-setup ()
+  "Setup for staleness header tests."
+  (testutil-wttrin-setup))
+
+(defun test-wttrin--format-staleness-header-teardown ()
+  "Teardown for staleness header tests."
+  (testutil-wttrin-teardown))
+
+;;; Normal Cases
+
+(ert-deftest test-wttrin--format-staleness-header-normal-returns-formatted-string ()
+  "Staleness header includes time and age when cache exists."
+  (test-wttrin--format-staleness-header-setup)
+  (unwind-protect
+      (let ((now 1000000.0))
+        (cl-letf (((symbol-function 'float-time) (lambda () now)))
+          ;; Add data cached 5 minutes ago
+          (testutil-wttrin-add-to-cache "Paris" "weather data" 300)
+          (let ((header (wttrin--format-staleness-header "Paris")))
+            (should header)
+            (should (string-match-p "Last updated:" header))
+            (should (string-match-p "5 minutes ago" header)))))
+    (test-wttrin--format-staleness-header-teardown)))
+
+(ert-deftest test-wttrin--format-staleness-header-normal-just-now ()
+  "Staleness header shows 'just now' for very recent data."
+  (test-wttrin--format-staleness-header-setup)
+  (unwind-protect
+      (let ((now 1000000.0))
+        (cl-letf (((symbol-function 'float-time) (lambda () now)))
+          (testutil-wttrin-add-to-cache "Paris" "weather data" 10)
+          (let ((header (wttrin--format-staleness-header "Paris")))
+            (should (string-match-p "just now" header)))))
+    (test-wttrin--format-staleness-header-teardown)))
+
+;;; Boundary Cases
+
+(ert-deftest test-wttrin--format-staleness-header-boundary-no-cache-returns-nil ()
+  "Returns nil when no cache entry exists for location."
+  (test-wttrin--format-staleness-header-setup)
+  (unwind-protect
+      (should-not (wttrin--format-staleness-header "Unknown City"))
+    (test-wttrin--format-staleness-header-teardown)))
+
+(ert-deftest test-wttrin--format-staleness-header-boundary-different-unit-system ()
+  "Cache lookup respects current unit system setting."
+  (test-wttrin--format-staleness-header-setup)
+  (unwind-protect
+      (let ((now 1000000.0))
+        (cl-letf (((symbol-function 'float-time) (lambda () now)))
+          ;; Add data under metric unit system
+          (let ((wttrin-unit-system "m"))
+            (testutil-wttrin-add-to-cache "Paris" "metric data" 300))
+          ;; Query under imperial unit system — should find nothing
+          (let ((wttrin-unit-system "u"))
+            (should-not (wttrin--format-staleness-header "Paris")))
+          ;; Query under metric — should find it
+          (let ((wttrin-unit-system "m"))
+            (should (wttrin--format-staleness-header "Paris")))))
+    (test-wttrin--format-staleness-header-teardown)))
+
+(provide 'test-wttrin--format-staleness-header)
+;;; test-wttrin--format-staleness-header.el ends here