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+;;; test-wttrin-hide-follow-line.el --- Tests for hiding the follow line -*- lexical-binding: t; -*-
+
+;; Copyright (C) 2026 Craig Jennings
+
+;;; Commentary:
+
+;; Unit tests for wttrin--effective-display-options and its effect on
+;; wttrin--build-url: the wttr.in F flag is added to the buffer request when
+;; wttrin-hide-follow-line is non-nil, composing with wttrin-display-options.
+
+;;; Code:
+
+(require 'ert)
+(require 'wttrin)
+
+;;; --------------------------------------------------------------------------
+;;; wttrin--effective-display-options
+;;; --------------------------------------------------------------------------
+
+;;; Normal Cases
+
+(ert-deftest test-wttrin--effective-display-options-normal-hide-empty-adds-f ()
+  "Normal: hiding with no other options yields just the F flag."
+  (let ((wttrin-display-options nil)
+        (wttrin-hide-follow-line t))
+    (should (equal (wttrin--effective-display-options) "F"))))
+
+(ert-deftest test-wttrin--effective-display-options-normal-hide-appends-f ()
+  "Normal: hiding appends F to existing options."
+  (let ((wttrin-display-options "0q")
+        (wttrin-hide-follow-line t))
+    (should (equal (wttrin--effective-display-options) "0qF"))))
+
+(ert-deftest test-wttrin--effective-display-options-normal-no-hide-unchanged ()
+  "Normal: not hiding leaves the options untouched."
+  (let ((wttrin-display-options "0q")
+        (wttrin-hide-follow-line nil))
+    (should (equal (wttrin--effective-display-options) "0q"))))
+
+;;; Boundary Cases
+
+(ert-deftest test-wttrin--effective-display-options-boundary-f-already-present ()
+  "Boundary: F already in the options is not duplicated."
+  (let ((wttrin-display-options "0Fq")
+        (wttrin-hide-follow-line t))
+    (should (equal (wttrin--effective-display-options) "0Fq"))))
+
+(ert-deftest test-wttrin--effective-display-options-boundary-no-hide-nil-options ()
+  "Boundary: not hiding with nil options yields an empty string."
+  (let ((wttrin-display-options nil)
+        (wttrin-hide-follow-line nil))
+    (should (equal (wttrin--effective-display-options) ""))))
+
+;;; --------------------------------------------------------------------------
+;;; wttrin--build-url integration
+;;; --------------------------------------------------------------------------
+
+;;; Normal Cases
+
+(ert-deftest test-wttrin--build-url-normal-hide-appends-f-to-url ()
+  "Normal: hiding the follow line puts the F flag at the end of the URL."
+  (let ((wttrin-display-options nil)
+        (wttrin-hide-follow-line t)
+        (wttrin-unit-system nil))
+    (should (string-suffix-p "F" (wttrin--build-url "Tokyo")))))
+
+(ert-deftest test-wttrin--build-url-normal-no-hide-no-trailing-f ()
+  "Normal: with hiding off, the URL does not end in the F flag."
+  (let ((wttrin-display-options nil)
+        (wttrin-hide-follow-line nil)
+        (wttrin-unit-system nil))
+    (should-not (string-suffix-p "F" (wttrin--build-url "Tokyo")))))
+
+(provide 'test-wttrin-hide-follow-line)
+;;; test-wttrin-hide-follow-line.el ends here