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+;;; test-wttrin--add-buffer-instructions.el --- Tests for wttrin--add-buffer-instructions -*- lexical-binding: t; -*-
+
+;; Copyright (C) 2024 Craig Jennings
+
+;;; Commentary:
+;; Unit tests for wttrin--add-buffer-instructions function.
+;; Tests adding user instructions to buffer content.
+
+;;; Code:
+
+(require 'ert)
+(require 'wttrin)
+
+;;; Normal Cases
+
+(ert-deftest test-wttrin--add-buffer-instructions-normal-empty-buffer ()
+  "Test adding instructions to empty buffer."
+  (with-temp-buffer
+    (wttrin--add-buffer-instructions)
+    (should (string= "\n\nPress: [a] for another location [g] to refresh [q] to quit"
+                     (buffer-string)))))
+
+(ert-deftest test-wttrin--add-buffer-instructions-normal-with-existing-content ()
+  "Test adding instructions to buffer with existing content."
+  (with-temp-buffer
+    (insert "Weather: Sunny\nTemperature: 20°C")
+    (wttrin--add-buffer-instructions)
+    (should (string= "Weather: Sunny\nTemperature: 20°C\n\nPress: [a] for another location [g] to refresh [q] to quit"
+                     (buffer-string)))))
+
+(ert-deftest test-wttrin--add-buffer-instructions-normal-preserves-point ()
+  "Test that point is moved to end after adding instructions."
+  (with-temp-buffer
+    (insert "Some content")
+    (goto-char (point-min))
+    (wttrin--add-buffer-instructions)
+    (should (= (point) (point-max)))))
+
+(ert-deftest test-wttrin--add-buffer-instructions-normal-idempotent-check ()
+  "Test that calling function twice adds instructions twice (not idempotent)."
+  (with-temp-buffer
+    (insert "Weather")
+    (wttrin--add-buffer-instructions)
+    (let ((first-result (buffer-string)))
+      (wttrin--add-buffer-instructions)
+      ;; Should add instructions again, not check if they already exist
+      (should-not (string= first-result (buffer-string)))
+      ;; Check for two occurrences of "Press:" by counting matches
+      (let ((count 0))
+        (with-temp-buffer
+          (insert first-result)
+          (goto-char (point-min))
+          (while (search-forward "Press:" nil t)
+            (setq count (1+ count))))
+        ;; After first call, should have 1 occurrence
+        (should (= 1 count)))
+      ;; After second call, check for 2 occurrences
+      (let ((count 0))
+        (save-excursion
+          (goto-char (point-min))
+          (while (search-forward "Press:" nil t)
+            (setq count (1+ count))))
+        (should (= 2 count))))))
+
+;;; Boundary Cases
+
+(ert-deftest test-wttrin--add-buffer-instructions-boundary-point-at-beginning ()
+  "Test adding instructions when point is at beginning of buffer."
+  (with-temp-buffer
+    (insert "Weather data here")
+    (goto-char (point-min))
+    (wttrin--add-buffer-instructions)
+    (should (string-suffix-p "Press: [a] for another location [g] to refresh [q] to quit"
+                             (buffer-string)))))
+
+(ert-deftest test-wttrin--add-buffer-instructions-boundary-point-in-middle ()
+  "Test adding instructions when point is in middle of buffer."
+  (with-temp-buffer
+    (insert "Line 1\nLine 2\nLine 3")
+    (goto-char (point-min))
+    (forward-line 1)
+    (wttrin--add-buffer-instructions)
+    (should (string-suffix-p "Press: [a] for another location [g] to refresh [q] to quit"
+                             (buffer-string)))))
+
+(ert-deftest test-wttrin--add-buffer-instructions-boundary-buffer-with-trailing-newlines ()
+  "Test adding instructions to buffer that already ends with newlines."
+  (with-temp-buffer
+    (insert "Weather\n\n\n")
+    (wttrin--add-buffer-instructions)
+    (should (string= "Weather\n\n\n\n\nPress: [a] for another location [g] to refresh [q] to quit"
+                     (buffer-string)))))
+
+(ert-deftest test-wttrin--add-buffer-instructions-boundary-very-large-buffer ()
+  "Test adding instructions to large buffer."
+  (with-temp-buffer
+    (insert (make-string 10000 ?x))
+    (wttrin--add-buffer-instructions)
+    (goto-char (point-max))
+    (should (looking-back "Press: \\[a\\] for another location \\[g\\] to refresh \\[q\\] to quit" nil))))
+
+;;; Error Cases
+
+(ert-deftest test-wttrin--add-buffer-instructions-error-read-only-buffer ()
+  "Test that function signals error when buffer is read-only."
+  (with-temp-buffer
+    (insert "Some content")
+    (read-only-mode 1)
+    (should-error (wttrin--add-buffer-instructions)
+                  :type 'buffer-read-only)))
+
+(ert-deftest test-wttrin--add-buffer-instructions-error-narrowed-buffer ()
+  "Test adding instructions to narrowed buffer adds at narrowed end."
+  (with-temp-buffer
+    (insert "Line 1\nLine 2\nLine 3\nLine 4")
+    (goto-char (point-min))
+    (forward-line 1)
+    (let ((start (point)))
+      (forward-line 1)
+      (narrow-to-region start (point))
+      (wttrin--add-buffer-instructions)
+      ;; Instructions should be added at end of narrowed region
+      (widen)
+      (goto-char start)
+      (forward-line 1)
+      (should (looking-at-p "\n\nPress:")))))
+
+(provide 'test-wttrin--add-buffer-instructions)
+;;; test-wttrin--add-buffer-instructions.el ends here