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+;;; test-wttrin--debug-log.el --- Tests for wttrin--debug-log -*- lexical-binding: t; -*-
+
+;; Copyright (C) 2025-2026 Craig Jennings
+
+;;; Commentary:
+
+;; Unit tests for wttrin--debug-log.  Verifies that messages are appended
+;; only when `wttrin-debug' is non-nil, that format args are interpolated,
+;; and that entries are timestamped (timestamp . message) cons cells.
+
+;;; Code:
+
+(require 'ert)
+(require 'wttrin)
+(require 'testutil-wttrin)
+
+(require 'wttrin-debug
+         (expand-file-name "wttrin-debug.el"
+                           (file-name-directory (locate-library "wttrin"))))
+
+;;; Normal Cases
+
+(ert-deftest test-wttrin--debug-log-normal-appends-entry-when-debug-enabled ()
+  "Calling debug-log with debug enabled appends a single entry."
+  (let ((wttrin-debug t)
+        (wttrin--debug-log nil))
+    (wttrin--debug-log "hello")
+    (should (= 1 (length wttrin--debug-log)))))
+
+(ert-deftest test-wttrin--debug-log-normal-interpolates-format-args ()
+  "Format args are passed through `format' before storage."
+  (let ((wttrin-debug t)
+        (wttrin--debug-log nil))
+    (wttrin--debug-log "value=%d name=%s" 42 "foo")
+    (should (string= "value=42 name=foo" (cdar wttrin--debug-log)))))
+
+(ert-deftest test-wttrin--debug-log-normal-prepends-most-recent-entry ()
+  "Most recent entry is at the head of the log (push semantics)."
+  (let ((wttrin-debug t)
+        (wttrin--debug-log nil))
+    (wttrin--debug-log "first")
+    (wttrin--debug-log "second")
+    (should (string= "second" (cdar wttrin--debug-log)))
+    (should (string= "first" (cdadr wttrin--debug-log)))))
+
+(ert-deftest test-wttrin--debug-log-normal-timestamp-has-millisecond-format ()
+  "Timestamp is a string in HH:MM:SS.mmm format."
+  (let ((wttrin-debug t)
+        (wttrin--debug-log nil))
+    (wttrin--debug-log "anything")
+    (should (string-match-p "^[0-9]\\{2\\}:[0-9]\\{2\\}:[0-9]\\{2\\}\\.[0-9]\\{3\\}$"
+                            (caar wttrin--debug-log)))))
+
+;;; Boundary Cases
+
+(ert-deftest test-wttrin--debug-log-boundary-empty-format-string-stores-empty-message ()
+  "An empty format string produces an entry with an empty message."
+  (let ((wttrin-debug t)
+        (wttrin--debug-log nil))
+    (wttrin--debug-log "")
+    (should (= 1 (length wttrin--debug-log)))
+    (should (string= "" (cdar wttrin--debug-log)))))
+
+(ert-deftest test-wttrin--debug-log-boundary-no-format-args-stores-literal-string ()
+  "Calling with only a format string and no args stores it as-is."
+  (let ((wttrin-debug t)
+        (wttrin--debug-log nil))
+    (wttrin--debug-log "literal text with no format directives")
+    (should (string= "literal text with no format directives"
+                     (cdar wttrin--debug-log)))))
+
+;;; Error Cases
+
+(ert-deftest test-wttrin--debug-log-error-no-entry-when-debug-disabled ()
+  "When wttrin-debug is nil, debug-log adds nothing."
+  (let ((wttrin-debug nil)
+        (wttrin--debug-log nil))
+    (wttrin--debug-log "should not appear")
+    (should-not wttrin--debug-log)))
+
+(provide 'test-wttrin--debug-log)
+;;; test-wttrin--debug-log.el ends here