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+;;; test-wttrin-additional-url-params.el --- Tests for wttrin-additional-url-params -*- lexical-binding: t; -*-
+
+;; Copyright (C) 2025 Craig Jennings
+
+;;; Commentary:
+
+;; Unit tests for wttrin-additional-url-params function.
+;; Tests URL parameter generation with different unit system configurations.
+
+;;; Code:
+
+(require 'ert)
+(require 'wttrin)
+(require 'testutil-wttrin)
+
+;;; Normal Cases
+
+(ert-deftest test-wttrin-additional-url-params-normal-metric-returns-param ()
+  "Test that metric unit system returns ?m parameter."
+  (testutil-wttrin-with-unit-system "m"
+    (should (equal "?m" (wttrin-additional-url-params)))))
+
+(ert-deftest test-wttrin-additional-url-params-normal-uscs-returns-param ()
+  "Test that USCS unit system returns ?u parameter."
+  (testutil-wttrin-with-unit-system "u"
+    (should (equal "?u" (wttrin-additional-url-params)))))
+
+(ert-deftest test-wttrin-additional-url-params-normal-wind-speed-returns-param ()
+  "Test that wind speed unit returns ?M parameter."
+  (testutil-wttrin-with-unit-system "M"
+    (should (equal "?M" (wttrin-additional-url-params)))))
+
+;;; Boundary Cases
+
+(ert-deftest test-wttrin-additional-url-params-boundary-nil-returns-question-mark ()
+  "Test that nil unit system returns just ? parameter."
+  (testutil-wttrin-with-unit-system nil
+    (should (equal "?" (wttrin-additional-url-params)))))
+
+(ert-deftest test-wttrin-additional-url-params-boundary-empty-string-returns-question-mark ()
+  "Test that empty string unit system returns just ? parameter."
+  (testutil-wttrin-with-unit-system ""
+    (should (equal "?" (wttrin-additional-url-params)))))
+
+(ert-deftest test-wttrin-additional-url-params-boundary-single-char-returns-param ()
+  "Test that single character unit system returns ?<char> parameter."
+  (testutil-wttrin-with-unit-system "x"
+    (should (equal "?x" (wttrin-additional-url-params)))))
+
+;;; Error Cases
+
+(ert-deftest test-wttrin-additional-url-params-error-number-signals-error ()
+  "Test that number unit system signals a type error."
+  ;; concat requires string or sequence, number causes wrong-type-argument error
+  (testutil-wttrin-with-unit-system 123
+    (should-error (wttrin-additional-url-params)
+                  :type 'wrong-type-argument)))
+
+(provide 'test-wttrin-additional-url-params)
+;;; test-wttrin-additional-url-params.el ends here