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+;;; test-wttrin--save-debug-data.el --- Tests for wttrin--save-debug-data -*- lexical-binding: t; -*-
+
+;; Copyright (C) 2025 Craig Jennings
+
+;;; Commentary:
+
+;; Unit tests for wttrin--save-debug-data function.
+;; Tests that debug data files are created correctly with the expected contents.
+
+;;; Code:
+
+(require 'ert)
+(require 'wttrin)
+(require 'testutil-wttrin)
+
+;;; Setup and Teardown
+
+(defun test-wttrin--save-debug-data-setup ()
+  "Setup for save-debug-data tests."
+  (testutil-wttrin-setup))
+
+(defun test-wttrin--save-debug-data-teardown ()
+  "Teardown for save-debug-data tests."
+  (testutil-wttrin-teardown))
+
+;;; Normal Cases
+
+(ert-deftest test-wttrin--save-debug-data-normal-creates-file-that-exists ()
+  "Returned filepath should point to a file that actually exists on disk."
+  (test-wttrin--save-debug-data-setup)
+  (unwind-protect
+      (let ((filepath (wttrin--save-debug-data "Paris" "some weather data")))
+        (unwind-protect
+            (should (file-exists-p filepath))
+          (when (file-exists-p filepath) (delete-file filepath))))
+    (test-wttrin--save-debug-data-teardown)))
+
+(ert-deftest test-wttrin--save-debug-data-normal-file-contains-location ()
+  "File should contain a Location header with the queried location."
+  (test-wttrin--save-debug-data-setup)
+  (unwind-protect
+      (let ((filepath (wttrin--save-debug-data "New Orleans, LA" "weather")))
+        (unwind-protect
+            (let ((contents (with-temp-buffer
+                              (insert-file-contents filepath)
+                              (buffer-string))))
+              (should (string-match-p "^Location: New Orleans, LA$" contents)))
+          (when (file-exists-p filepath) (delete-file filepath))))
+    (test-wttrin--save-debug-data-teardown)))
+
+(ert-deftest test-wttrin--save-debug-data-normal-file-contains-unit-system ()
+  "File should record the wttrin-unit-system value at the time of capture."
+  (test-wttrin--save-debug-data-setup)
+  (unwind-protect
+      (let* ((wttrin-unit-system "m")
+             (filepath (wttrin--save-debug-data "Paris" "weather data")))
+        (unwind-protect
+            (let ((contents (with-temp-buffer
+                              (insert-file-contents filepath)
+                              (buffer-string))))
+              (should (string-match-p "wttrin-unit-system: m" contents)))
+          (when (file-exists-p filepath) (delete-file filepath))))
+    (test-wttrin--save-debug-data-teardown)))
+
+(ert-deftest test-wttrin--save-debug-data-normal-file-contains-raw-response ()
+  "File should contain the raw weather response body after the separator."
+  (test-wttrin--save-debug-data-setup)
+  (unwind-protect
+      (let ((filepath (wttrin--save-debug-data "Berlin" "Clear skies 72°F")))
+        (unwind-protect
+            (let ((contents (with-temp-buffer
+                              (insert-file-contents filepath)
+                              (buffer-string))))
+              (should (string-match-p "--- Raw Response ---" contents))
+              (should (string-match-p "Clear skies 72°F" contents)))
+          (when (file-exists-p filepath) (delete-file filepath))))
+    (test-wttrin--save-debug-data-teardown)))
+
+(ert-deftest test-wttrin--save-debug-data-normal-file-contains-timestamp ()
+  "File should contain a human-readable timestamp."
+  (test-wttrin--save-debug-data-setup)
+  (unwind-protect
+      (let ((filepath (wttrin--save-debug-data "Tokyo" "data")))
+        (unwind-protect
+            (let ((contents (with-temp-buffer
+                              (insert-file-contents filepath)
+                              (buffer-string))))
+              ;; Timestamp should look like YYYY-MM-DD HH:MM:SS
+              (should (string-match-p "^Timestamp: [0-9]\\{4\\}-[0-9]\\{2\\}-[0-9]\\{2\\} [0-9]\\{2\\}:[0-9]\\{2\\}:[0-9]\\{2\\}$"
+                                      contents)))
+          (when (file-exists-p filepath) (delete-file filepath))))
+    (test-wttrin--save-debug-data-teardown)))
+
+;;; Boundary Cases
+
+(ert-deftest test-wttrin--save-debug-data-boundary-unicode-location ()
+  "File should preserve unicode characters in the location name."
+  (test-wttrin--save-debug-data-setup)
+  (unwind-protect
+      (let ((filepath (wttrin--save-debug-data "São Paulo, BR" "data")))
+        (unwind-protect
+            (let ((contents (with-temp-buffer
+                              (insert-file-contents filepath)
+                              (buffer-string))))
+              (should (string-match-p "São Paulo" contents)))
+          (when (file-exists-p filepath) (delete-file filepath))))
+    (test-wttrin--save-debug-data-teardown)))
+
+(ert-deftest test-wttrin--save-debug-data-boundary-empty-raw-string ()
+  "Empty raw-string should still produce a valid file (just no response body)."
+  (test-wttrin--save-debug-data-setup)
+  (unwind-protect
+      (let ((filepath (wttrin--save-debug-data "Paris" "")))
+        (unwind-protect
+            (progn
+              (should (file-exists-p filepath))
+              (let ((contents (with-temp-buffer
+                                (insert-file-contents filepath)
+                                (buffer-string))))
+                ;; Should still have the headers and separator
+                (should (string-match-p "Location: Paris" contents))
+                (should (string-match-p "--- Raw Response ---" contents))))
+          (when (file-exists-p filepath) (delete-file filepath))))
+    (test-wttrin--save-debug-data-teardown)))
+
+;;; Error Cases
+
+(ert-deftest test-wttrin--save-debug-data-error-nil-raw-string-should-not-crash ()
+  "Nil raw-string should be handled gracefully, not cause an insert error.
+This can happen when wttrin--display-weather is called with nil data
+and debug mode is on — save-debug-data is called before validation."
+  (test-wttrin--save-debug-data-setup)
+  (unwind-protect
+      ;; wttrin--save-debug-data calls (insert raw-string) which errors on nil.
+      ;; This test documents the bug: a nil raw-string should produce a file
+      ;; with an empty or placeholder response body, not crash.
+      (let ((filepath (wttrin--save-debug-data "Paris" nil)))
+        (unwind-protect
+            (should (file-exists-p filepath))
+          (when (and filepath (file-exists-p filepath)) (delete-file filepath))))
+    (test-wttrin--save-debug-data-teardown)))
+
+(provide 'test-wttrin--save-debug-data)
+;;; test-wttrin--save-debug-data.el ends here